[Python-Dev] Another case for frozendict

dw+python-dev at hmmz.org dw+python-dev at hmmz.org
Sun Jul 13 20:43:28 CEST 2014

On Sun, Jul 13, 2014 at 02:04:17PM +0000, Jason R. Coombs wrote:

> PEP-416 mentions a MappingProxyType, but that’s no help.

Well, it kindof is. By combining MappingProxyType and UserDict the
desired effect can be achieved concisely:

    import collections
    import types

    class frozendict(collections.UserDict):
        def __init__(self, d, **kw):
            if d:
                d = d.copy()
                d = kw
            self.data = types.MappingProxyType(d)

        _h = None
        def __hash__(self):
            if self._h is None:
                self._h = sum(map(hash, self.data.items()))
            return self._h

        def __repr__(self):
            return repr(dict(self))

> Although hashability is mentioned in the PEP under constraints, there are many
> use-cases that fall out of the ability to hash a dict, such as the one
> described above, which are not mentioned at all in use-cases for the PEP.

> If there’s ever any interest in reviving that PEP, I’m in favor of its
> implementation.

In its previous form, the PEP seemed more focused on some false
optimization capabilities of a read-only type, rather than as here, the
far more interesting hashability properties. It might warrant a fresh
PEP to more thoroughly investigate this angle.


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