ArmenianChinese (Traditional)EnglishGermanRussian
Python Tutorial -

Python Tutorial

Python courses with W3s interpretation & Video Lessons

Learn python in 7 hours! Python Lessons A complete course of learning to program in python from scratch

  1.   00:00  Introduction to the course of python lessons
  2.   00:45  Installing python, the first program
  3.   10:47  Variables
  4.   20:56  Data types in pytho
  5.   30:02  Conditional statements if, elif, else
  6.   39:03  Conditional statements 2
  7.   45:53  Conditional Operators Practice
  8.   56:44  while loop
  9.   1:12:37  for loop
  10.   1:21:23  A for loop, a loop within a loop
  11.   1:29:34  Lists, list data type
  12.   1:41:29  Lists, methods of the list data type
  13.   1:53:21  Tuples, tuple data type
  14.   2:08:42  Practice python, os module, walk function
  15.   2:21:16  def functions, definition, and call
  16.   2:30:07  def functions, parameters, and arguments
  17.   2:43:49  Functions variable number of arguments, *args parameter
  18.   2:56:28  Functions, variable scope
  19.   3:08:49  Functions and code structure
  20.   3:22:07  Dictionaries, data type dict
  21.   3:37:22  Dictionaries, dict methods
  22.   3:49:21  Reading and writing files
  23.   4:05:29  Sets, set data type
  24.   4:22:30  Strings escaped characters
  25.   4:30:17  Strings, str methods
  26.   4:43:13  f-string, string formatting
  27.   4:50:10  Exception handling, try, except
  28.   5:09:59  Context manager with as
  29.   5:16:19  Import modules, if __name__ == ‘__main__”
  30.   5:34:58  Dice game in python Tkinter
  31.   5:56:53  Decorators
  32.   6:10:36  List, dictionary, set generators
  33.   6:30:56  Generator expression
  34.   6:43:03  Generator function, the yield statement   
  35.   6:51:51  lambda function

Python Tutorial

Python lessons for beginners from scratch – a complete course for learning to program in Python. Learn the python programming language for free, basics from scratch, complete python learning lecture course. The course video covers all the main topics with examples and practice necessary for a junior python developer. At the end of the course, you will know all the basic python syntax and be free to write your own code!

Python is a popular programming language. Python can be used on a server to create web applications.

1. Whetting Your Appetite

If you do much work on computers, eventually you find that there’s some task you’d like to automate. For example, you may wish to perform a search-and-replace over a large number of text files, or rename and rearrange a bunch of photo files in a complicated way. Perhaps you’d like to write a small custom database, or a specialized GUI application, or a simple game.

>>> fruits = ['orange', 'apple', 'pear', 'banana', 'kiwi', 'apple', 'banana']
>>> fruits.count('apple')
2
>>> fruits.count('tangerine')
0
>>> fruits.index('banana')
3
>>> fruits.index('banana', 4)  # Find next banana starting a position 4
6
>>> fruits.reverse()
>>> fruits
['banana', 'apple', 'kiwi', 'banana', 'pear', 'apple', 'orange']
>>> fruits.append('grape')
>>> fruits
['banana', 'apple', 'kiwi', 'banana', 'pear', 'apple', 'orange', 'grape']
>>> fruits.sort()
>>> fruits
['apple', 'apple', 'banana', 'banana', 'grape', 'kiwi', 'orange', 'pear']
>>> fruits.pop()
'pear'

If you’re a professional software developer, you may have to work with several C/C++/Java libraries but find the usual write/compile/test/re-compile cycle is too slow. Perhaps you’re writing a test suite for such a library and find writing the testing code a tedious task. Or maybe you’ve written a program that could use an extension language, and you don’t want to design and implement a whole new language for your application.

Python is just the language for you.

You could write a Unix shell script or Windows batch files for some of these tasks, but shell scripts are best at moving around files and changing text data, not well-suited for GUI applications or games. You could write a C/C++/Java program, but it can take a lot of development time to get even a first-draft program. Python is simpler to use, available on Windows, macOS, and Unix operating systems, and will help you get the job done more quickly.

Python is simple to use, but it is a real programming language, offering much more structure and support for large programs than shell scripts or batch files can offer. On the other hand, Python also offers much more error checking than C, and, being a very-high-level language, it has high-level data types built in, such as flexible arrays and dictionaries. Because of its more general data types Python is applicable to a much larger problem domain than Awk or even Perl, yet many things are at least as easy in Python as in those languages.

Python allows you to split your program into modules that can be reused in other Python programs. It comes with a large collection of standard modules that you can use as the basis of your programs — or as examples to start learning to program in Python. Some of these modules provide things like file I/O, system calls, sockets, and even interfaces to graphical user interface toolkits like Tk.

Python is an interpreted language, which can save you considerable time during program development because no compilation and linking is necessary. The interpreter can be used interactively, which makes it easy to experiment with features of the language, to write throw-away programs, or to test functions during bottom-up program development. It is also a handy desk calculator.

Python enables programs to be written compactly and readably. Programs written in Python are typically much shorter than equivalent C, C++, or Java programs, for several reasons:

  • the high-level data types allow you to express complex operations in a single statement;

  • statement grouping is done by indentation instead of beginning and ending brackets;

  • no variable or argument declarations are necessary.

Python is extensible: if you know how to program in C it is easy to add a new built-in function or module to the interpreter, either to perform critical operations at maximum speed, or to link Python programs to libraries that may only be available in binary form (such as a vendor-specific graphics library). Once you are really hooked, you can link the Python interpreter into an application written in C and use it as an extension or command language for that application.

By the way, the language is named after the BBC show “Monty Python’s Flying Circus” and has nothing to do with reptiles. Making references to Monty Python skits in documentation is not only allowed, it is encouraged!

Now that you are all excited about Python, you’ll want to examine it in some more detail. Since the best way to learn a language is to use it, the tutorial invites you to play with the Python interpreter as you read.

In the next chapter, the mechanics of using the interpreter are explained. This is rather mundane information, but essential for trying out the examples shown later.

The rest of the tutorial introduces various features of the Python language and system through examples, beginning with simple expressions, statements and data types, through functions and modules, and finally touching upon advanced concepts like exceptions and user-defined classes.

Python Tutorial

Learn Python

Python is a popular programming language.

Python can be used on a server to create web applications.


Learning by Examples

With our “Try it Yourself” editor, you can edit Python code and view the result.

Example

print("Hello, World!")

Try it Yourself »

Click on the “Try it Yourself” button to see how it works.


 

Python File Handling

In our File Handling section you will learn how to open, read, write, and delete files.

Python File Handling


 

Python Database Handling

In our database section you will learn how to access and work with MySQL and MongoDB databases:

Python MySQL Tutorial

Python MongoDB Tutorial


 

Python Exercises

Test Yourself With Exercises

Exercise:

Insert the missing part of the code below to output “Hello World”.

("Hello World")

Start the Exercise


 

Python Examples

Learn by examples! This tutorial supplements all explanations with clarifying examples.

See All Python Examples


Python Quiz

Test your Python skills with a quiz.

Python Quiz


Python Reference

You will also find complete function and method references:

Reference Overview

Built-in Functions

String Methods

List/Array Methods

Dictionary Methods

Tuple Methods

Set Methods

File Methods

Python Keywords

Python Exceptions

Python Glossary

Random Module

Requests Module

Math Module

CMath Module


Download Python

Download Python from the official Python web site: https://python.org

Python Introduction

 

What is Python?

Python is a popular programming language. It was created by Guido van Rossum, and released in 1991.

It is used for:

  • web development (server-side),
  • software development,
  • mathematics,
  • system scripting.

What can Python do?

  • Python can be used on a server to create web applications.
  • Python can be used alongside software to create workflows.
  • Python can connect to database systems. It can also read and modify files.
  • Python can be used to handle big data and perform complex mathematics.
  • Python can be used for rapid prototyping, or for production-ready software development.

Why Python?

  • Python works on different platforms (Windows, Mac, Linux, Raspberry Pi, etc).
  • Python has a simple syntax similar to the English language.
  • Python has syntax that allows developers to write programs with fewer lines than some other programming languages.
  • Python runs on an interpreter system, meaning that code can be executed as soon as it is written. This means that prototyping can be very quick.
  • Python can be treated in a procedural way, an object-oriented way or a functional way.

Good to know

  • The most recent major version of Python is Python 3, which we shall be using in this tutorial. However, Python 2, although not being updated with anything other than security updates, is still quite popular.
  • In this tutorial Python will be written in a text editor. It is possible to write Python in an Integrated Development Environment, such as Thonny, Pycharm, Netbeans or Eclipse which are particularly useful when managing larger collections of Python files.

Python Syntax compared to other programming languages

  • Python was designed for readability, and has some similarities to the English language with influence from mathematics.
  • Python uses new lines to complete a command, as opposed to other programming languages which often use semicolons or parentheses.
  • Python relies on indentation, using whitespace, to define scope; such as the scope of loops, functions and classes. Other programming languages often use curly-brackets for this purpose.

Example

print("Hello, World!")

Try it Yourself »

Python Getting Started

 

Python Install

Many PCs and Macs will have python already installed.

To check if you have python installed on a Windows PC, search in the start bar for Python or run the following on the Command Line (cmd.exe):

C:\Users\Your Name>python –version
 

To check if you have python installed on a Linux or Mac, then on linux open the command line or on Mac open the Terminal and type:

python –version
 

If you find that you do not have Python installed on your computer, then you can download it for free from the following website: https://www.python.org/


 

Python Quickstart

Python is an interpreted programming language, this means that as a developer you write Python (.py) files in a text editor and then put those files into the python interpreter to be executed.

