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

dw+python-dev at hmmz.org dw+python-dev at hmmz.org
Wed Jul 16 16:04:29 CEST 2014

On Wed, Jul 16, 2014 at 09:47:59AM -0400, R. David Murray wrote:

It would be nice to be able to return a frozendict instead of having the overhead of building a new dict on each call.

There already is an in-between available both to Python and C: PyDictProxy_New() / types.MappingProxyType. It's a one line change in each case to return a temporary intermediary, using something like (C): Py_INCREF(self->dict) return self->dict;

To return PyDictProxy_New(self->dict);

Or Python: return self.dct

To return types.MappingProxyType(self.dct)

Which is cheaper than a copy, and avoids having to audit every use of self->dict to ensure the semantics required for a "frozendict" are respected, i.e. no mutation occurs after the dict becomes visible to the user, and potentially has hash called.

That by itself might not be enough reason. But, if the user wants to use the data in modified form elsewhere, they would then have to construct a new regular dict out of it, making the decision to vary the data from what matches the state of the object it came from an explicit one. That seems to fit the Python zen ("explicit is better than implicit").

So I'm changing my mind, and do consider this a valid use case, even absent the crash.

Avoiding crashes seems a better use for a read-only proxy, rather than a hashable immutable type.

David

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