Python List: How To Create, Sort, Append, Remove, And More

The Python list is one of the most used data structures, together with dictionaries. The Python list is not just a list, but can also be used as a stack and even a queue. In this article, I’ll explain everything you might possibly want to know about Python lists:

  • how to create lists,
  • modify them,
  • how to sort lists,
  • loop over elements of a list with a for-loop or a list comprehension,
  • how to slice a list,
  • append to Python lists,
  • … and more!

I’ve included lots of working code examples to demonstrate.

How to create a Python list

Let’s start by creating a list:

my_list = [1, 2, 3]
empty_list = []

Lists contain regular Python objects, separated by commas and surrounded by brackets. The elements in a list can have any data type, and they can be mixed. You can even create a list of lists. The following lists are all valid:

my_list = [1, "Erik", { 'age': 39 }]

# This is a list with three lists inside
game_board = [[], [], []]

Using the list() function

Python lists, like all Python data types, are objects. The class of a list is called ‘list’, with a lower case L. If you want to convert another Python object to a list, you can use the list() function, which is actually the constructor of the list class itself. This function takes one argument: an object that is iterable.

So you can convert anything that is iterable into a list. E.g., you can materialize the range function into a list of actual values, or convert a Python set or tuple into a list:

>>> list(range(1, 4))
[1, 2, 3]
>>> list({1, 2, 2, 2, 3})
[1, 2, 3]
>>> list( (1, 2, 3) )
[1, 2, 3]

Accessing Python list elements

To access an individual list element, you need to know the position of that element. Since computers start counting at 0, the first element is in position 0, the second element is in position 1, etcetera.

Here are a few examples:

>>> my_list = [1, 2, 3]
>>> my_list[1]
2
>> my_list[0]
1
>> my_list[4]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: list index out of range

As you can see, you can’t access elements that don’t exist. In this case, Python throws an IndexError exception, with the explanation ‘list index out of range’. In my article on exceptions and the try and except statements, I’ve written about this subject more in-depth in the section on best practices. I recommend you to read it.

Get the last element of a list

If you want to get elements from the end of the list, you can supply a negative value. In this case, we start counting at -1 instead of 0. E.g., to get the last element of a list, you can do this:

>>> my_list = [1, 2, 3]
>>> my_list[-1]  # get the last element
3
>>> my_list[-2]  # get the 2nd last element
2

Accessing nested list elements

Accessing nested list elements is not that much different. When you access an element that is a list, that list is returned. So to request an element in that list, you need to again use a couple of brackets:

>>> my_list = [[1, 2], [3, 4], [5, 6]]
>>> my_list[0]
[1, 2]
>> my_list[0][0]
1
>>> my_list[2][1]
6

Adding and removing elements

Let’s see how we can add and remove data. There are several ways to remove data from a list. What you use, depends on the situation you’re in. I’ll describe and demonstrate them all in this section.

Append to a Python list

List objects have a number of useful built-in methods, one of which is the append method. When calling append on a list, we append an object to the end of the list:

>>> my_list = [1, 2]
>>> my_list.append('a')
>>> my_list
[1, 2, 'a']
>>> my_list.append(4)
>>> my_list
[1, 2, 'a', 4]

Combine or merge two lists

Another way of adding elements is adding all the elements from one list to the other. There are two ways to combine lists:

  1. ‘Add’ them together with the + operator.
  2. Add all elements of one list to the other with the extend method.

Here’s how you can add two lists together. The result is a new, third list:

>>> l1 = [1, 2]
>>> l2 = [3, 4]
>>> l3 = l1 + l2
>>> l3
[1, 2, 3, 4]

The original lists are kept intact. The alternative is to extend one list with another, using the extend method:

>>> l1 = [1, 2]
>>> l2 = [3, 4]
>>> l1.extend(l2)
>>> l1
[1, 2, 3, 4]
>>> l2
[3, 4]

While l1 got extended with the elements of l2, l2 stayed the same. Extend appended all values from l2 to l1.

