Core List Operations
Once you’ve created a list, Python offers a rich set of operations to access, modify, and inspect its contents. Understanding these operations is key to using lists effectively.
1. Accessing Elements: Indexing and Slicing
Accessing elements is a fundamental list operation. Python’s syntax is both powerful and flexible.
Indexing: Accessing a Single Element
You can retrieve any element from a list using its zero-based index in square brackets [].
- Positive Indexing: Starts from
0for the first element. - Negative Indexing: Starts from
-1for the last element,-2for the second-to-last, and so on. This is incredibly useful for accessing items at the end of a list without needing to know its length.
PygroundTry It Out
From a list of tasks, retrieve the second task using a positive index and the last task using a negative index.
Expected Output:
The second task is: 'Write tests' The last task is: 'Update docs'
Output:
Accessing an index that doesn’t exist will raise an IndexError. For example, todos[4] would fail for the list above.
Slicing: Extracting a Sub-List
Slicing lets you create a new list containing a portion of the original. The syntax is list[start:stop:step].
start: The index to begin the slice (inclusive, defaults to0).stop: The index to end the slice (exclusive, defaults to the end of the list).step: The amount to jump between elements (defaults to1).
Basic Slice
Extract a sub-list from the middle.
PygroundTry It Out
Get the middle three numbers from a list of digits.
Expected Output:
[3, 4, 5]
Output:
A common use for slicing is to create a shallow copy of a list: new_list = original_list[:].
2. Combining and Repeating Lists
You can easily combine or duplicate lists using standard operators.
Concatenation (+) vs. Extension (.extend())
Concatenation (+)
The + operator combines two lists to create a new third list, leaving the original lists unchanged.
PygroundTry It Out
Combine two lists of numbers into a single new list.
Expected Output:
New list: [1, 2, 3, 4, 5, 6] Original first_half is unchanged: [1, 2, 3]
Output:
Repetition (*)
The * operator creates a new list by repeating its elements a specified number of times.
PygroundTry It Out
Create a simple 'loading' animation bar of size 5.
Expected Output:
['-', '-', '-', '-', '-']
Output:
Warning: Shared References
When using * on a list containing mutable objects (like other lists), it copies references, not the objects themselves. Modifying one nested element will affect all its “copies”.
Example
Output:
Incorrect: [[99], [99], [99]]Correct: [[99], [0], [0]]
3. Membership Testing (in)
Use the in and not in keywords to check if an item exists within a list. This is highly readable and efficient.
PygroundTry It Out
Check if 'Python' is in a list of required skills.
Expected Output:
Is Python a required skill? True Is Java a required skill? False
Output:
4. Essential Built-in Functions
Python provides several built-in functions that are very useful for working with lists.
len()
The len() function returns the number of items in a list.
PygroundTry It Out
Find out how many items are in your shopping list.
Expected Output:
There are 4 items on the shopping list.