mutation

`dict[key] = value`

Assigns value to key using bracket subscription syntax, invoking __setitem__ under the hood. Creates the entry if key is absent, or overwrites the existing value; preserves the original insertion order when the key already exists.

Syntax

d[key] = value

Parameters

Parameter	Purpose
key	Any hashable value (str, int, tuple of hashables, ...)
value	Any Python object — no restriction

Examples

>>> d = {}
>>> d['a'] = 1
>>> d
{'a': 1}

Assignment creates the key

>>> d = {'a': 1, 'b': 2}
>>> d['a'] = 99
>>> d
{'a': 99, 'b': 2}

Overwrites without changing insertion position

>>> d = {}
>>> d[[1,2]] = 'x'
Traceback (most recent call last):
  ...
TypeError: unhashable type: 'list'

Keys must be hashable — lists and dicts are not

>>> d = {}
>>> d[(1, 2)] = 'point'
>>> d
{(1, 2): 'point'}

Tuples of hashables are valid keys

Gotcha

Keys must be hashable (immutable) — using a list, dict, or set as a key raises TypeError. Overwriting an existing key preserves its original insertion order; new keys go to the end.

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.update

Updates the dictionary in place with key/value pairs from other, overwriting existing keys. Accepts another mapping, an iterable of key/value pairs, and/or keyword arguments; returns None.

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.

dict.pop

Removes the specified key and returns its value. If the key is missing, returns default if provided; otherwise raises KeyError.

dict.popitem

Removes and returns the last inserted (key, value) pair as a 2-tuple in LIFO order. Raises KeyError if the dictionary is empty.

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