The way to run a python file is like this on the command line:

C:\Users\Your Name>python helloworld.py
 

Where “helloworld.py” is the name of your python file.

Let’s write our first Python file, called helloworld.py, which can be done in any text editor.

helloworld.py

print("Hello, World!")

Try it Yourself »

Simple as that. Save your file. Open your command line, navigate to the directory where you saved your file, and run:

C:\Users\Your Name>python helloworld.py

The output should read:

Hello, World!

Congratulations, you have written and executed your first Python program.

The Python Command Line

To test a short amount of code in python sometimes it is quickest and easiest not to write the code in a file. This is made possible because Python can be run as a command line itself.

Type the following on the Windows, Mac or Linux command line:

C:\Users\Your Name>python

Or, if the “python” command did not work, you can try “py”:

C:\Users\Your Name>py

From there you can write any python, including our hello world example from earlier in the tutorial:

C:\Users\Your Name>python
Python 3.6.4 (v3.6.4:d48eceb, Dec 19 2017, 06:04:45) [MSC v.1900 32 bit (Intel)] on win32
Type “help”, “copyright”, “credits” or “license” for more information.
>>> print(“Hello, World!”)

Which will write “Hello, World!” in the command line:

C:\Users\Your Name>python
Python 3.6.4 (v3.6.4:d48eceb, Dec 19 2017, 06:04:45) [MSC v.1900 32 bit (Intel)] on win32
Type “help”, “copyright”, “credits” or “license” for more information.
>>> print(“Hello, World!”)
Hello, World!

Whenever you are done in the python command line, you can simply type the following to quit the python command line interface:

exit()

Python Syntax

Execute Python Syntax

As we learned in the previous page, Python syntax can be executed by writing directly in the Command Line:

>>> print(“Hello, World!”)
Hello, World!
 
On this page

Or by creating a python file on the server, using the .py file extension, and running it in the Command Line:

C:\Users\Your Name>python myfile.py

Python Indentation

Indentation refers to the spaces at the beginning of a code line.

Where in other programming languages the indentation in code is for readability only, the indentation in Python is very important.

Python uses indentation to indicate a block of code.

Example

if 5 2:
  print("Five is greater than two!")

Try it Yourself »

Python will give you an error if you skip the indentation:

Example

Syntax Error:

if 5 2:
print("Five is greater than two!")

Try it Yourself »

The number of spaces is up to you as a programmer, the most common use is four, but it has to be at least one.

Example

if 5 2:
  print("Five is greater than two!")
if 5 2:
print("Five is greater than two!")

Try it Yourself »

You have to use the same number of spaces in the same block of code, otherwise Python will give you an error:

Example

Syntax Error:

if 5 2:
  print("Five is greater than two!")
print("Five is greater than two!")

Try it Yourself »


Python Variables

In Python, variables are created when you assign a value to it:

Example

Variables in Python:

x = 5
y = "Hello, World!"

Try it Yourself »

Python has no command for declaring a variable.

You will learn more about variables in the Python Variables chapter.


Comments

Python has commenting capability for the purpose of in-code documentation.

Comments start with a #, and Python will render the rest of the line as a comment:

Example

Comments in Python:

#This is a comment.
print("Hello, World!")

Try it Yourself »


 

Test Yourself With Exercises

Exercise:

Insert the missing part of the code below to output “Hello World”.

("Hello World")

Start the Exercise

Python Comments

Comments can be used to explain Python code.

Comments can be used to make the code more readable.

Comments can be used to prevent execution when testing code.


Creating a Comment

Comments starts with a #, and Python will ignore them:

Example

#This is a comment
print("Hello, World!")

Try it Yourself »

Comments can be placed at the end of a line, and Python will ignore the rest of the line:

Example

print("Hello, World!"#This is a comment

Try it Yourself »

A comment does not have to be text that explains the code, it can also be used to prevent Python from executing code:

Example

#print("Hello, World!")
print("Cheers, Mate!")

Try it Yourself »


 

Multi Line Comments

Python does not really have a syntax for multi line comments.

To add a multiline comment you could insert a # for each line:

Example

#This is a comment
#written in
#more than just one line
print("Hello, World!")

Try it Yourself »

Or, not quite as intended, you can use a multiline string.

Since Python will ignore string literals that are not assigned to a variable, you can add a multiline string (triple quotes) in your code, and place your comment inside it:

Example

"""
This is a comment
written in
more than just one line
"
""

print("Hello, World!")

Try it Yourself »

 
As long as the string is not assigned to a variable, Python will read the code, but then ignore it, and you have made a multiline comment.

Test Yourself With Exercises

Exercise:

Comments in Python are written with a special character, which one?

This is a comment

Start the Exercise

Python Data Types

 

Built-in Data Types

In programming, data type is an important concept.

Variables can store data of different types, and different types can do different things.

Python has the following data types built-in by default, in these categories:

Text Type:str
Numeric Types:intfloatcomplex
Sequence Types:listtuplerange
Mapping Type:dict
Set Types:setfrozenset
Boolean Type:bool
Binary Types:bytesbytearraymemoryview

 

Getting the Data Type

You can get the data type of any object by using the type() function:

Example

Print the data type of the variable x:

x = 5
print(type(x))

Try it Yourself »


 

Setting the Data Type

In Python, the data type is set when you assign a value to a variable:

ExampleData TypeTry it
x = “Hello World”strTry it »
x = 20intTry it »
x = 20.5floatTry it »
x = 1jcomplexTry it »
x = [“apple”, “banana”, “cherry”]listTry it »
x = (“apple”, “banana”, “cherry”)tupleTry it »
x = range(6)rangeTry it »
x = {“name” : “John”, “age” : 36}dictTry it »
x = {“apple”, “banana”, “cherry”}setTry it »
x = frozenset({“apple”, “banana”, “cherry”})frozensetTry it »
x = TrueboolTry it »
x = b”Hello”bytesTry it »
x = bytearray(5)bytearrayTry it »
x = memoryview(bytes(5))memoryviewTry it »

 

Setting the Specific Data Type

If you want to specify the data type, you can use the following constructor functions:

ExampleData TypeTry it
x = str(“Hello World”)strTry it »
x = int(20)intTry it »
x = float(20.5)floatTry it »
x = complex(1j)complexTry it »
x = list((“apple”, “banana”, “cherry”))listTry it »
x = tuple((“apple”, “banana”, “cherry”))tupleTry it »
x = range(6)rangeTry it »
x = dict(name=”John”, age=36)dictTry it »
x = set((“apple”, “banana”, “cherry”))setTry it »
x = frozenset((“apple”, “banana”, “cherry”))frozensetTry it »
x = bool(5)boolTry it »
x = bytes(5)bytesTry it »
x = bytearray(5)bytearrayTry it »
x = memoryview(bytes(5))memoryviewTry it »

 

Test Yourself With Exercises

Exercise:

The following code example would print the data type of x, what data type would that be?

x = 5
print(type(x))

Start the Exercise

Python Numbers

Python Numbers

There are three numeric types in Python:

  • int
  • float
  • complex

Variables of numeric types are created when you assign a value to them:

Example

x = 1    # int
y = 2.8  # float
z = 1j   # complex

To verify the type of any object in Python, use the type() function:

Example

print(type(x))
print(type(y))
print(type(z))

Try it Yourself »


Int

Int, or integer, is a whole number, positive or negative, without decimals, of unlimited length.

Example

Integers:

x = 1
y = 35656222554887711
z = -3255522


print(type(x))
print(type(y))
print(type(z))

Try it Yourself »


Float

Float, or “floating point number” is a number, positive or negative, containing one or more decimals.

Example

Floats:

x = 1.10
y = 1.0
z = -35.59


print(type(x))
print(type(y))
print(type(z))

Try it Yourself »

Float can also be scientific numbers with an “e” to indicate the power of 10.

Example

Floats:

x = 35e3
y = 12E4
z = -87.7e100


print(type(x))
print(type(y))
print(type(z))

Try it Yourself »


 

Complex

Complex numbers are written with a “j” as the imaginary part:

Example

Complex:

x = 3+5j
y = 5j
z = -5j


print(type(x))
print(type(y))
print(type(z))

Try it Yourself »


Type Conversion

You can convert from one type to another with the int()float(), and complex() methods:

Example

Convert from one type to another:

x = 1    # int
y = 2.8  # float
z = 1j   # complex

#convert from int to float:
a = float(x)


#convert from float to int:
b = int(y)

#convert from int to complex:
c = complex(x)

print(a)
print(b)
print(c)

print(type(x))
print(type(y))
print(type(z))

Try it Yourself »

Note: You cannot convert complex numbers into another number type.


Random Number

Python does not have a random() function to make a random number, but Python has a built-in module called random that can be used to make random numbers:

Example

Import the random module, and display a random number between 1 and 9:

import random

print(random.randrange(110))

Try it Yourself »

In our Random Module Reference you will learn more about the Random module.


Test Yourself With Exercises

Exercise:

Insert the correct syntax to convert x into a floating point number.

x = 5
x = (x)

Start the Exercise

Python Casting

Specify a Variable Type

There may be times when you want to specify a type on to a variable. This can be done with casting. Python is an object-orientated language, and as such it uses classes to define data types, including its primitive types.