Pop items from a list

The pop() method removes and returns the last item by default unless you give it an index argument.

Here are a couple of examples that demonstrate both the default behavior and the behavior when given an index:

>>> my_list = [1, 2, 3, 4, 5]
>>> my_list.pop()
5
>>> my_list.pop()
4
>>> my_list.pop(0)
1
>>> my_list
[2, 3]

If you’re familiar with the concept of a stack, you can now build one using only the append and pop methods on a list!

Using del() to delete items

There are multiple ways to delete or remove items from a list. While pop returns the item that is deleted from the list, del removes it without returning anything. In fact, you can delete any object, including the entire list, using del:

>>> my_list = [1, 2, 3, 4, 5]
>>> del(my_list[0])
>>> my_list
[2, 3, 4, 5]
>>> del(my_list[2])
>>> my_list
[2, 3, 5]
>>> del(my_list)
>>> my_list
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'my_list' is not defined

Remove specific values from a Python list

If you want to remove a specific value instead, you use the remove method. E.g. if you want to remove the first occurrence of the number 2 in a list, you can do so as follows:

>>> my_list = [1, 2, 3, 2, 5]
>>> my_list.remove(2)
>>> my_list
[1, 3, 2, 5]
>>> my_list.remove(2)
>>> my_list
[1, 3, 5]
>>> my_list.remove(2)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: list.remove(x): x not in list

As you can see, repeated calls to remove will remove additional twos, until there are none left, in which case Python throws a ValueError exception.

Remove or clear all items from a Python list

To remove all items from a list, use the clear() method:

>>> my_list = [1, 2, 3]
>>> my_list.clear()
>>> my_list
[]

Remove duplicates from a list

There is no special function or method to remove duplicates from a list, but there are multiple tricks that we can use to do so anyway. The simplest, in my opinion, is using a Python set. Sets are collections of objects, like lists, but can only contain one of each element. More formally: sets are unordered collections of distinct objects.

By converting the list to a set, and then back to a list again, we’ve effectively removed all duplicates:

>>> my_list = [1, 2, 2, 3, 5]
>>> my_set = set(my_list)
>>> my_set
{1, 2, 3, 5}
>>> list(my_set)
[1, 2, 3, 5]

In your own code, you probably want to use this more compact version:

>>> my_list = [1, 2, 2, 3, 5]
>>> list(set(my_list))
[1, 2, 3, 5]

Since sets are very similar to lists, you may not even have to convert them back into a list. If the set offers what you need, use it instead to prevent a double conversion, making your program a little bit faster and more efficient.

Replace items in a list

To replace list items, we assign a new value to a given list index, like so:

>>> my_list = [1, 2, 3, 4, 5]
>>> my_list[0] = 400
>>> my_list
[400, 2, 3, 4, 5]

How to get List length in Python

In Python, we use the len function to get the length of objects. This is true for lists as well:

>>> my_list = [1, 2, 3, 4, 5]
>>> len(my_list)
5

If you’re familiar with other programming languages, like Java, you might wonder why Python has a function for this. After all, it could have been one of the built-in methods of a list too, like my_list.len(). This is because, in other languages, this often results in all kinds of ways to get an object’s length. E.g., some will call this function len, others will call it length, and yet someone else won’t even implement the function but simply offer a public member variable. And this is exactly the reason why Python chose to standardize the naming of such a common operation!

Counting element occurrence in a list

Don’t confuse the count function with getting the list length, it’s totally different. The built-in count function counts occurrences of a particular value inside that list. Here’s an example:

>>> my_list = [1, 2, 1, 4, 1, 7]
>>> my_list.count(1)
3
>>> my_list.count(4)
1

Since the number 1 occurs three times in the list, my_list.count(1) returned 3.

