merge

`dict.fromkeys`

Class method that builds a new dict whose keys come from iterable and whose values are all set to value (default None). The same value object is shared across every key — beware mutable defaults.

Syntax

dict.fromkeys(iterable[, value])

Parameters

Parameter	Purpose
iterable	Any iterable providing the keys
value	Value assigned to every key (defaults to None)

Examples

>>> dict.fromkeys(['a', 'b', 'c'])
{'a': None, 'b': None, 'c': None}

Default value is None

>>> dict.fromkeys(range(3), 0)
{0: 0, 1: 0, 2: 0}

Initialize a counter-style dict

>>> d = dict.fromkeys(['a', 'b'], [])
>>> d['a'].append(1)
>>> d
{'a': [1], 'b': [1]}

GOTCHA — the SAME list is shared by every key

>>> dict.fromkeys('abca')
{'a': None, 'b': None, 'c': None}

Duplicate keys in the iterable collapse to one entry

Gotcha

Mutable defaults (list, dict, set) are shared across all keys — use a dict comprehension {k: [] for k in keys} instead. Duplicate keys in the iterable silently deduplicate.

Related methods

dict.setdefault

If key is in the dictionary, returns its value. If not, inserts key with a value of default (None if omitted) and returns default.

dict.copy

Returns a shallow copy of the dictionary — a new dict object with the same key-value pairs. Values themselves are not copied, so nested mutable objects are still shared between the original and the copy.

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.

dict1 | dict2

Merge operator (Python 3.9+) that returns a new dict containing the entries of d1 followed by d2, with d2's values winning on shared keys. Both operands must be dicts; neither is mutated.

dict1 |= dict2

In-place merge operator (Python 3.9+) that updates d1 with entries from other, with other's values winning on shared keys. Unlike |, the right operand can be any iterable of pairs or a mapping — matching update()'s flexibility.

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