Python Forum

we have a couple immutable types. why no immutable dictionaries? yeah, yeah we have named tuples. but those are awkward to create. it should be as simple as creating a literal dictionary.

There was a rejected PEP 416 to create a frozendict type. You can find there the arguments why it was rejected.

Also note the use of types.MappingProxyType() to create a read-only view of an existing dictionary.

Guido Van Rossum Wrote:On the other hand, exposing the existing read-only dict proxy as a built-in type sounds good to me. (It would need to be changed to allow calling the constructor.)

There is also a frozendict module in Pypi. Could be worth checking.

>>> import types
>>> d = types.MappingProxyType({'foo': 1, 'bar': 10, 'spam': 100})
>>> 
>>> d
mappingproxy({'foo': 1, 'bar': 10, 'spam': 100})
>>> d['bar']
10
>>> d['bar'] = 3
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'mappingproxy' object does not support item assignment

in that language i created 3 decades ago, its mapping type also served as sets. it had a way to place a key with no value (and to delete a value while keeping the key in place). every type had mutable and immutable forms. i remember when i first encountered Python, i backed away because str was strictly immutable. now i realize that every use case for a "mutable str" is better handled by a list.