[Python-Dev] Another case for frozendict (original) (raw)

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.update(kw)
        else:
            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.

David

More information about the Python-Dev mailing list