Casting in python is therefore done using constructor functions:

  • int() – constructs an integer number from an integer literal, a float literal (by removing all decimals), or a string literal (providing the string represents a whole number)
  • float() – constructs a float number from an integer literal, a float literal or a string literal (providing the string represents a float or an integer)
  • str() – constructs a string from a wide variety of data types, including strings, integer literals and float literals

Example

Integers:

x = int(1)   # x will be 1
y = int(2.8) # y will be 2
z = int("3"# z will be 3

Try it Yourself »

Example

Floats:

x = float(1)     # x will be 1.0
y = float(2.8)   # y will be 2.8
z = float("3")   # z will be 3.0
w = float("4.2"# w will be 4.2

Try it Yourself »

Example

Strings:

x = str("s1"# x will be 's1'
y = str(2)    # y will be '2'
z = str(3.0 # z will be '3.0'

Try it Yourself »

Python Booleans

Booleans represent one of two values: True or False.


Boolean Values

In programming you often need to know if an expression is True or False.

You can evaluate any expression in Python, and get one of two answers, True or False.

When you compare two values, the expression is evaluated and Python returns the Boolean answer:

Example

print(10 9)
print(10 == 9)
print(10 9)

Try it Yourself »

When you run a condition in an if statement, Python returns True or False:

Example

Print a message based on whether the condition is True or False:

a = 200
b = 33


if b > a:
  print("b is greater than a")
else:
  print("b is not greater than a")

Try it Yourself »


Evaluate Values and Variables

The bool() function allows you to evaluate any value, and give you True or False in return,

Example

Evaluate a string and a number:

print(bool(“Hello”))
print(bool(15))

Try it Yourself »

Example

Evaluate two variables:

x = "Hello"
y = 15


print(bool(x))
print(bool(y))

Try it Yourself »


 

Most Values are True

Almost any value is evaluated to True if it has some sort of content.

Any string is True, except empty strings.

Any number is True, except 0.

Any list, tuple, set, and dictionary are True, except empty ones.

Example

The following will return True:

bool("abc")
bool(123)
bool(["apple""cherry""banana"])

Try it Yourself »


Some Values are False

In fact, there are not many values that evaluate to False, except empty values, such as ()[]{}"", the number 0, and the value None. And of course the value False evaluates to False.

Example

The following will return False:

bool(False)
bool(None)
bool(0)
bool("")
bool(())
bool([])
bool({})

Try it Yourself »

One more value, or object in this case, evaluates to False, and that is if you have an object that is made from a class with a __len__ function that returns 0 or False:

Example

class myclass():
  def __len__(self):
    return 0


myobj = myclass()
print(bool(myobj))

Try it Yourself »


Functions can Return a Boolean

You can create functions that returns a Boolean Value:

Example

Print the answer of a function:

def myFunction() :
  return True


print(myFunction())

Try it Yourself »

You can execute code based on the Boolean answer of a function:

Example

Print “YES!” if the function returns True, otherwise print “NO!”:

def myFunction() :
  return True


if myFunction():
  print("YES!")
else:
  print("NO!")

Try it Yourself »

Python also has many built-in functions that return a boolean value, like the isinstance() function, which can be used to determine if an object is of a certain data type:

Example

Check if an object is an integer or not:

x = 200
print(isinstance(x, int))

Try it Yourself »


Test Yourself With Exercises

Exercise:

The statement below would print a Boolean value, which one?

print(10 > 9)


Start the Exercise

Python Operators

Python Operators

Operators are used to perform operations on variables and values.

In the example below, we use the + operator to add together two values:

Example

print(10 + 5)

Run example »

Python divides the operators in the following groups:

  • Arithmetic operators
  • Assignment operators
  • Comparison operators
  • Logical operators
  • Identity operators
  • Membership operators
  • Bitwise operators

Python Arithmetic Operators

Arithmetic operators are used with numeric values to perform common mathematical operations:

OperatorNameExampleTry it
+Additionx + yTry it »
Subtractionx – yTry it »
*Multiplicationx * yTry it »
/Divisionx / yTry it »
%Modulusx % yTry it »
**Exponentiationx ** yTry it »
//Floor divisionx // yTry it »

Python Assignment Operators

Assignment operators are used to assign values to variables:

OperatorExampleSame AsTry it
=x = 5x = 5Try it »
+=x += 3x = x + 3Try it »
-=x -= 3x = x – 3Try it »
*=x *= 3x = x * 3Try it »
/=x /= 3x = x / 3Try it »
%=x %= 3x = x % 3Try it »
//=x //= 3x = x // 3Try it »
**=x **= 3x = x ** 3Try it »
&=x &= 3x = x & 3Try it »
|=x |= 3x = x | 3Try it »
^=x ^= 3x = x ^ 3Try it »
>>=x >>= 3x = x >> 3Try it »
<<=x <<= 3x = x << 3Try it »

Python Comparison Operators

Comparison operators are used to compare two values:

OperatorNameExampleTry it
==Equalx == yTry it »
!=Not equalx != yTry it »
>Greater thanx > yTry it »
<Less thanx < yTry it »
>=Greater than or equal tox >= yTry it »
<=Less than or equal tox <= yTry it »

Python Logical Operators

Logical operators are used to combine conditional statements:

OperatorDescriptionExampleTry it
and Returns True if both statements are truex < 5 and  x < 10Try it »
orReturns True if one of the statements is truex < 5 or x < 4Try it »
notReverse the result, returns False if the result is truenot(x < 5 and x < 10)Try it »

Python Identity Operators

Identity operators are used to compare the objects, not if they are equal, but if they are actually the same object, with the same memory location:

OperatorDescriptionExampleTry it
is Returns True if both variables are the same objectx is yTry it »
is notReturns True if both variables are not the same objectx is not yTry it »

Python Membership Operators

Membership operators are used to test if a sequence is presented in an object:

OperatorDescriptionExampleTry it
in Returns True if a sequence with the specified value is present in the objectx in yTry it »
not inReturns True if a sequence with the specified value is not present in the objectx not in yTry it »

Python Bitwise Operators

Bitwise operators are used to compare (binary) numbers:

OperatorNameDescription
ANDSets each bit to 1 if both bits are 1
|ORSets each bit to 1 if one of two bits is 1
 ^XORSets each bit to 1 if only one of two bits is 1
NOTInverts all the bits
<<Zero fill left shiftShift left by pushing zeros in from the right and let the leftmost bits fall off
>>Signed right shiftShift right by pushing copies of the leftmost bit in from the left, and let the rightmost bits fall off

Test Yourself With Exercises

Exercise:

Multiply 10 with 5, and print the result.

print(10  5)

Start the Exercise

Python If … Else

Python Conditions and If statements

Python supports the usual logical conditions from mathematics:

  • Equals: a == b
  • Not Equals: a != b
  • Less than: a < b
  • Less than or equal to: a <= b
  • Greater than: a > b
  • Greater than or equal to: a >= b

These conditions can be used in several ways, most commonly in “if statements” and loops.

An “if statement” is written by using the if keyword.

Example

If statement:

a = 33
b = 200
if b > a:
  print("b is greater than a")

Try it Yourself »

In this example we use two variables, a and b, which are used as part of the if statement to test whether b is greater than a. As a is 33, and b is 200, we know that 200 is greater than 33, and so we print to screen that “b is greater than a”.

Indentation

Python relies on indentation (whitespace at the beginning of a line) to define scope in the code. Other programming languages often use curly-brackets for this purpose.

Example

If statement, without indentation (will raise an error):

a = 33
b = 200
if b > a:
print("b is greater than a"# you will get an error

Try it Yourself »


Elif

The elif keyword is pythons way of saying “if the previous conditions were not true, then try this condition”.

Example

a = 33
b = 33
if b > a:
  print("b is greater than a")
elif a == b:
  print("a and b are equal")

Try it Yourself »

In this example a is equal to b, so the first condition is not true, but the elif condition is true, so we print to screen that “a and b are equal”.


Else

The else keyword catches anything which isn’t caught by the preceding conditions.

Example

a = 200
b = 33
if b > a:
  print("b is greater than a")
elif a == b:
  print("a and b are equal")
else:
  print("a is greater than b")

Try it Yourself »

In this example a is greater than b, so the first condition is not true, also the elif condition is not true, so we go to the else condition and print to screen that “a is greater than b”.

You can also have an else without the elif:

Example

a = 200
b = 33
if b > a:
  print("b is greater than a")
else:
  print("b is not greater than a")

Try it Yourself »


Short Hand If

If you have only one statement to execute, you can put it on the same line as the if statement.

Example

One line if statement:

if a > b: print("a is greater than b")

Try it Yourself »


Short Hand If … Else

If you have only one statement to execute, one for if, and one for else, you can put it all on the same line:

Example

One line if else statement:

a = 2
b = 330
print("A"if a > b else print("B")

Try it Yourself »

This technique is known as Ternary Operators, or Conditional Expressions.

You can also have multiple else statements on the same line:

Example

One line if else statement, with 3 conditions:

a = 330
b = 330
print("A"if a > b else print("="if a == b else print("B")

Try it Yourself »


And

The and keyword is a logical operator, and is used to combine conditional statements:

Example

Test if a is greater than b, AND if c is greater than a:

a = 200
b = 33
c = 500
if a > b and c > a:
  print("Both conditions are True")

Try it Yourself »


Or

The or keyword is a logical operator, and is used to combine conditional statements:

Example

Test if a is greater than b, OR if a is greater than c:

a = 200
b = 33
c = 500
if a > b or a > c:
  print("At least one of the conditions is True")

Try it Yourself »


Nested If

You can have if statements inside if statements, this is called nested if statements.

Example

x = 41

if x > 10:
  print("Above ten,")
  if x > 20:
    print("and also above 20!")
  else:
    print("but not above 20.")

Try it Yourself »


The pass Statement

if statements cannot be empty, but if you for some reason have an if statement with no content, put in the pass statement to avoid getting an error.

