List Methods Reference

Python’s lists come with a powerful set of built-in methods. These methods can be categorized into two groups: those that mutate (modify) the list in-place, and those that are non-mutating, returning a new value without changing the original list.

1. Mutating Methods (Modify the List In-Place)

These methods change the list they are called on. They typically return None to signal that the change happened in-place.

.append()

`list.append(item)`

Adds a single item to the very end of the list. This is one of the most commonly used list methods.

Pyground

You have a list of tasks. Add a new task, 'File TPS reports', to the end.

Expected Output:

['Review code', 'Write documentation', 'File TPS reports']

Output:

.extend()

`list.extend(iterable)`

Adds all items from an iterable (like another list, a tuple, or a string) to the end of the list.

Pyground

Combine a list of morning tasks with a list of afternoon tasks.

Expected Output:

['Team stand-up', 'Answer emails', 'Develop feature', 'Deploy to staging']

Output:

.insert()

`list.insert(index, item)`

Inserts an item at a specific index. All subsequent items are shifted to the right.

This can be slow on large lists because every element after the insertion point must be moved.

Pyground

Insert 'Urgent: Fix bug' at the beginning of your task list.

Expected Output:

['Urgent: Fix bug', 'Task 1', 'Task 2']

Output:

.remove()

`list.remove(value)`

Searches for the first occurrence of value and removes it.

🚨

If the value is not found in the list, this method will raise a ValueError.

Pyground

A party guest has left. Remove 'Charlie' from the guest list.

Expected Output:

['Alice', 'Bob', 'David']

Output:

.pop()

`list.pop(index=-1)`

Removes and returns the item at the given index. If no index is specified, it defaults to -1, removing and returning the last item. This makes it easy to use a list as a stack (Last-In, First-Out).

Pyground

Process the last task added to a list and show which task was processed.

Expected Output:

Processed: Task C
Remaining tasks: ['Task A', 'Task B']

Output:

.clear()

`list.clear()`

Removes all items from the list, making it empty.

Pyground

You've completed all your tasks for the day. Clear your task list.

Expected Output:

Tasks left: []

Output:

.reverse()

`list.reverse()`

Reverses the order of the elements in the list in-place.

Pyground

Reverse a list of countdown numbers.

Expected Output:

['Blast off!', 1, 2, 3]

Output:

.sort()

`list.sort(*, key=None, reverse=False)`

Sorts the list in-place. This is a powerful method with two optional, keyword-only arguments:

key: A function to be called on each element before making comparisons.
reverse: If True, sorts in descending order.

Pyground

Sort a list of words alphabetically, but case-insensitively.

Expected Output:

['Apple', 'Banana', 'kiwi', 'zebra']

Output:

For sorting, the built-in function sorted(iterable) provides the same functionality but returns a new, sorted list instead of modifying the original in-place.

2. Non-Mutating Methods (Return a Value)

These methods inspect the list and return a value without changing the original list.

.count()

`list.count(value)`

Returns the number of times a value appears in the list.

Pyground

Count how many 'A' grades are in a list of student scores.

Expected Output:

There are 3 'A' grades.

Output:

.index()

`list.index(value, start=0, stop=len(list))`

Returns the zero-based index of the first occurrence of value. You can optionally provide start and stop indices to search within a sub-section of the list.

🚨

Like .remove(), this method raises a ValueError if the value is not found.

Pyground

Find the position of 'Python' in a list of skills.

Expected Output:

'Python' is at index 3.

Output:

.copy()

`list.copy()`

Returns a shallow copy of the list. This is equivalent to a_list[:].

🤔

Shallow Copy Explained: A shallow copy creates a new list, but it fills it with references to the same objects found in the original list. If the list contains mutable objects (like other lists), modifying the nested object in one list will affect the other.

Pyground

Demonstrate the effect of modifying a nested list in a shallow copy.

Expected Output:

Original after modification: [1, 2, [3, 4, 5]]
Copied list: [1, 2, [3, 4, 5]]