add-remove

`set.pop`

Removes and returns an arbitrary element from the set. Raises KeyError if the set is empty.

Syntax

s.pop()

Parameters

Parameter	Purpose
(no args)	pop() takes no arguments — the element chosen is implementation-defined
returns	The removed element

Examples

>>> s = {10, 20, 30}
>>> x = s.pop()
>>> x in {10, 20, 30}
True

Returns some element; which one is not guaranteed.

>>> s = {'only'}
>>> s.pop()
'only'
>>> s
set()

On a one-element set the result is deterministic.

>>> s = set()
>>> s.pop()
KeyError: 'pop from an empty set'

Empty set raises KeyError.

>>> s = {1, 2, 3}
>>> while s: s.pop()
>>> s
set()

Common pattern to drain a set.

Gotcha

pop returns an ARBITRARY element — NOT the last inserted or smallest. Do not rely on which element you get; iteration order in CPython is not part of the language spec.

Related methods

set.remove

Removes the specified element from the set. Raises KeyError if the element is not present.

set.discard

Removes an element from the set if it is present, and does nothing otherwise. Unlike remove(), never raises an error.

set.clear

Removes all elements from the set, leaving it empty. Mutates the set in place; other references to the same set see it emptied.

set.add

Adds a single hashable element to the set in place. If the element is already present, the set is unchanged (no error).

set.update

Adds every element from each iterable argument to s in place. Equivalent to a union but mutates s instead of returning a new set.

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