Example

a = 33
b = 200


if b > a:
  pass

Try it Yourself »


Test Yourself With Exercises

Exercise:

Print “Hello World” if a is greater than b.

a = 50
b = 10
 a  b
  print("Hello World")

Start the Exercise

Python While Loops

Python Loops

Python has two primitive loop commands:

  • while loops
  • for loops

The while Loop

With the while loop we can execute a set of statements as long as a condition is true.

Example

Print i as long as i is less than 6:

i = 1
while i < 6:
  print(i)
  i += 1

Try it Yourself »

Note: remember to increment i, or else the loop will continue forever.

The while loop requires relevant variables to be ready, in this example we need to define an indexing variable, i, which we set to 1.


The break Statement

With the break statement we can stop the loop even if the while condition is true:

Example

Exit the loop when i is 3:

i = 1
while i < 6:
  print(i)
  if i == 3:
    break
  i += 1

Try it Yourself »


 

The continue Statement

With the continue statement we can stop the current iteration, and continue with the next:

Example

Continue to the next iteration if i is 3:

i = 0
while i < 6:
  i += 1
  if i == 3:
    continue
  print(i)

Try it Yourself »


The else Statement

With the else statement we can run a block of code once when the condition no longer is true:

Example

Print a message once the condition is false:

i = 1
while i < 6:
  print(i)
  i += 1
else:
  print("i is no longer less than 6")

Try it Yourself »


Test Yourself With Exercises

Exercise:

Print i as long as i is less than 6.

i = 1
 i < 6
  print(i)
  i += 1

Start the Exercise

Python For Loops

Python For Loops

for loop is used for iterating over a sequence (that is either a list, a tuple, a dictionary, a set, or a string).

This is less like the for keyword in other programming languages, and works more like an iterator method as found in other object-orientated programming languages.

With the for loop we can execute a set of statements, once for each item in a list, tuple, set etc.

Example

Print each fruit in a fruit list:

fruits = ["apple", "banana""cherry"]
for in fruits:
  print(x)

Try it Yourself »

The for loop does not require an indexing variable to set beforehand.


Looping Through a String

Even strings are iterable objects, they contain a sequence of characters:

Example

Loop through the letters in the word “banana”:

for in "banana":
  print(x)

Try it Yourself »


The break Statement

With the break statement we can stop the loop before it has looped through all the items:

Example

Exit the loop when x is “banana”:

fruits = ["apple""banana""cherry"]
for x in fruits:
  print(x)
  if x == "banana":
    break

Try it Yourself »

Example

Exit the loop when x is “banana”, but this time the break comes before the print:

fruits = ["apple""banana", "cherry"]
for in fruits:
  if x == "banana":
    break
  print(x)

Try it Yourself »


 

The continue Statement

With the continue statement we can stop the current iteration of the loop, and continue with the next:

Example

Do not print banana:

fruits = ["apple""banana""cherry"]
for in fruits:
  if x == "banana":
    continue
  print(x)

Try it Yourself »


The range() Function

To loop through a set of code a specified number of times, we can use the range() function,

The range() function returns a sequence of numbers, starting from 0 by default, and increments by 1 (by default), and ends at a specified number.

Example

Using the range() function:

for in range(6):
  print(x)

Try it Yourself »

Note that range(6) is not the values of 0 to 6, but the values 0 to 5.

The range() function defaults to 0 as a starting value, however it is possible to specify the starting value by adding a parameter: range(2, 6), which means values from 2 to 6 (but not including 6):

Example

Using the start parameter:

for in range(26):
  print(x)

Try it Yourself »

The range() function defaults to increment the sequence by 1, however it is possible to specify the increment value by adding a third parameter: range(2, 30, 3):

Example

Increment the sequence with 3 (default is 1):

for in range(2303):
  print(x)

Try it Yourself »


Else in For Loop

The else keyword in a for loop specifies a block of code to be executed when the loop is finished:

Example

Print all numbers from 0 to 5, and print a message when the loop has ended:

for in range(6):
  print(x)
else:
  print("Finally finished!")

Try it Yourself »

Note: The else block will NOT be executed if the loop is stopped by a break statement.

Example

Break the loop when x is 3, and see what happens with the else block:

for in range(6):
  if x == 3break
  print(x)
else:
  print("Finally finished!")

Try it Yourself »


Nested Loops

A nested loop is a loop inside a loop.

The “inner loop” will be executed one time for each iteration of the “outer loop”:

Example

Print each adjective for every fruit:

adj = ["red", "big", "tasty"]
fruits = ["apple""banana""cherry"]


for in adj:
  for in fruits:
    print(x, y)

Try it Yourself »


The pass Statement

for loops cannot be empty, but if you for some reason have a for loop with no content, put in the pass statement to avoid getting an error.

Example

for in [012]:
  pass

Try it Yourself »


Test Yourself With Exercises

Exercise:

Loop through the items in the fruits list.

fruits = ["apple", "banana", "cherry"]
 x  fruits
  print(x)

Start the Exercise

Python Functions

A function is a block of code which only runs when it is called.

You can pass data, known as parameters, into a function.

A function can return data as a result.


Creating a Function

In Python a function is defined using the def keyword:

Example

def my_function():
  print(“Hello from a function”)

Calling a Function

To call a function, use the function name followed by parenthesis:

Example

def my_function():
  print(“Hello from a function”)

my_function()
Try it Yourself »

Arguments

Information can be passed into functions as arguments.

Arguments are specified after the function name, inside the parentheses. You can add as many arguments as you want, just separate them with a comma.

The following example has a function with one argument (fname). When the function is called, we pass along a first name, which is used inside the function to print the full name:

Example

def my_function(fname):
  print(fname + ” Refsnes”)

my_function(“Emil”)
my_function(“Tobias”)
my_function(“Linus”)
Try it Yourself »

Arguments are often shortened to args in Python documentations.


 

Parameters or Arguments?

The terms parameter and argument can be used for the same thing: information that are passed into a function.

From a function’s perspective:

A parameter is the variable listed inside the parentheses in the function definition.

An argument is the value that is sent to the function when it is called.


Number of Arguments

By default, a function must be called with the correct number of arguments. Meaning that if your function expects 2 arguments, you have to call the function with 2 arguments, not more, and not less.

Example

This function expects 2 arguments, and gets 2 arguments:

def my_function(fname, lname):
  print(fname + ” “ + lname)

my_function(“Emil”“Refsnes”)
Try it Yourself »

If you try to call the function with 1 or 3 arguments, you will get an error:

Example

This function expects 2 arguments, but gets only 1:

def my_function(fname, lname):
  print(fname + ” “ + lname)

my_function(“Emil”)
Try it Yourself »

Arbitrary Arguments, *args

If you do not know how many arguments that will be passed into your function, add a * before the parameter name in the function definition.

This way the function will receive a tuple of arguments, and can access the items accordingly:

Example

If the number of arguments is unknown, add a * before the parameter name:

def my_function(*kids):
  print(“The youngest child is “ + kids[2])

my_function(“Emil”“Tobias”“Linus”)
Try it Yourself »

Arbitrary Arguments are often shortened to *args in Python documentations.


Keyword Arguments

You can also send arguments with the key = value syntax.

This way the order of the arguments does not matter.

Example

def my_function(child3, child2, child1):
  print(“The youngest child is “ + child3)

my_function(child1 = “Emil”, child2 = “Tobias”, child3 = “Linus”)
Try it Yourself »

The phrase Keyword Arguments are often shortened to kwargs in Python documentations.


Arbitrary Keyword Arguments, **kwargs

If you do not know how many keyword arguments that will be passed into your function, add two asterisk: ** before the parameter name in the function definition.

This way the function will receive a dictionary of arguments, and can access the items accordingly:

Example

If the number of keyword arguments is unknown, add a double ** before the parameter name:

def my_function(**kid):
  print(“His last name is “ + kid[“lname”])

my_function(fname = “Tobias”, lname = “Refsnes”)
Try it Yourself »

Arbitrary Kword Arguments are often shortened to **kwargs in Python documentations.


Default Parameter Value

The following example shows how to use a default parameter value.

If we call the function without argument, it uses the default value:

Example

def my_function(country = “Norway”):
  print(“I am from “ + country)

my_function(“Sweden”)
my_function(“India”)
my_function()
my_function(“Brazil”)
Try it Yourself »

Passing a List as an Argument

You can send any data types of argument to a function (string, number, list, dictionary etc.), and it will be treated as the same data type inside the function.

E.g. if you send a List as an argument, it will still be a List when it reaches the function:

Example

def my_function(food):
  for x in food:
    print(x)

fruits = [“apple”“banana”“cherry”]

my_function(fruits)
Try it Yourself »

Return Values

To let a function return a value, use the return statement:

Example

def my_function(x):
  return 5 * x

print(my_function(3))
print(my_function(5))
print(my_function(9))
Try it Yourself »

The pass Statement

function definitions cannot be empty, but if you for some reason have a function definition with no content, put in the pass statement to avoid getting an error.

Example

def myfunction():
  pass
Try it Yourself »

Recursion

Python also accepts function recursion, which means a defined function can call itself.

Recursion is a common mathematical and programming concept. It means that a function calls itself. This has the benefit of meaning that you can loop through data to reach a result.

The developer should be very careful with recursion as it can be quite easy to slip into writing a function which never terminates, or one that uses excess amounts of memory or processor power. However, when written correctly recursion can be a very efficient and mathematically-elegant approach to programming.

In this example, tri_recursion() is a function that we have defined to call itself (“recurse”). We use the k variable as the data, which decrements (-1) every time we recurse. The recursion ends when the condition is not greater than 0 (i.e. when it is 0).

