Python Lists

Python Lists


List

Lists are used to store multiple items in a single variable.

Lists are one of 4 built-in data types in Python used to store collections of data, the other 3 are TupleSet, and Dictionary, all with different qualities and usage.

Lists are created using square brackets:

Example

Create a List:

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List Items

List items are ordered, changeable, and allow duplicate values.

List items are indexed, the first item has index [0], the second item has index [1] etc.


Ordered

When we say that lists are ordered, it means that the items have a defined order, and that order will not change.

If you add new items to a list, the new items will be placed at the end of the list.

Note: There are some list methods that will change the order, but in general: the order of the items will not change.


Changeable

The list is changeable, meaning that we can change, add, and remove items in a list after it has been created.


Allow Duplicates

Since lists are indexed, lists can have items with the same value:

Example

Lists allow duplicate values:

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List Length

To determine how many items a list has, use the len() function:

Example

Print the number of items in the list:

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List Items – Data Types

List items can be of any data type:

Example

String, int and boolean data types:

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A list can contain different data types:

Example

A list with strings, integers and boolean values:

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type()

From Python’s perspective, lists are defined as objects with the data type ‘list’:

Example

What is the data type of a list?

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The list() Constructor

It is also possible to use the list() constructor when creating a new list.

Example

Using the list() constructor to make a List:

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Python Collections (Arrays)

There are four collection data types in the Python programming language:

  • List is a collection which is ordered and changeable. Allows duplicate members.
  • Tuple is a collection which is ordered and unchangeable. Allows duplicate members.
  • Set is a collection which is unordered, unchangeable*, and unindexed. No duplicate members.
  • Dictionary is a collection which is ordered** and changeable. No duplicate members.

*Set items are unchangeable, but you can remove and/or add items whenever you like.

**As of Python version 3.7, dictionaries are ordered. In Python 3.6 and earlier, dictionaries are unordered.

When choosing a collection type, it is useful to understand the properties of that type. Choosing the right type for a particular data set could mean retention of meaning, and, it could mean an increase in efficiency or security.

Python – Access List Items


Access Items

List items are indexed and you can access them by referring to the index number:

Example

Print the second item of the list:

thislist = [“apple”“banana”“cherry”]
print(thislist[1])

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Note: The first item has index 0.

Negative Indexing

Negative indexing means start from the end

-1 refers to the last item, -2 refers to the second last item etc.

Example

Print the last item of the list:

thislist = [“apple”“banana”“cherry”]
print(thislist[-1])

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Range of Indexes

You can specify a range of indexes by specifying where to start and where to end the range.

When specifying a range, the return value will be a new list with the specified items.

Example

Return the third, fourth, and fifth item:

thislist = [“apple”“banana”“cherry”“orange”“kiwi”“melon”“mango”]
print(thislist[2:5])

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Note: The search will start at index 2 (included) and end at index 5 (not included).

Remember that the first item has index 0.

By leaving out the start value, the range will start at the first item:

Example

This example returns the items from the beginning to, but NOT including, “kiwi”:

thislist = [“apple”“banana”“cherry”“orange”“kiwi”“melon”“mango”]
print(thislist[:4])

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By leaving out the end value, the range will go on to the end of the list:

Example

This example returns the items from “cherry” to the end:

thislist = [“apple”“banana”“cherry”“orange”“kiwi”“melon”“mango”]
print(thislist[2:])

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Range of Negative Indexes

Specify negative indexes if you want to start the search from the end of the list:

Example

This example returns the items from “orange” (-4) to, but NOT including “mango” (-1):

thislist = [“apple”“banana”“cherry”“orange”“kiwi”“melon”“mango”]
print(thislist[-4:-1])

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Check if Item Exists

To determine if a specified item is present in a list use the in keyword:

Example

Check if “apple” is present in the list:

thislist = [“apple”“banana”“cherry”]
if “apple” in thislist:
print(“Yes, ‘apple’ is in the fruits list”)

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Python – Change List Items


Change Item Value

To change the value of a specific item, refer to the index number:

Example

Change the second item:

thislist = [“apple”“banana”“cherry”]
thislist[1] = “blackcurrant”
print(thislist)

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Change a Range of Item Values

To change the value of items within a specific range, define a list with the new values, and refer to the range of index numbers where you want to insert the new values:

Example

Change the values “banana” and “cherry” with the values “blackcurrant” and “watermelon”:

thislist = [“apple”“banana”“cherry”“orange”“kiwi”“mango”]
thislist[1:3] = [“blackcurrant”“watermelon”]
print(thislist)

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If you insert more items than you replace, the new items will be inserted where you specified, and the remaining items will move accordingly:

Example

Change the second value by replacing it with two new values:

thislist = [“apple”“banana”“cherry”]
thislist[1:2] = [“blackcurrant”“watermelon”]
print(thislist)

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Note: The length of the list will change when the number of items inserted does not match the number of items replaced.

If you insert less items than you replace, the new items will be inserted where you specified, and the remaining items will move accordingly:

Example

Change the second and third value by replacing it with one value:

thislist = [“apple”“banana”“cherry”]
thislist[1:3] = [“watermelon”]
print(thislist)

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Insert Items

To insert a new list item, without replacing any of the existing values, we can use the insert() method.

The insert() method inserts an item at the specified index:

Example

Insert “watermelon” as the third item:

thislist = [“apple”“banana”“cherry”]
thislist.insert(2“watermelon”)
print(thislist)

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Note: As a result of the example above, the list will now contain 4 items.

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