On Wednesday, February 5, 2020 at 7:41:13 PM UTC+1, Dieter Maurer wrote: > Stefan Halfpap wrote at 2020-2-5 14:57 +0100: > >I do not understand the following statement from the python (2 and 3) > >documentation regarding __hash__ and __eq__ methods: > >"If a class does not define an __eq__() > ><https://docs.python.org/3/reference/datamodel.html#object.__eq__> method it > >should not define a __hash__() > ><https://docs.python.org/3/reference/datamodel.html#object.__hash__> > >operation either;” > >(see https://docs.python.org/3/reference/datamodel.html#object.__hash__ > ><https://docs.python.org/3/reference/datamodel.html#object.__hash__> ) > > > >I thought it relates to the second part (“if it defines __eq__() > ><https://docs.python.org/3/reference/datamodel.html#object.__eq__> but not > >__hash__() > ><https://docs.python.org/3/reference/datamodel.html#object.__hash__>, its > >instances will not be usable as items in hashable collections”), which is > >totally clear to me. > >But then the implication should be the other way around. > > "if not A then not B" is equivalent to "if B then A". > > In your case: "__eq__ not defined, then __hash__ not defined" > is equivalent to "__hash__ definied requires __eq__ defined".
You are right. This is what the (first part of the) documentation says, but I do not know why. I thought it should be: "if __hash__ is not defined, then __eq__ should not be defined" which is equivalent to "__eq__ defined then __hash__ should be defined" (the second part, which is clear to me) If not, why should a class not define __hash__, if it does not define __eq__? -- https://mail.python.org/mailman/listinfo/python-list
