Python Lists and How to Use Their Built-In Methods

Python includes many built-in methods and operations that help you manipulate lists. This guide shows you how to use the append(), insert(), and remove() built-in list methods. You also learn how to write a list comprehensions, and how to sort lists.

Before You Begin

1. This guide uses Python 3 syntax. Although there is some overlap, some of the code may not work with Python 2. To install Python 3, follow our How to Install Python 3 guide, using the Distribution drop down to select your Linux distribution.

2. All commands in this guide can be entered either in a Python script or in the Python interpreter (also known as the Python Interactive Shell). You can learn more about either method through the Python 3 installation guide linked above.

How to Create a List in Python

To create a list in Python wrap one or more comma-separated items in square brackets. Create a Python list as shown in the following example:

example_list = ["item_1", "item_2", "item_3"]

To view the contents of the list, issue the following command:

print(example_list)

['item_1', 'item_2', 'item_3']

The items in a list can be integers, strings, or any kind of other objects — even other lists. You can also mix and match different types within the same list.

List Comprehensions

Python list comprehensions give you a succinct but advanced way of creating new lists from existing lists. A list comprehension creates a new list by applying a set of conditions and/or operations on each item in an existing list.

For example, the code below uses a list comprehension on the range of numbers between zero and nine (inclusive). The range function’s return value is a list. The list comprehension selects all even numbers (x % 2 == 0) in the range and multiplies each matching number by two (x * 2).

In your Python interpreter run the following code to create the example list comprehension:

example_list = [(x * 2) for x in range(10) if x % 2 == 0]

Print the contents of example_list to view the result:

print(example_list)

[0, 4, 8, 12, 16]

A list comprehension’s succinct syntax is convenient when writing code. It lets you quickly rework a list from complicated criteria, and do so using a single line of code.

How to Add or Append to a List in Python

Lists in Python have a built-in append() method that allows you to append an item to the end of an existing list. Using the example_list created in the section above, issue the following command to add the string item_4 to the end of the list.

example_list.append("item_4")

Print the contents of the list to view the appended item:

print(example_list)

[0, 4, 8, 12, 16, 'item_4']

Python lists have a built-in insert() method. This method lets you add an item to a specific index position in a list. In the following example, an item is added to the beginning of the example_list list. The rest of the list items are moved one position to the right.

example_list.insert(0, "item_0")

Print the contents of the list to view the inserted item:

print(example_list)

['item_0', 0, 4, 8, 12, 16, 'item_4']

Finally, you can also concatenate lists using the + operator. This allows you to create a new list, rather than modifying the existing list. Create a new list named new_example_list with the contents of example_list:

new_example_list = example_list + ["item_5", "item_6", "item_7"]

Print the contents of example_list and new_example_list: print(example_list) print(new_example_list)

['item_0', 0, 4, 8, 12, 16, 'item_4']
['item_0', 0, 4, 8, 12, 16, 'item_4', 'item_5', 'item_6', 'item_7']

You can also use the + operator to add an individual item to a list, as long as that item is presented as a list.

  example_list = new_example_list + ["item_8"]

View the contents of example_list:

  print(example_list)

['item_0', 0, 4, 8, 12, 16, 'item_4', 'item_5', 'item_6', 'item_7', 'item_8']

How to Remove an Item from a List in Python

Like with the append() method, Python lists include a remove() method that removes an item from a list. The method takes the contents of the item to be deleted as an argument. For example, remove item_0 from example_list:

example_list.remove("item_0")

View the contents of example_list:

print(example_list)

[0, 4, 8, 12, 16, 'item_4', 'item_5', 'item_6', 'item_7', 'item_8']

Alternatively, you can use the pop() method on a list to delete an item based on its index. In the example below, the item at index 3 (12) is removed.

example_list.pop(3)

Print the contents of example_list:

print(example_list)

[0, 4, 8, 16, 'item_4', 'item_5', 'item_6', 'item_7', 'item_8']

One useful feature of the pop() method is that it returns the item it deletes. This means you can assign the deleted value to a variable. For example, remove the item at index 6 and assign it to the variable pop_output:

pop_output = example_list.pop(6)

Print the values of pop_output and example_list:

print(pop_output)
print(example_list)

'item_6'
[0, 4, 8, 16, 'item_4', 'item_5', 'item_7', 'item_8']

Finally, Python has one other option for removing an item from a list, del keyword. This option identifies the item or items to be deleted by using Python’s slice notation. In the example below, the items from index 3 to the end of the list are deleted.

del example_list[3:]

Print the remaining items in example_list:

print(example_list)

[0, 4, 8]

You can learn more about Python’s slice notation in the Reverse a List section below. The links provided at the end of this guide also provide more information.

How to Sort a List in Python

Sort a List

Python lists include a sort() method dedicated to sorting the content of lists in place. In the example below, a list of fruits is sorted alphabetically, in ascending order (default):

example_fruit_list = ["strawberry", "apricot", "cranberry", "banana"]
example_fruit_list.sort()

Print the contents of example_fruit_list:

print(example_fruit_list)

['apricot', 'banana', 'cranberry', 'strawberry']

The sort method has two optional arguments. First, a key argument allows you to reference a function to use when sorting the list. Second, a reverse option which, if set to True, allows you to sort the list in descending order.

