A workaround for Python's missing frozen-dict

Question

This question already has answers here:

Immutable dictionary in Python 3: how to make keys(), items(), and values() dictionary views immutable (2 answers)

What would a “frozen dict” be? (14 answers)

Closed 3 years ago.

In Python, when you want to use lists as keys of some dictionary, you can turn them into tuples, which are immutable and hence are hashable.

>>> a = {}
>>> a[tuple(list_1)] = some_value
>>> a[tuple(list_2)] = some_other_value

The same happens when you want to use set objects as keys of some dictionary - you can build a frozenset, that is again immutable and hence is hashable.

>>> a = {}
>>> a[frozenset(set_1)] = some_value
>>> a[frozenset(set_2)] = some_other_value

But it seems that for dictionary there is no equivalent.

A first idea I thought about (and found it bad finally), is to use str(some_dict) as a key. But, dictionaries always use different hash functions, so strings of equal dictionaries may be different.

Is there any workaround known as a good practice, or does anyone have other ideas how to use dictionary-like objects as keys of other dictionaries?

Answer 1

I've found a nice workaround for this problem, which is building a frozenset containing the dictionary items:

>>> a = {'key1' : 'val1', 'key2' : 'val2'}
>>> b = frozenset(a.items())
>>> frozenset_restored_to_dict = dict(b)
>>> frozenset_restored_to_dict
{'key2': 'val2', 'key1': 'val1'}

As can be seen in the code, b is a frozenset, which is immutable and hashable, and can be totally restored to be a regular dictionary like a.

Answer 2

You can try ordered dict or look on these answers:

• What would a "frozen dict" be?
• Immutable dictionary, only use as a key for another dictionary
• How to create an immutable dictionary in python?

and there is even a package on PyPI: https://pypi.python.org/pypi/frozendict

You can also simply convert dict to tuples(sorted(your_dict.items())) and then use as a hash.

UPD: as mentioned in comments, OrderedDict is unhashable. My bad, it is really should not be hashable since it is mutable.