Python Lists
A list in Python is a sequence data type used for storing a comma-separated collection of objects in a single variable. Lists are always ordered and can contain different types of objects (strings, integers, booleans, etc.). Since they are mutable data types, lists are a good choice for dynamic data (that may be added or removed over time).
Syntax
# With square brackets
list_a = []
# With built-in function
list_b = list()
Lists can either be defined with square brackets ([]) or with the built-in list() constructor method. In any case, the values initially passed to the new list must be comma-separated.
The following example showcases how lists can hold items of the same type or a mix of different types:
list_1 = [1, 2, 3, 4, 5, 6, 7, 8, 9]list_2 = ["one", 2, "3"]
Stacks and Queues
Python lists can also be made to behave like stacks and queues.
Stacks follow a “last-in, first-out” insertion order. This behavior can be showcased with the .append() and .pop() methods for adding to and removing from the top of the stack, respectively:
stack_example = ["a", "b", "c"]print(stack_example)# Output: ['a', 'b', 'c']stack_example.append(1)stack_example.append(2)stack_example.append(3)print(stack_example)# Output: ['a', 'b', 'c', 1, 2, 3]stack_example.pop()stack_example.pop()print(stack_example)# Output: ['a', 'b', 'c', 1]
Queues follow a “first-in, first-out” insertion order and also utilize the .append() and .pop() methods:
queue_example = ["a", "b", "c"]print(queue_example)# Output: ['a', 'b', 'c']queue_example.append(1)queue_example.append(2)print(queue_example)# Output: ['a', 'b', 'c', 1, 2]queue_example.pop(0)print(queue_example)# Output: ['b', 'c', 1, 2]
While using a list as a queue is possible, it is not efficient because applying .pop() to the first item requires shifting all remaining items by one spot and reassigning indices. Instead, a deque object from the collections module should be used to efficiently add/remove from a queue.
Video Walkthrough
Watch this video for a step-by-step demonstration on how to create and edit Python lists.
Lists
- .append()
- Adds a single item to the end of a list in Python.
- .clear()
- Removes all items from the list.
- .copy()
- Returns a shallow copy of a list.
- .count()
- Returns the number of occurrences of a specified element in a list.
- .extend()
- Adds the elements of an iterable (like a list, tuple, or string) to the end of the current list.
- .index()
- Returns the index of the first occurrence of a specified element in a list.
- .insert()
- Adds an element to a specific index in a list.
- .pop()
- Removes an element at the specified position from a list and returns it.
- .remove()
- Removes an item from a list by passing in the value of the item to be removed as an argument.
- .reverse()
- Reverse the elements in the list.
- .sort()
- Sorts the elements of a list in ascending or descending order.
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