To a new developer it can take some time to work out how exactly this works, best way to find out is by testing and modifying it.

Example

Recursion Example

def tri_recursion(k):
  if(k > 0):
    result = k + tri_recursion(k – 1)
    print(result)
  else:
    result = 0
  return result

print(“\n\nRecursion Example Results”)
tri_recursion(6)
Try it Yourself »

Test Yourself With Exercises

Exercise:

Create a function named my_function.

:
  print("Hello from a function")

Start the Exercise

Python Lambda

A lambda function is a small anonymous function.

A lambda function can take any number of arguments, but can only have one expression.


Syntax

lambda arguments expression

The expression is executed and the result is returned:

Example

Add 10 to argument a, and return the result:

x = lambda a : a + 10
print(x(5))
Try it Yourself »

Lambda functions can take any number of arguments:

Example

Multiply argument a with argument b and return the result:

x = lambda a, b : a * b
print(x(56))
Try it Yourself »

Example

Summarize argument ab, and c and return the result:

x = lambda a, b, c : a + b + c
print(x(562))
Try it Yourself »

 

Why Use Lambda Functions?

The power of lambda is better shown when you use them as an anonymous function inside another function.

Say you have a function definition that takes one argument, and that argument will be multiplied with an unknown number:

def myfunc(n):
  return lambda a : a * n

Use that function definition to make a function that always doubles the number you send in:

Example

def myfunc(n):
  return lambda a : a * n

mydoubler = myfunc(2)

print(mydoubler(11))
Try it Yourself »

Or, use the same function definition to make a function that always triples the number you send in:

Example

def myfunc(n):
  return lambda a : a * n

mytripler = myfunc(3)

print(mytripler(11))
Try it Yourself »

Or, use the same function definition to make both functions, in the same program:

Example

def myfunc(n):
  return lambda a : a * n

mydoubler = myfunc(2)
mytripler = myfunc(3)

print(mydoubler(11))
print(mytripler(11))
Try it Yourself »

Use lambda functions when an anonymous function is required for a short period of time.


Test Yourself With Exercises

Exercise:

Create a lambda function that takes one parameter (a) and returns it.

x =    

Start the Exercise

Python Arrays

Note: Python does not have built-in support for Arrays, but Python Lists can be used instead.


Arrays

Note: This page shows you how to use LISTS as ARRAYS, however, to work with arrays in Python you will have to import a library, like the NumPy library.

Arrays are used to store multiple values in one single variable:

Example

Create an array containing car names:

cars = [“Ford”“Volvo”“BMW”]
Try it Yourself »

What is an Array?

An array is a special variable, which can hold more than one value at a time.

If you have a list of items (a list of car names, for example), storing the cars in single variables could look like this:

car1 = “Ford”
car2 = “Volvo”
car3 = “BMW”

However, what if you want to loop through the cars and find a specific one? And what if you had not 3 cars, but 300?

The solution is an array!

An array can hold many values under a single name, and you can access the values by referring to an index number.


Access the Elements of an Array

You refer to an array element by referring to the index number.

Example

Get the value of the first array item:

x = cars[0]
Try it Yourself »

Example

Modify the value of the first array item:

cars[0] = “Toyota”
Try it Yourself »

The Length of an Array

Use the len() method to return the length of an array (the number of elements in an array).

Example

Return the number of elements in the cars array:

x = len(cars)
Try it Yourself »

Note: The length of an array is always one more than the highest array index.


 

Looping Array Elements

You can use the for in loop to loop through all the elements of an array.

Example

Print each item in the cars array:

for x in cars:
  print(x)
Try it Yourself »

Adding Array Elements

You can use the append() method to add an element to an array.

Example

Add one more element to the cars array:

cars.append(“Honda”)
Try it Yourself »

Removing Array Elements

You can use the pop() method to remove an element from the array.

Example

Delete the second element of the cars array:

cars.pop(1)
Try it Yourself »

You can also use the remove() method to remove an element from the array.

Example

Delete the element that has the value “Volvo”:

cars.remove(“Volvo”)
Try it Yourself »

Note: The list’s remove() method only removes the first occurrence of the specified value.


Array Methods

Python has a set of built-in methods that you can use on lists/arrays.

MethodDescription
append()Adds an element at the end of the list
clear()Removes all the elements from the list
copy()Returns a copy of the list
count()Returns the number of elements with the specified value
extend()Add the elements of a list (or any iterable), to the end of the current list
index()Returns the index of the first element with the specified value
insert()Adds an element at the specified position
pop()Removes the element at the specified position
remove()Removes the first item with the specified value
reverse()Reverses the order of the list
sort()Sorts the list

Note: Python does not have built-in support for Arrays, but Python Lists can be used instead.

Python Classes and Objects

Python Classes/Objects

Python is an object oriented programming language.

Almost everything in Python is an object, with its properties and methods.

A Class is like an object constructor, or a “blueprint” for creating objects.


Create a Class

To create a class, use the keyword class:

Example

Create a class named MyClass, with a property named x:

class MyClass:
  x = 5
Try it Yourself »

Create Object

Now we can use the class named MyClass to create objects:

Example

Create an object named p1, and print the value of x:

p1 = MyClass()
print(p1.x)
Try it Yourself »

The __init__() Function

The examples above are classes and objects in their simplest form, and are not really useful in real life applications.

To understand the meaning of classes we have to understand the built-in __init__() function.

All classes have a function called __init__(), which is always executed when the class is being initiated.

Use the __init__() function to assign values to object properties, or other operations that are necessary to do when the object is being created:

Example

Create a class named Person, use the __init__() function to assign values for name and age:

class Person:
  def __init__(self, name, age):
    self.name = name
    self.age = age

p1 = Person(“John”36)

print(p1.name)
print(p1.age)
Try it Yourself »

Note: The __init__() function is called automatically every time the class is being used to create a new object.


 

Object Methods

Objects can also contain methods. Methods in objects are functions that belong to the object.

Let us create a method in the Person class:

Example

Insert a function that prints a greeting, and execute it on the p1 object:

class Person:
  def __init__(self, name, age):
    self.name = name
    self.age = age

  def myfunc(self):
    print(“Hello my name is “ + self.name)

p1 = Person(“John”36)
p1.myfunc()
Try it Yourself »

Note: The self parameter is a reference to the current instance of the class, and is used to access variables that belong to the class.


The self Parameter

The self parameter is a reference to the current instance of the class, and is used to access variables that belongs to the class.

It does not have to be named self , you can call it whatever you like, but it has to be the first parameter of any function in the class:

Example

Use the words mysillyobject and abc instead of self:

class Person:
  def __init__(mysillyobject, name, age):
    mysillyobject.name = name
    mysillyobject.age = age

  def myfunc(abc):
    print(“Hello my name is “ + abc.name)

p1 = Person(“John”36)
p1.myfunc()
Try it Yourself »

Modify Object Properties

You can modify properties on objects like this:

Example

Set the age of p1 to 40:

p1.age = 40
Try it Yourself »

Delete Object Properties

You can delete properties on objects by using the del keyword:

Example

Delete the age property from the p1 object:

del p1.age
Try it Yourself »

Delete Objects

You can delete objects by using the del keyword:

Example

Delete the p1 object:

del p1
Try it Yourself »

The pass Statement

class definitions cannot be empty, but if you for some reason have a class definition with no content, put in the pass statement to avoid getting an error.

Example

class Person:
  pass
Try it Yourself »

Test Yourself With Exercises

Exercise:

Create a class named MyClass:

 MyClass:
  x = 5

Start the Exercise

Python Inheritance

Python Inheritance

Inheritance allows us to define a class that inherits all the methods and properties from another class.

Parent class is the class being inherited from, also called base class.

Child class is the class that inherits from another class, also called derived class.


Create a Parent Class

Any class can be a parent class, so the syntax is the same as creating any other class:

Example

Create a class named Person, with firstname and lastname properties, and a printname method:

class Person:
  def __init__(self, fname, lname):
    self.firstname = fname
    self.lastname = lname

  def printname(self):
    print(self.firstname, self.lastname)

#Use the Person class to create an object, and then execute the printname method:

x = Person(“John”“Doe”)
x.printname()
Try it Yourself »

Create a Child Class

To create a class that inherits the functionality from another class, send the parent class as a parameter when creating the child class:

Example

Create a class named Student, which will inherit the properties and methods from the Person class:

class Student(Person):
  pass

Note: Use the pass keyword when you do not want to add any other properties or methods to the class.

Now the Student class has the same properties and methods as the Person class.

Example

Use the Student class to create an object, and then execute the printname method:

x = Student(“Mike”“Olsen”)
x.printname()
Try it Yourself »

Add the __init__() Function

So far we have created a child class that inherits the properties and methods from its parent.

We want to add the __init__() function to the child class (instead of the pass keyword).

Note: The __init__() function is called automatically every time the class is being used to create a new object.

Example

Add the __init__() function to the Student class:

class Student(Person):
  def __init__(self, fname, lname):
    #add properties etc.

When you add the __init__() function, the child class will no longer inherit the parent’s __init__() function.

Note: The child’s __init__() function overrides the inheritance of the parent’s __init__() function.

To keep the inheritance of the parent’s __init__() function, add a call to the parent’s __init__() function:

Example

class Student(Person):
  def __init__(self, fname, lname):
    Person.__init__(self, fname, lname)
Try it Yourself »

Now we have successfully added the __init__() function, and kept the inheritance of the parent class, and we are ready to add functionality in the __init__() function.


