inspection

`key in dict`

Membership test invoking __contains__ — returns True if key exists in the dictionary. The check is against keys only (never values) and runs in O(1) average time.

Syntax

key in d   /   key not in d

Parameters

Parameter	Purpose
in	True when the key is present
not in	True when the key is absent

Examples

>>> d = {'a': 1, 'b': 2}
>>> 'a' in d
True
>>> 'z' in d
False

Tests key presence, not value

>>> d = {'a': 1}
>>> 1 in d
False
>>> 1 in d.values()
True

Use .values() to test for a value (O(n) scan)

>>> d = {'a': 1}
>>> 'x' not in d
True

'not in' is the idiomatic negative form

>>> d = {'a': None}
>>> 'a' in d
True
>>> d.get('a') is None
True

Prefer 'in' over d.get(k) is not None when the value might legitimately be None

Gotcha

'in' checks keys only — 'v in d' with a value returns False unless a key equals it. Prefer 'k in d' over 'k in d.keys()' — same result, slightly faster and more idiomatic.

Related methods

dict.get

Returns the value for key if key is in the dictionary, else default. If default is not given, it defaults to None so this method never raises a KeyError.

dict.keys

Returns a dynamic view object of the dictionary's keys. The view reflects any subsequent changes made to the dictionary and supports set-like operations.

len(dict)

Built-in that invokes __len__ to return the number of key-value pairs in the dictionary. Runs in O(1) — the count is maintained internally, not recomputed.

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