[Python-Dev] Add a frozendict builtin type (original) (raw)

M.-A. Lemburg mal at egenix.com
Tue Feb 28 13:44:20 CET 2012

Victor Stinner wrote:

See also the PEP 351. I read the PEP and the email explaining why it was rejected. Just to be clear: the PEP 351 tries to freeze an object, try to convert a mutable or immutable object to an immutable object. Whereas my frozendict proposition doesn't convert anything: it just raises a TypeError if you use a mutable key or value. For example, frozendict({'list': ['a', 'b', 'c']}) doesn't create frozendict({'list': ('a', 'b', 'c')}) but raises a TypeError.

I fail to see the use case you're trying to address with this kind of frozendict().

The purpose of frozenset() is to be able to use a set as dictionary key (and to some extent allow for optimizations and safe iteration). Your implementation can be used as dictionary key as well, but why would you want to do that in the first place ?

If you're thinking about disallowing changes to the dictionary structure, e.g. in order to safely iterate over its keys or items, "freezing" the keys is enough.

Requiring the value objects not to change is too much of a restriction to make the type useful in practice, IMHO.

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