Python

Python Lists: A Complete Guide

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Python lists are a fundamental data type in Python. They are similar to Arrays in other languages, such as Javascript. You can learn more about the main Python data structures here.

To create a list in Python is super easy. Let’s start with some of the basics, and then dive into useful ways you’ll be able to work with lists. To start a new list, simply define a variable in Python, with the contents encapsulated in [] square brackets:

myList = [ "some", "list", "contents" ]

As Python is a language that is dynamically typed, there are no major limitations on what can go inside a list, so feel free to add other types such as integers or floats too:

myList = [ "some", "list", "contents", 1, 2.5, "etc" ]

Empty lists can be defined as [] empty square brackets should you need to do that too:

myList = []

Lists can also be nested, or contain lists within lists (… within lists). So this is also valid python:

myList = [ [ "some", "list" ], "inside", "a", "list" ]

Getting the length of a list is done using the standalone len() function. Here’s an example where we try to get the length of our nested list:

myList = [ [ "some", "list" ], "inside", "a", "list" ] print(len(myList)) # returns 4

Finally, we can reference items in a list using the square bracket notation. For example, to get the first item of a list:

myList = [ "some", "list", "contents", 1, 2.5, "etc" ] print(myList[0]) # "some"

Or, to get the first two items:

myList = [ "some", "list", "contents", 1, 2.5, "etc" ] print(myList[0:2]) # [ "some", "list" ]

List Methods in Python

Lists come with a bunch of built in methods in Python, to allow us to fully realise their potential as data stores. These methods are:

  • list.append("newItem") - appends an item with the value newItem to the end of the lists.
  • list.extend(["newItem"]) - appends another iterable item onto the list, for example - combining two lists.
  • list.insert(0, "newItem") - inserts a list item newItem and index 0. You can change the value of the index, to decide where the item should be inserted.
  • list.clear() - deletes the lists contents entirely.
  • list.remove("someItem") - removes the first item of the list with a value of item. Will throw an error if no value item exists.
  • list.count("someItem") - counts any instances of someItem in the list. If no item in the list has the value someItem then it will return 0.
  • list.copy() - creates a shallow copy of the list.
  • list.reverse() - reverse the elements of the list.
  • list.pop(10) - removes an item at position 10 of the list, and returns it. If you don’t define a number, it’ll remove the last item, i.e. list.pop().
  • list.sort() - for sorting lists.

To bring this to life, let’s look at a few examples of how it works in practice:

Appending and Inserting new list items

One of the most common things you’ll want to do with a list is adding new data to it. If we’ve created a list, we can append data using the append method as mentioned, or insert to insert items at a certain position:

myList = [ "inside", "a", "list" ] myList.append("friend") myList.insert(2, "new") print(myList); # ['inside', 'a', 'new', 'list', 'friend']

You can also add to a list using +=:

myList = [ "inside", "a", "list" ] myList += "friend" print(myList) # ['inside', 'a', 'list', 'friend']

Or, if you have two lists, you can add the second onto the first using extend:

myList = [ "some", "list" ] otherList = [ "other", "list" ] myList.extend(otherList) print(myList) # ['some', 'list', 'other', 'list']

Deleting lists and lists content

An equally common thing you’ll want to do, once you add everything to your list, is to delete items. You can delete with remove or clear to simply delete the entire list:

myList = [ "inside", "a", "list" ] myList.remove("inside") print(myList) # ['a', 'new'] myList.clear() print(myList) # returns []

Reversing a List in Python

You’ll also want to reverse lists, in some situations. Python has a build in method for this, so no need to define your own:

myList = [ "inside", "a", "list" ] myList.reverse() print(myList) # ['list', 'a', 'inside']

Making copies of lists

In Python, we use == to compare by value, and is to compare by reference. We can use copy() to make a new reference for a list. This will make a new reference point in memory pointing to the same value. Below, myList and otherList are equal in value, but now their reference is different, so myList is otherList returns false:

myList = [ "a", "list" ] otherList = myList.copy() print(myList == otherList) # True print(myList is otherList) # False

This can also be written as myList[:], if you want to avoid using the copy() method:

myList = [ "a", "list" ] otherList = myList[:] print(myList == otherList) # True print(myList is otherList) # False

Sorting Lists in Python

Sorting a list in ascending order is easy using the sort function, and all items are of the same type:

myList = [ "a", "c", "e", "b", "f", "d", "g", "z", "w", "x" ] myNumberList = [ 1, 3, 5, 2, 7, 4, 6 ] myList.sort() myNumberList.sort() print(myList) # ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'w', 'x', 'z'] print(myNumberList) # [1, 2, 3, 4, 5, 6, 7]

If you try to sort where the list contains different types - like integers and strings, you’ll end up getting an error. If you want to sort a list based on another feature of the list, you can define its key, and reverse arguments:

  • key gives us a number which will be used to compare the list content
  • reverse if set to true will reverse the order.

For example, to put all values which are a at the start, we could try something like this:

def isA(letter): if(letter == "a"): return 1 else: return 0 myList = [ "a", "c", "a", 5, "f", "a", 2, "z", "a", "x" ] myList.sort(key=isA, reverse=True) print(myList) # ['a', 'a', 'a', 'a', 'c', 5, 'f', 2, 'z', 'x']

Here, we define a function isA, which takes each item in the list as its first argument (letter). If the letter is a, then it returns 1, otherwise it returns 0. Then we reverse the list using the reverse=True argument, to get all the as at the start.

Lists function best as stacks of data!

Since lists are ordered, they function best as stacks, which means adding and removing items from the end of a list is super fast, while adding or removing items from the start is kind of slow. That means it’s recommended to use pop and append where possible, as this is going to be a lot faster on large data sets than other methods on lists.

Conclusion

Lists are super powerful data structures in Python, and used everywhere. You can learn more about Python data structures here. To learn more about other engineering topics, check out the rest of my content. You can read more about Python data structures below:

Last Updated Saturday, 1 October 2022

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