Check if an item is in a list

To check if an item is in a list, use the following syntax:

>>> my_list = [1, 2]
>>> if 1 in my_list:
...    print("It's in the list")
...
It's in the list

Find the index of an item in a list

We can find where an item is inside a list with the index method. For example, in the following list the 4 is located at position 3 (remember that we start counting at zero):

>>> my_list = [1, 2, 3, 4, 5]
>>> my_list.index(4)
3

The index method takes two optional parameters, start and stop. With these, we can continue looking for more of the same values. If we supply a start value, we don’t need to supply an end value. Now let’s find both 4’s in the list below:

my_list = [1, 2, 3, 4, 5, 4]
print(my_list.index(4))
# Output: 3

# We know there's a 4 at position 3 so
# let's continue our search from position 4

print(my_list.index(4, 4))
# Output: 5

If you want to do more advanced filtering of lists, you should head over to my article on list comprehensions.

Loop over list elements

A list is iterable, so we can use a for-loop over the elements of a list just like we can with any other iterable with the ‘for <element> in <iterable>’ syntax:

>>> my_list = [1, 2, 3, 4, 5]
>>> for n in my_list:
...     print(n)
... 
1
2
3
4
5

Python list to string

In Python, you can convert most objects to a string with the str function:

>>> str([1, 'abc', 2.3])
"[1, 'abc', 2.3]"

If you’re interested, str is actually the Python string‘s base class and calling str() constructs a new str object by calling the constructor of the str class. This constructor inspects the provided object and looks for a special method (also called dunder method) called __str__. If it is present, this method is called. There’s nothing more to it.

If you create your own classes and objects, you can implement the __str__ function yourself to offer a printable version of your objects.

Sorting Python lists

To sort a Python list, we have two options:

  1. Use the built-in sort method of the list itself.
  2. Use Python’s built-in sorted() function.

Option one, the built-in method, offers an in-place sort, which means the list itself is modified. In other words, this function does not return anything. Instead, it modifies the list itself.

Option two returns a new list, leaving the original intact. Which one you use depends on the situation you’re in.

In-place list sort in ascending order

Let’s start with the simplest use-case: sorting in ascending order:

>>> my_list = [10, 2, 5, 4, 2]
>>> my_list.sort()
>>> my_list
[2, 2, 4, 5, 10]

In-place list sort in descending order

We can call the sort method with a reverse parameter. If this is set to True, sort reverses the order:

>>> my_list = [10, 2, 5, 4, 2]
>>> my_list.sort(reverse=True)
>>> my_list
[10, 5, 4, 2, 2]

Using sorted()

The following example demonstrates how to sort lists in ascending order, returning a new list with the result:

>>> my_list = [10, 2, 5, 4, 2]
>>> sorted(my_list)
[2, 2, 4, 5, 10]
>>> my_list
[10, 2, 5, 4, 2]

As you can see from the last statement, the original list is unchanged. Let’s do that again, but now in descending order:

>>> my_list = [10, 2, 5, 4, 2]
>>> sorted(my_list, reverse=True)
[10, 5, 4, 2, 2]
>>> my_list
[10, 2, 5, 4, 2]

Unsortable lists

We can not sort all lists, since Python can not compare all types with each other. For example, we can sort a list of numbers, like integers and floats, because they have a defined order. We can sort a list of strings as well since Python is able to compare strings too.

However, lists can contain any type of object, and we can’t compare completely different objects, like numbers and strings, to each other. In such cases, Python throws a TypeError:

>>> my_mixed_list = [1, 'a', 2, 'B']
>>> my_mixed_list.sort()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: '<' not supported between instances of 'str' and 'int'

Although the error might look a bit cryptic, it’s only logical when you know what’s going on. To sort a list, Python needs to compare the objects to each other. So in its sorting algorithm, at some point, it checks if ‘a’ < 1, hence the error: '<' not supported between instances of 'str' and 'int'.