Use the super() Function

Python also has a super() function that will make the child class inherit all the methods and properties from its parent:

Example

class Student(Person):
  def __init__(self, fname, lname):
    super().__init__(fname, lname)
Try it Yourself »

By using the super() function, you do not have to use the name of the parent element, it will automatically inherit the methods and properties from its parent.


Add Properties

Example

Add a property called graduationyear to the Student class:

class Student(Person):
  def __init__(self, fname, lname):
    super().__init__(fname, lname)
    self.graduationyear = 2019
Try it Yourself »

In the example below, the year 2019 should be a variable, and passed into the Student class when creating student objects. To do so, add another parameter in the __init__() function:

Example

Add a year parameter, and pass the correct year when creating objects:

class Student(Person):
  def __init__(self, fname, lname, year):
    super().__init__(fname, lname)
    self.graduationyear = year

x = Student(“Mike”“Olsen”2019)
Try it Yourself »

Add Methods

Example

Add a method called welcome to the Student class:

class Student(Person):
  def __init__(self, fname, lname, year):
    super().__init__(fname, lname)
    self.graduationyear = year

  def welcome(self):
    print(“Welcome”, self.firstname, self.lastname, “to the class of”, self.graduationyear)
Try it Yourself »

If you add a method in the child class with the same name as a function in the parent class, the inheritance of the parent method will be overridden.


Test Yourself With Exercises

Exercise:

What is the correct syntax to create a class named Student that will inherit properties and methods from a class named Person?

class :

Start the Exercise

Python Iterators

Python Iterators

An iterator is an object that contains a countable number of values.

An iterator is an object that can be iterated upon, meaning that you can traverse through all the values.

Technically, in Python, an iterator is an object which implements the iterator protocol, which consist of the methods __iter__() and __next__().


Iterator vs Iterable

Lists, tuples, dictionaries, and sets are all iterable objects. They are iterable containers which you can get an iterator from.

All these objects have a iter() method which is used to get an iterator:

Example

Return an iterator from a tuple, and print each value:

mytuple = (“apple”“banana”“cherry”)
myit = iter(mytuple)

print(next(myit))
print(next(myit))
print(next(myit))
Try it Yourself »

Even strings are iterable objects, and can return an iterator:

Example

Strings are also iterable objects, containing a sequence of characters:

mystr = “banana”
myit = iter(mystr)

print(next(myit))
print(next(myit))
print(next(myit))
print(next(myit))
print(next(myit))
print(next(myit))
Try it Yourself »

Looping Through an Iterator

We can also use a for loop to iterate through an iterable object:

Example

Iterate the values of a tuple:

mytuple = (“apple”“banana”“cherry”)

for x in mytuple:
  print(x)
Try it Yourself »

Example

Iterate the characters of a string:

mystr = “banana”

for x in mystr:
  print(x)
Try it Yourself »

The for loop actually creates an iterator object and executes the next() method for each loop.


 

Create an Iterator

To create an object/class as an iterator you have to implement the methods __iter__() and __next__() to your object.

As you have learned in the Python Classes/Objects chapter, all classes have a function called __init__(), which allows you to do some initializing when the object is being created.

The __iter__() method acts similar, you can do operations (initializing etc.), but must always return the iterator object itself.

The __next__() method also allows you to do operations, and must return the next item in the sequence.

Example

Create an iterator that returns numbers, starting with 1, and each sequence will increase by one (returning 1,2,3,4,5 etc.):

class MyNumbers:
  def __iter__(self):
    self.a = 1
    return self

  def __next__(self):
    x = self.a
    self.a += 1
    return x

myclass = MyNumbers()
myiter = iter(myclass)

print(next(myiter))
print(next(myiter))
print(next(myiter))
print(next(myiter))
print(next(myiter))
Try it Yourself »

StopIteration

The example above would continue forever if you had enough next() statements, or if it was used in a for loop.

To prevent the iteration to go on forever, we can use the StopIteration statement.

In the __next__() method, we can add a terminating condition to raise an error if the iteration is done a specified number of times:

Example

Stop after 20 iterations:

class MyNumbers:
  def __iter__(self):
    self.a = 1
    return self

  def __next__(self):
    if self.a <= 20:
      x = self.a
      self.a += 1
      return x
    else:
      raise StopIteration

myclass = MyNumbers()
myiter = iter(myclass)

for x in myiter:
  print(x)
Try it Yourself »

Python Scope

A variable is only available from inside the region it is created. This is called scope.


Local Scope

A variable created inside a function belongs to the local scope of that function, and can only be used inside that function.

Example

A variable created inside a function is available inside that function:

def myfunc():
  x = 300
  print(x)

myfunc()
Try it Yourself »

Function Inside Function

As explained in the example above, the variable x is not available outside the function, but it is available for any function inside the function:

Example

The local variable can be accessed from a function within the function:

def myfunc():
  x = 300
  def myinnerfunc():
    print(x)
  myinnerfunc()

myfunc()
Try it Yourself »

 

Global Scope

A variable created in the main body of the Python code is a global variable and belongs to the global scope.

Global variables are available from within any scope, global and local.

Example

A variable created outside of a function is global and can be used by anyone:

x = 300

def myfunc():
  print(x)

myfunc()

print(x)
Try it Yourself »

Naming Variables

If you operate with the same variable name inside and outside of a function, Python will treat them as two separate variables, one available in the global scope (outside the function) and one available in the local scope (inside the function):

Example

The function will print the local x, and then the code will print the global x:

x = 300

def myfunc():
  x = 200
  print(x)

myfunc()

print(x)
Try it Yourself »

Global Keyword

If you need to create a global variable, but are stuck in the local scope, you can use the global keyword.

The global keyword makes the variable global.

Example

If you use the global keyword, the variable belongs to the global scope:

def myfunc():
  global x
  x = 300

myfunc()

print(x)
Try it Yourself »

Also, use the global keyword if you want to make a change to a global variable inside a function.

Example

To change the value of a global variable inside a function, refer to the variable by using the global keyword:

x = 300

def myfunc():
  global x
  x = 200

myfunc()

print(x)
Try it Yourself »

 

 

Python Modules

What is a Module?

Consider a module to be the same as a code library.

A file containing a set of functions you want to include in your application.


Create a Module

To create a module just save the code you want in a file with the file extension .py:

Example

Save this code in a file named mymodule.py

def greeting(name):
  print(“Hello, “ + name)

Use a Module

Now we can use the module we just created, by using the import statement:

Example

Import the module named mymodule, and call the greeting function:

import mymodule

mymodule.greeting(“Jonathan”)
Run Example »

Note: When using a function from a module, use the syntax: module_name.function_name.


Variables in Module

The module can contain functions, as already described, but also variables of all types (arrays, dictionaries, objects etc):

Example

Save this code in the file mymodule.py

person1 = {
  “name”“John”,
  “age”36,
  “country”“Norway”
}

Example

Import the module named mymodule, and access the person1 dictionary:

import mymodule

a = mymodule.person1[“age”]
print(a)
Run Example »

 

Naming a Module

You can name the module file whatever you like, but it must have the file extension .py

Re-naming a Module

You can create an alias when you import a module, by using the as keyword:

Example

Create an alias for mymodule called mx:

import mymodule as mx

a = mx.person1[“age”]
print(a)
Run Example »

Built-in Modules

There are several built-in modules in Python, which you can import whenever you like.

Example

Import and use the platform module:

import platform

x = platform.system()
print(x)
Try it Yourself »

Using the dir() Function

There is a built-in function to list all the function names (or variable names) in a module. The dir() function:

Example

List all the defined names belonging to the platform module:

import platform

x = dir(platform)
print(x)
Try it Yourself »

Note: The dir() function can be used on all modules, also the ones you create yourself.


Import From Module

You can choose to import only parts from a module, by using the from keyword.

Example

The module named mymodule has one function and one dictionary:

def greeting(name):
  print(“Hello, “ + name)

person1 = {
  “name”“John”,
  “age”36,
  “country”“Norway”
}

Example

Import only the person1 dictionary from the module:

from mymodule import person1

print (person1[“age”])
Run Example »

Note: When importing using the from keyword, do not use the module name when referring to elements in the module. Example: person1["age"]not mymodule.person1["age"]


Test Yourself With Exercises

Exercise:

What is the correct syntax to import a module named “mymodule”?

 mymodule

Start the Exercise

Python Datetime

Python Dates

A date in Python is not a data type of its own, but we can import a module named datetime to work with dates as date objects.

Example

Import the datetime module and display the current date:

import datetime

x = datetime.datetime.now()
print(x)
Try it Yourself »

Date Output

When we execute the code from the example above the result will be:

2022-04-28 23:32:48.609004

The date contains year, month, day, hour, minute, second, and microsecond.

The datetime module has many methods to return information about the date object.

Here are a few examples, you will learn more about them later in this chapter:

Example

Return the year and name of weekday:

import datetime

x = datetime.datetime.now()

print(x.year)
print(x.strftime(“%A”))
Try it Yourself »

Creating Date Objects

To create a date, we can use the datetime() class (constructor) of the datetime module.

The datetime() class requires three parameters to create a date: year, month, day.

Example

Create a date object:

import datetime

x = datetime.datetime(2020517)

print(x)
Try it Yourself »

The datetime() class also takes parameters for time and timezone (hour, minute, second, microsecond, tzone), but they are optional, and has a default value of 0, (None for timezone).


ADVERTISEMENT

The strftime() Method

The datetime object has a method for formatting date objects into readable strings.

