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Python 3 reference · Built-in Functions

Python frozendict() Function

Create an immutable, hashable dictionary (new in Python 3.15). It can be a dict key or set member and can't be changed.

Python 3 Since 3.15

Syntax

frozendict(**kwargs)
frozendict(mapping, **kwargs)
frozendict(iterable, **kwargs)

Parameters

ParameterDescription
mapping / iterable optional Another mapping, or an iterable of (key, value) pairs.
**kwargs optional Extra keys as keyword arguments.

Return value

A new frozendict.

Examples

Read-only configuration
config = frozendict(host="localhost", port=8080)
print(config["port"], len(config))
try:
    config["port"] = 9000
except TypeError as e:
    print("TypeError:", e)

Output

8080 2
TypeError: 'frozendict' object does not support item assignment

Run this on your computer — see “Get Started” for setup.

Use it as a dictionary key
cache = {}
key = frozendict(user=1, page=2)
cache[key] = "rendered page"
print(cache[frozendict(page=2, user=1)])  # order doesn't matter

Output

rendered page

Run this on your computer — see “Get Started” for setup.

Combine without changing the original
base = frozendict(a=1)
print(base | {"b": 2})  # creates a new frozendict
print(isinstance(base, dict))

Output

frozendict({'a': 1, 'b': 2})
False

Run this on your computer — see “Get Started” for setup.

Good to know

  • frozendict is not a subclass of dict, so isinstance(x, dict) is False. Check for collections.abc.Mapping to accept both.
  • It is hashable only if all its values are hashable.
  • On Python 3.14 and older, types.MappingProxyType(dict(...)) gives a read-only view, but it isn't hashable.

Version history

VersionChange
Python 3.15Added
  • dict() · Create a dictionary: a mapping of keys to values. Build it from keyword arguments, pairs or another mapping.
  • frozenset() · Create an immutable set: unique items that can't be added or removed, so it is hashable and can be a dict key.
  • hash() · Return the hash value of an object: an integer used by dicts and sets to find keys quickly. Only immutable values are hashable.
  • tuple() · Create a tuple: an ordered, immutable sequence. Tuples are hashable when their items are, so they can be dict keys.
  • Python Dictionaries · tutorial