Slicing

Sometimes you need to get parts of a list. Python has a powerful syntax to do so, called slicing, and it makes working with lists a lot easier compared to some other programming languages. Slicing works on Python lists, but it also works on any other sequence type like strings, tuples, and ranges.

The slicing syntax is as follows:

my_list[start:stop:step]

A couple of notes:

  • start is the first element position to include
  • stop is exclusive, meaning that the element at position stop won’t be included.
  • step is the step size. more on this later.
  • start, stop, and step are all optional.
  • Negative values can be used too.

To explain how slicing works, it’s best to just look at examples, and try for yourself, so that’s what we’ll do:

>>> my_list = [1, 2, 3, 4, 5, 6, 7, 8]

>>> my_list[0:3] # get the first three elements of a list
[1, 2, 3]

>>> my_list[:3]  # start is 0 by default
[1, 2, 3]

>>> my_list[4:]  # skip the first 4 elements
[5, 6, 7, 8]

The step value

The step value is 1 by default. If you increase it, you can step over elements. Let’s try this:

>>> my_list = [1, 2, 3, 4, 5, 6, 7, 8]

>>> my_list[::2]  # skip one each time
[1, 3, 5, 7]

Going backward

Like with list indexing, we can also supply negative numbers with slicing. Here’s a little ASCII art to show you how counting backward in a list works:

my_list = [1 , 2, 3, 4, 5, 6]
           -6 -5 -4 -3 -2 -1   (these are the index numbers)

Just remember that you need to set a negative step size in order to go backward:

>>> my_list = [1, 2, 3, 4, 5, 6]
>>> my_list[-1:-3:-1]
[6, 5]

Reversing Python lists

There are actually three methods you can use to reverse a list in Python:

  1. An in-place reverse, using the built-in reverse method that every list has natively
  2. Using list slicing with a negative step size results in a new list
  3. Create a reverse iterator, with the reversed() function

In the following code crumb, I demonstrate all three. They are explained in detail in the following sections:

Three ways to revert a Python list

Using the built-in reverse method

The list.reverse() method does an in-place reverse, meaning it reorders the list. In other words, the method does not return a new list object, with a reversed order. Here’s how to use reverse():

>>> lst = [1, 2, 3, 4]
>>> lst.reverse()
>>> lst
[4, 3, 2, 1]
>>> lst.reverse()
>>> lst
[1, 2, 3, 4]

Reverse a list with list slicing

Although you can reverse a list with the list.reverse() method that every list has, you can do it with list slicing too, using a negative step size of -1. The difference here is that list slicing results in a new, second list. It keeps the original list intact:

>>> lst = [1, 2, 3, 4]
>>> lst[::-1]
[4, 3, 2, 1]
>>> lst
[1, 2, 3, 4]

Creating a reverse iterator

Finally, you can use the reversed() built-in function, which creates an iterator that returns all elements of the given iterable (our list) in reverse. This method is quite cheap in terms of CPU and memory usage. All it needs to do is walk backward over the iterable object. It’s doesn’t need to move around data and it doesn’t need to reserve extra memory for a second list. So if you need to iterate over a (large) list in reverse, this should be your choice.

Here’s how you can use this function. Keep in mind, that you can only use the iterator once, but that it’s cheap to make a new one:

>>> lst = [1, 2, 3, 4]
>>> rev = reversed(lst)
>>> rev
<list_reverseiterator object at 0x0000023DB96A25C0>
>>> for i in rev:
...     print(i)
...
4
3
2
1

Learn more about Python lists

Because there’s a lot to tell about list comprehensions, I created a dedicated article for the topic. A Python list comprehension is a language construct that we use to create a list based on an existing list, without creating for-loops.

Some other resources you might like:

  • The official Python docs about lists.
  • If you are interested in the internals: lists are often implemented internally as a linked list.
  • Python also has arrays, which are very similar and the term will be familiar to people coming from other programming languages. They are more efficient at storing data, but they can only store one type of data.
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