The method is called strftime(), and takes one parameter, format, to specify the format of the returned string:

Example

Display the name of the month:

import datetime

x = datetime.datetime(201861)

print(x.strftime(“%B”))
Try it Yourself »

A reference of all the legal format codes:

DirectiveDescriptionExampleTry it
%aWeekday, short versionWedTry it »
%AWeekday, full versionWednesdayTry it »
%wWeekday as a number 0-6, 0 is Sunday3Try it »
%dDay of month 01-3131Try it »
%bMonth name, short versionDecTry it »
%BMonth name, full versionDecemberTry it »
%mMonth as a number 01-1212Try it »
%yYear, short version, without century18Try it »
%YYear, full version2018Try it »
%HHour 00-2317Try it »
%IHour 00-1205Try it »
%pAM/PMPMTry it »
%MMinute 00-5941Try it »
%SSecond 00-5908Try it »
%fMicrosecond 000000-999999548513Try it »
%zUTC offset+0100 
%ZTimezoneCST 
%jDay number of year 001-366365Try it »
%UWeek number of year, Sunday as the first day of week, 00-5352Try it »
%WWeek number of year, Monday as the first day of week, 00-5352Try it »
%cLocal version of date and timeMon Dec 31 17:41:00 2018Try it »
%CCentury20Try it »
%xLocal version of date12/31/18Try it »
%XLocal version of time17:41:00Try it »
%%A % character%Try it »
%GISO 8601 year2018Try it »
%uISO 8601 weekday (1-7)1Try it »
%VISO 8601 weeknumber (01-53)01Try it »

 

Python Math

Python has a set of built-in math functions, including an extensive math module, that allows you to perform mathematical tasks on numbers.


Built-in Math Functions

The min() and max() functions can be used to find the lowest or highest value in an iterable:

Example

x = min(51025)
y = max(51025)

print(x)
print(y)
Try it Yourself »

The abs() function returns the absolute (positive) value of the specified number:

Example

x = abs(-7.25)

print(x)
Try it Yourself »

The pow(xy) function returns the value of x to the power of y (xy).

Example

Return the value of 4 to the power of 3 (same as 4 * 4 * 4):

x = pow(43)

print(x)
Try it Yourself »

 

The Math Module

Python has also a built-in module called math, which extends the list of mathematical functions.

To use it, you must import the math module:

import math

When you have imported the math module, you can start using methods and constants of the module.

The math.sqrt() method for example, returns the square root of a number:

Example

import math

x = math.sqrt(64)

print(x)
Try it Yourself »

The math.ceil() method rounds a number upwards to its nearest integer, and the math.floor() method rounds a number downwards to its nearest integer, and returns the result:

Example

import math

x = math.ceil(1.4)
y = math.floor(1.4)

print(x) # returns 2
print(y) # returns 1
Try it Yourself »

The math.pi constant, returns the value of PI (3.14…):

Example

import math

x = math.pi

print(x)
Try it Yourself »

Complete Math Module Reference

In our Math Module Reference you will find a complete reference of all methods and constants that belongs to the Math module.

Python JSON

JSON is a syntax for storing and exchanging data.

JSON is text, written with JavaScript object notation.


JSON in Python

Python has a built-in package called json, which can be used to work with JSON data.

Example

Import the json module:

import json

Parse JSON – Convert from JSON to Python

If you have a JSON string, you can parse it by using the json.loads() method.

The result will be a Python dictionary.

Example

Convert from JSON to Python:

import json

# some JSON:
x =  ‘{ “name”:”John”, “age”:30, “city”:”New York”}’

# parse x:
y = json.loads(x)

# the result is a Python dictionary:
print(y[“age”])
Try it Yourself »

Convert from Python to JSON

If you have a Python object, you can convert it into a JSON string by using the json.dumps() method.

Example

Convert from Python to JSON:

import json

# a Python object (dict):
x = {
  “name”“John”,
  “age”30,
  “city”“New York”
}

# convert into JSON:
y = json.dumps(x)

# the result is a JSON string:
print(y)
Try it Yourself »

 

You can convert Python objects of the following types, into JSON strings:

  • dict
  • list
  • tuple
  • string
  • int
  • float
  • True
  • False
  • None

Example

Convert Python objects into JSON strings, and print the values:

import json

print(json.dumps({“name”“John”“age”30}))
print(json.dumps([“apple”“bananas”]))
print(json.dumps((“apple”“bananas”)))
print(json.dumps(“hello”))
print(json.dumps(42))
print(json.dumps(31.76))
print(json.dumps(True))
print(json.dumps(False))
print(json.dumps(None))
Try it Yourself »

When you convert from Python to JSON, Python objects are converted into the JSON (JavaScript) equivalent:

PythonJSON
dictObject
listArray
tupleArray
strString
intNumber
floatNumber
Truetrue
Falsefalse
Nonenull

Example

Convert a Python object containing all the legal data types:

import json

x = {
  “name”“John”,
  “age”30,
  “married”True,
  “divorced”False,
  “children”: (“Ann”,“Billy”),
  “pets”: None,
  “cars”: [
    {“model”“BMW 230”“mpg”27.5},
    {“model”“Ford Edge”“mpg”24.1}
  ]
}

print(json.dumps(x))
Try it Yourself »

Format the Result

The example above prints a JSON string, but it is not very easy to read, with no indentations and line breaks.

The json.dumps() method has parameters to make it easier to read the result:

Example

Use the indent parameter to define the numbers of indents:

json.dumps(x, indent=4)
Try it Yourself »

You can also define the separators, default value is (“, “, “: “), which means using a comma and a space to separate each object, and a colon and a space to separate keys from values:

Example

Use the separators parameter to change the default separator:

json.dumps(x, indent=4, separators=(“. “” = “))
Try it Yourself »

Order the Result

The json.dumps() method has parameters to order the keys in the result:

Example

Use the sort_keys parameter to specify if the result should be sorted or not:

json.dumps(x, indent=4, sort_keys=True)
Try it Yourself »

Python RegEx

A RegEx, or Regular Expression, is a sequence of characters that forms a search pattern.

RegEx can be used to check if a string contains the specified search pattern.


RegEx Module

Python has a built-in package called re, which can be used to work with Regular Expressions.

Import the re module:

import re

RegEx in Python

When you have imported the re module, you can start using regular expressions:

Example

Search the string to see if it starts with “The” and ends with “Spain”:

import re

txt = “The rain in Spain”
x = re.search(“^The.*Spain$”, txt)
Try it Yourself »

RegEx Functions

The re module offers a set of functions that allows us to search a string for a match:

FunctionDescription
findallReturns a list containing all matches
searchReturns a Match object if there is a match anywhere in the string
splitReturns a list where the string has been split at each match
subReplaces one or many matches with a string

 

Metacharacters

Metacharacters are characters with a special meaning:

CharacterDescriptionExampleTry it
[]A set of characters“[a-m]”Try it »
\Signals a special sequence (can also be used to escape special characters)“\d”Try it »
.Any character (except newline character)“he..o”Try it »
^Starts with“^hello”Try it »
$Ends with“planet$”Try it »
*Zero or more occurrences“he.*o”Try it »
+One or more occurrences“he.+o”Try it »
?Zero or one occurrences“he.?o”Try it »
{}Exactly the specified number of occurrences“he.{2}o”Try it »
|Either or“falls|stays”Try it »
()Capture and group  

Special Sequences

A special sequence is a \ followed by one of the characters in the list below, and has a special meaning:

CharacterDescriptionExampleTry it
\AReturns a match if the specified characters are at the beginning of the string“\AThe”Try it »
\bReturns a match where the specified characters are at the beginning or at the end of a word
(the “r” in the beginning is making sure that the string is being treated as a “raw string”)
r”\bain”
r”ain\b”
Try it »
Try it »
\BReturns a match where the specified characters are present, but NOT at the beginning (or at the end) of a word
(the “r” in the beginning is making sure that the string is being treated as a “raw string”)
r”\Bain”
r”ain\B”
Try it »
Try it »
\dReturns a match where the string contains digits (numbers from 0-9)“\d”Try it »
\DReturns a match where the string DOES NOT contain digits“\D”Try it »
\sReturns a match where the string contains a white space character“\s”Try it »
\SReturns a match where the string DOES NOT contain a white space character“\S”Try it »
\wReturns a match where the string contains any word characters (characters from a to Z, digits from 0-9, and the underscore _ character)“\w”Try it »
\WReturns a match where the string DOES NOT contain any word characters“\W”Try it »
\ZReturns a match if the specified characters are at the end of the string“Spain\Z”Try it »

Sets

A set is a set of characters inside a pair of square brackets [] with a special meaning:

SetDescriptionTry it
[arn]Returns a match where one of the specified characters (ar, or n) are presentTry it »
[a-n]Returns a match for any lower case character, alphabetically between a and nTry it »
[^arn]Returns a match for any character EXCEPT ar, and nTry it »
[0123]Returns a match where any of the specified digits (012, or 3) are presentTry it »
[0-9]Returns a match for any digit between 0 and 9Try it »
[0-5][0-9]Returns a match for any two-digit numbers from 00 and 59Try it »
[a-zA-Z]Returns a match for any character alphabetically between a and z, lower case OR upper caseTry it »
[+]In sets, +*.|()$,{} has no special meaning, so [+] means: return a match for any + character in the stringTry it »

 

The findall() Function

The findall() function returns a list containing all matches.

Example

Print a list of all matches:

import re

txt = “The rain in Spain”
x = re.findall(“ai”, txt)
print(x)
Try it Yourself »

The list contains the matches in the order they are found.

If no matches are found, an empty list is returned:

Example

Return an empty list if no match was found:

import re

txt = “The rain in Spain”
x = re.findall(“Portugal”, txt)
print(x)
Try it Yourself »

 

The search() Function

The search() function searches the string for a match, and returns a Match object if there is a match.

