Materialize iterable data
Materialize an iterable so it can be indexed, sliced, or traversed repeatedly.
Python · Class
list
list is Python’s mutable sequence type. Create one with a literal, comprehension, or list(iterable); construction preserves the iterable’s item order.
How it works
list creates a mutable sequence, either empty or populated from an iterable. Its elements preserve the iterable’s order, and the sequence supports indexing, slicing, iteration, concatenation, repetition, and in-place mutation.
Syntax
list([iterable])Parameters
| Name | Type | Description |
|---|---|---|
iterable | iterable | Optional iterable whose items populate the new list in iteration order. |
Returns
A new list containing the iterable’s items in the same order; with no argument, an empty list.
Key points
listis one of Python’s three basic sequence types, alongsidetupleandrange.- Lists are mutable and are typically used to hold collections whose elements have broadly similar roles.
- Use
[]to create an empty list. - List literals place comma-separated values inside square brackets.
- List comprehensions provide a compact construction form that iterates over another iterable.
- The constructor supports both an empty list and a list populated from an iterable.
- Construction preserves each input item and its iteration order.
Common mistakes
- In-place mutating methods such as
append,extend,sort, andreversereturnNone, not the list. - Constructing from an iterable copies its references into a new list in iteration order; it does not deep-copy the contained objects.
Example: Construct and mutate a list
numbers = list(range(1, 4))
numbers.append(4)
print(numbers) # [1, 2, 3, 4]Creates a list from an iterable, then mutates it by appending one item.
When to use it
Build computed collections
Collect values produced by a comprehension or loop for later processing.
Manage mutable sequences
Maintain an ordered, mutable collection whose contents change over time.
Interview questions
QWhat does the list constructor do?
list(iterable) accepts an iterable and creates a new list containing the iterable’s items in the same order. The iterable can be a sequence, another container that supports iteration, or an iterator object.
Follow-up: What happens with an iterator? It is consumed as the list is built, so later iteration begins wherever that iterator was left.
QWhy choose a list instead of a tuple?
A list is a mutable sequence, so its elements can be changed after creation. Lists are commonly used for collections whose items are generally homogeneous, although Python does not enforce a single element type.
Follow-up: How would you prevent accidental mutation? Use a tuple for a fixed sequence, or expose an immutable interface where appropriate.
QHow can a list be created?
Lists support square-bracket literals, list comprehensions, and the list() or list(iterable) constructor. An empty list can be written as [], while items in a literal are separated by commas.
Follow-up: When is a comprehension preferable? Use it when transforming or filtering an iterable concisely; use list(iterable) when direct materialization is all that is needed.
Related
tuplerangedictsetcollections