How Can You Implement Immutable, Hashable Dictionaries in Python?

Immutable, Hashable Dictionaries in Python

Frozen sets and tuples provide immutable, hashable counterparts to lists in Python. However, a similar concept for dictionaries is lacking. A "frozendict" would provide an immutable and hashable representation of a dictionary.

Implementation and Usage

Although Python does not natively offer a frozen dictionary type, it is possible to create a custom implementation using a wrapper class:

<code class="python">class FrozenDict(collections.Mapping):
    # ... (code as provided in the reference answer)</code>

Behavior and Comparison

FrozenDict instances behave similarly to regular dictionaries, supporting iteration, item access, and membership testing. However, they are immutable, meaning that once created, they cannot be modified.

Despite their immutability, FrozenDict instances can be compared for equality based on their hashable nature:

>>> x = FrozenDict(a=1, b=2)
>>> y = FrozenDict(a=1, b=2)
>>> x is y
False
>>> x == y
True

Utility

FrozenDict is particularly useful for caching and memoization, where immutable and hashable keys are required. For instance, it can be used to store a hashed version of a dictionary's values for efficient comparisons:

>>> cache = {}
>>> def memoized_function(args):
>>>     key = FrozenDict(args)
>>>     if key in cache:
>>>         return cache[key]
>>>     else:
>>>         result = ...  # Computation here
>>>         cache[key] = result
>>>         return result

PEP 603

It is worth noting that PEP 603 proposed a native frozendict type in Python, but it was withdrawn due to concerns about its potential usefulness. Nevertheless, custom implementations like FrozenDict provide a practical solution for situations where immutable, hashable dictionaries are required.

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