If there is more than one match, only the first occurrence of the match will be returned:

Example

Search for the first white-space character in the string:

import re

txt = “The rain in Spain”
x = re.search(“\s”, txt)

print(“The first white-space character is located in position:”, x.start())
Try it Yourself »

If no matches are found, the value None is returned:

Example

Make a search that returns no match:

import re

txt = “The rain in Spain”
x = re.search(“Portugal”, txt)
print(x)
Try it Yourself »

 

The split() Function

The split() function returns a list where the string has been split at each match:

Example

Split at each white-space character:

import re

txt = “The rain in Spain”
x = re.split(“\s”, txt)
print(x)
Try it Yourself »

You can control the number of occurrences by specifying the maxsplit parameter:

Example

Split the string only at the first occurrence:

import re

txt = “The rain in Spain”
x = re.split(“\s”, txt, 1)
print(x)
Try it Yourself »

 

The sub() Function

The sub() function replaces the matches with the text of your choice:

Example

Replace every white-space character with the number 9:

import re

txt = “The rain in Spain”
x = re.sub(“\s”“9”, txt)
print(x)
Try it Yourself »

You can control the number of replacements by specifying the count parameter:

Example

Replace the first 2 occurrences:

import re

txt = “The rain in Spain”
x = re.sub(“\s”“9”, txt, 2)
print(x)
Try it Yourself »

 

Match Object

A Match Object is an object containing information about the search and the result.

Note: If there is no match, the value None will be returned, instead of the Match Object.

Example

Do a search that will return a Match Object:

import re

txt = “The rain in Spain”
x = re.search(“ai”, txt)
print(x) #this will print an object
Try it Yourself »

The Match object has properties and methods used to retrieve information about the search, and the result:

.span() returns a tuple containing the start-, and end positions of the match.
.string returns the string passed into the function
.group() returns the part of the string where there was a match

Example

Print the position (start- and end-position) of the first match occurrence.

The regular expression looks for any words that starts with an upper case “S”:

import re

txt = “The rain in Spain”
x = re.search(r“\bS\w+”, txt)
print(x.span())
Try it Yourself »

Example

Print the string passed into the function:

import re

txt = “The rain in Spain”
x = re.search(r“\bS\w+”, txt)
print(x.string)
Try it Yourself »

Example

Print the part of the string where there was a match.

The regular expression looks for any words that starts with an upper case “S”:

import re

txt = “The rain in Spain”
x = re.search(r“\bS\w+”, txt)
print(x.group())
Try it Yourself »

Note: If there is no match, the value None will be returned, instead of the Match Object.

 

Python PIP

What is PIP?

PIP is a package manager for Python packages, or modules if you like.

Note: If you have Python version 3.4 or later, PIP is included by default.


What is a Package?

A package contains all the files you need for a module.

Modules are Python code libraries you can include in your project.


Check if PIP is Installed

Navigate your command line to the location of Python’s script directory, and type the following:

Example

Check PIP version:

C:\Users\Your Name\AppData\Local\Programs\Python\Python36-32\Scripts>pip –version

Install PIP

If you do not have PIP installed, you can download and install it from this page: https://pypi.org/project/pip/


Download a Package

Downloading a package is very easy.

Open the command line interface and tell PIP to download the package you want.

Navigate your command line to the location of Python’s script directory, and type the following:

Example

Download a package named “camelcase”:

C:\Users\Your Name\AppData\Local\Programs\Python\Python36-32\Scripts>pip install camelcase

Now you have downloaded and installed your first package!


 

Using a Package

Once the package is installed, it is ready to use.

Import the “camelcase” package into your project.

Example

Import and use “camelcase”:

import camelcase

c = camelcase.CamelCase()

txt = “hello world”

print(c.hump(txt))
Run Example »

Find Packages

Find more packages at https://pypi.org/.


Remove a Package

Use the uninstall command to remove a package:

Example

Uninstall the package named “camelcase”:

C:\Users\Your Name\AppData\Local\Programs\Python\Python36-32\Scripts>pip uninstall camelcase

The PIP Package Manager will ask you to confirm that you want to remove the camelcase package:

Uninstalling camelcase-02.1:
  Would remove:
    c:\users\Your Name\appdata\local\programs\python\python36-32\lib\site-packages\camecase-0.2-py3.6.egg-info
    c:\users\Your Name\appdata\local\programs\python\python36-32\lib\site-packages\camecase\*
Proceed (y/n)?

Press y and the package will be removed.


List Packages

Use the list command to list all the packages installed on your system:

Example

List installed packages:

C:\Users\Your Name\AppData\Local\Programs\Python\Python36-32\Scripts>pip list

Result:

Package         Version
———————–
camelcase       0.2
mysql-connector 2.1.6
pip             18.1
pymongo         3.6.1
setuptools      39.0.1
 

Python Try Except

The try block lets you test a block of code for errors.

The except block lets you handle the error.

The else block lets you execute code when there is no error.

The finally block lets you execute code, regardless of the result of the try- and except blocks.


Exception Handling

When an error occurs, or exception as we call it, Python will normally stop and generate an error message.

These exceptions can be handled using the try statement:

Example

The try block will generate an exception, because x is not defined:

try:
  print(x)
except:
  print(“An exception occurred”)
Try it Yourself »

Since the try block raises an error, the except block will be executed.

Without the try block, the program will crash and raise an error:

Example

This statement will raise an error, because x is not defined:

print(x)
Try it Yourself »

Many Exceptions

You can define as many exception blocks as you want, e.g. if you want to execute a special block of code for a special kind of error:

Example

Print one message if the try block raises a NameError and another for other errors:

try:
  print(x)
except NameError:
  print(“Variable x is not defined”)
except:
  print(“Something else went wrong”)
Try it Yourself »

 

Else

You can use the else keyword to define a block of code to be executed if no errors were raised:

Example

In this example, the try block does not generate any error:

try:
  print(“Hello”)
except:
  print(“Something went wrong”)
else:
  print(“Nothing went wrong”)
Try it Yourself »

Finally

The finally block, if specified, will be executed regardless if the try block raises an error or not.

Example

try:
  print(x)
except:
  print(“Something went wrong”)
finally:
  print(“The ‘try except’ is finished”)
Try it Yourself »

This can be useful to close objects and clean up resources:

Example

Try to open and write to a file that is not writable:

try:
  f = open(“demofile.txt”)
  try:
    f.write(“Lorum Ipsum”)
  except:
    print(“Something went wrong when writing to the file”)
  finally:
    f.close()
except:
  print(“Something went wrong when opening the file”)
Try it Yourself »

The program can continue, without leaving the file object open.


Raise an exception

As a Python developer you can choose to throw an exception if a condition occurs.

To throw (or raise) an exception, use the raise keyword.

Example

Raise an error and stop the program if x is lower than 0:

x = –1

if x < 0:
  raise Exception(“Sorry, no numbers below zero”)
Try it Yourself »

The raise keyword is used to raise an exception.

You can define what kind of error to raise, and the text to print to the user.

Example

Raise a TypeError if x is not an integer:

x = “hello”

if not type(x) is int:
  raise TypeError(“Only integers are allowed”)
Try it Yourself »

Python User Input

User Input

Python allows for user input.

That means we are able to ask the user for input.

The method is a bit different in Python 3.6 than Python 2.7.

Python 3.6 uses the input() method.

Python 2.7 uses the raw_input() method.

The following example asks for the username, and when you entered the username, it gets printed on the screen:

Python 3.6

username = input(“Enter username:”)
print(“Username is: “ + username)
Run Example »

Python 2.7

username = raw_input(“Enter username:”)
print(“Username is: “ + username)
Run Example »

Python stops executing when it comes to the input() function, and continues when the user has given some input.

Python String Formatting

To make sure a string will display as expected, we can format the result with the format() method.


String format()

The format() method allows you to format selected parts of a string.

Sometimes there are parts of a text that you do not control, maybe they come from a database, or user input?

To control such values, add placeholders (curly brackets {}) in the text, and run the values through the format() method:

Example

Add a placeholder where you want to display the price:

price = 49
txt = “The price is {} dollars”
print(txt.format(price))
Try it Yourself »

You can add parameters inside the curly brackets to specify how to convert the value:

Example

Format the price to be displayed as a number with two decimals:

txt = “The price is {:.2f} dollars”
Try it Yourself »

Check out all formatting types in our String format() Reference.


Multiple Values

If you want to use more values, just add more values to the format() method:

print(txt.format(price, itemno, count))

And add more placeholders:

Example

quantity = 3
itemno = 567
price = 49
myorder = “I want {} pieces of item number {} for {:.2f} dollars.”
print(myorder.format(quantity, itemno, price))
Try it Yourself »

 

Index Numbers

You can use index numbers (a number inside the curly brackets {0}) to be sure the values are placed in the correct placeholders:

Example

quantity = 3
itemno = 567
price = 49
myorder = “I want {0} pieces of item number {1} for {2:.2f} dollars.”
print(myorder.format(quantity, itemno, price))
Try it Yourself »

Also, if you want to refer to the same value more than once, use the index number:

Example

age = 36
name = “John”
txt = “His name is {1}. {1} is {0} years old.”
print(txt.format(age, name))
Try it Yourself »

Named Indexes

You can also use named indexes by entering a name inside the curly brackets {carname}, but then you must use names when you pass the parameter values txt.format(carname = "Ford"):

Example

myorder = “I have a {carname}, it is a {model}.”
print(myorder.format(carname = “Ford”, model = “Mustang”))
Try it Yourself »

 

Python Modules

ArmenianChinese (Traditional)EnglishGermanRussian