add

`list.extend`

Extends the list by appending every element from an iterable, in order. Mutates in place and returns None; equivalent to `list += iterable`.

Syntax

list.extend(iterable)

Parameters

Parameter	Purpose
iterable	Any iterable (list, tuple, set, str, generator, range) whose elements are appended one by one.
str argument	A string is iterable — extend('ab') adds 'a' then 'b', not the whole string. Often a bug source.

Examples

>>> lst = [1, 2]
>>> lst.extend([3, 4])
>>> lst
[1, 2, 3, 4]

Adds each element of the iterable individually.

>>> lst = [1, 2]
>>> lst.extend('ab')
>>> lst
[1, 2, 'a', 'b']

Strings iterate as characters — a common gotcha.

>>> lst = []
>>> lst.extend(range(3))
>>> lst
[0, 1, 2]

Any iterable works — including generators and range.

>>> lst = [1]
>>> lst.extend(5)
TypeError: 'int' object is not iterable

Non-iterable args raise TypeError — use append for a single value.

Gotcha

extend(non_iterable) raises TypeError; extend('abc') iterates the string into characters. Returns None — do not chain or assign. `a += b` is the operator form.

Related methods

list.append

Appends a single item x to the end of the list, mutating it in place. Always returns None — it does not return the modified list.

list.insert

Inserts item x before index i, shifting subsequent elements right. Mutates in place, returns None; O(n) because it shifts trailing elements.

list.copy

Returns a shallow copy of the list — a new list containing the same element references. Equivalent to lst[:] or list(lst); does not recursively copy nested mutables.

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