On Fri, Jan 6, 2017 at 2:07 AM Stephen J. Turnbull < turnbull.stephen...@u.tsukuba.ac.jp> wrote:
> Neil Girdhar writes: > > > I don't understand this? How is providing a default method in an > > abstract base class a pessimization? If it happens to be slower > > than the code in the current methods, it can still be overridden. > > How often will people override until it's bitten them? How many > people will not even notice until they've lost business due to slow > response? If you don't have a default, that's much more obvious. > Note that if there is a default, the collections are "large", and > equality comparisons are "rare", this could be a substantial overhead. > I still don't understand what you're talking about here. You're saying that we shouldn't provide a __hash__ in case the default hash happens to be slower than what the user wants and so by not providing it, we force the user to write a fast one? Doesn't that argument apply to all methods provided by abcs? > > > BTW, it occurs to me that now that dictionaries are versioned, in some > > > cases it *may* make sense to hash dictionaries even though they are > > > mutable, although the "hash" would need to somehow account for the > > > version changing. Seems messy but maybe someone has an idea? > > > I think it's important to keep in mind that dictionaries are not > versioned > > in Python. They happen to be versioned in CPython as an unexposed > > implementation detail. I don't think that such details should have any > > bearing on potential changes to Python. > > AFAIK the use of the hash member for equality checking is an > implementation detail too, although the language reference does > mention that set, frozenset and dict are "hashed collections". The > basic requirements on hashes are that (1) objects that compare equal > must hash to the same value, and (2) the hash bucket must not change > over an object's lifetime (this is the "messy" aspect that probably > kills the idea -- you'd need to fix up all hashed collections that contain the object as a key). > Are you saying that __hash__ is called by __eq__?
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