In this next example, a length_sort() function is defined to return the length of an item. Referencing the length_sort() function as the key argument causes sort() to order the list by the length of fruit names. Setting reverse=True has sort() use a descending order, from longest name to shortest name.

Create the length_sort() function:

def length_sort(item):
    return len(item)

Sort example_fruit_list using length_sort() as the key:

example_fruit_list.sort(key=length_sort, reverse=True)

Print the newly sorted contents of example_fruit_list:

print(example_fruit_list)

['strawberry', 'cranberry', 'apricot', 'banana']

Python also has a sorted() function. The function takes an iterable (like a list) and returns a sorted list. It even includes the same optional arguments as the sort() method (key and reverse). This function is useful when you want to create a new list from the sort, rather than sorting a list in place. The example below, creates a new list named new_example_fruit_list that is sorted in the same way as the previous example:

    new_example_fruit_list = sorted(example_fruit_list, key=length_sort)

Print the contents of both lists:

    print(example_fruit_list)
    print(new_example_fruit_list)

['strawberry', 'cranberry', 'apricot', 'banana']
['banana', 'apricot', 'cranberry', 'strawberry']

Both sort and sorted can work on lists containing either numbers or strings. However, neither works on lists containing a mixture of numbers and strings.

sorted([198, "strawberry", 46.8, "apricot"])

TypeError: '<' not supported between instances of 'int' and 'str'

Reverse a List

Reversing a list in Python can be achieved by using slice notation. The example below, creates a list, then creates a new version of the list in reversed order:

example_list = ["item_1", "item_2", "item_3", "item_4]

reversed_example_list = example_list[::-1]

Print the two lists to view their difference in order:

print(example_list)
print(reversed_example_list)

['item_1', 'item_2', 'item_3', 'item_4']
['item_4', 'item_3', 'item_2', 'item_1']

Python’s slice notation allows you to select a sub-list or a reordered version of a list. It follows the following format: [start:stop:step]

The start portion defines what index to begin with. The stop portion indicates the index to end with. Leaving these values empty defaults to the beginning and ending of the list, respectively. The optional step portion defines how many items to step at a time. It defaults to 1, meaning each item is captured, whereas 2 would capture every other item.

Each portion can take a negative number. For start and stop, negative numbers identify items starting from the end of the list, -1 being the last item, -2 the next to last, etc. For step, negative numbers mean stepping backward through the list, from the end to the beginning.

So, in the example above, the slice notation — [::-1] — returns the whole list but steps through it in reverse order. This results in a reversed version of the list.

Convert a Python List to a String

Several approaches are available in Python for converting a list to a string. However, the join() method is probably the most versatile and convenient.

The join() method is invoked on a string and takes a list as an argument. The string becomes the separator for each item in the list as these items are concatenated into a new string. The example below uses the join() method to convert example_list into a string:

example_list = ["This", "string", "was", "a", "list"]
example_string_from_list = " ".join(example_list)

View the created string stored in the example_string_from_list variable:

print(example_string_from_list)

This string was a list

If your list contains numbers rather than strings, you need to first convert the numbers to strings. You can do this in the same line using a simple list comprehension, as shown below:

example_list = [1, 2, 3, 4, 5]
example_string_from_list = " ".join([str(x) for x in example_list])

Print the strings stored in the example_string_from_list variable:

print(example_string_from_list)

1 2 3 4 5

How to Find an Item in a List in Python

You have a few options at your disposal for searching a list in Python.

You can use the in syntax if you just want Python to tell you whether a certain value is in a list. The example below prints true if the value 3 is in example_list.

example_list = [1, 2, 3, 4, 1, 2, 3, 4]

print(3 in example_list)

True

If you want to know the index of a particular value in a list, use the list’s index() method. It takes the desired value as an argument and returns its index in the list, or an error if it cannot find the value. This method only returns the first index it finds with the matching value. For example, to find index for the number 2 in example_list use the following code:

print(example_list.index(2))

1

If you want the indices for all matching items in a list, you can use a list comprehension like in the example below:

print([key for key, value in enumerate(example_list) if value == 2])

[1, 5]

All the above operations work no matter what type the value has — integer, string, another list, etc. However, these options only work for exact matches.

example_fruit_list = ["strawberry", "apricot", "cranberry", "banana"]

print("straw" in example_fruit_list)
print("strawberry" in example_fruit_list)

False
True

To find partial-string matches, you can use another list comprehension. This one iterates through the list and returns the indices of strings containing the desired substring — straw in this example.

matching_items = [key for key, value in enumerate(example_fruit_list) if value.find("straw") != -1]

print(matching_items)

for item in matching_items:
    print(example_fruit_list[item])

[0]
strawberry

More Information

You may wish to consult the following resources for additional information on this topic. While these are provided in the hope that they will be useful, please note that we cannot vouch for the accuracy or timeliness of externally hosted materials.

• Python 3 Documentation: List Comprehensions
• O’Reilly: How Do I Use the Slice Notation in Python?