On Thu, 7 May 2020 at 08:54, Greg Ewing <greg.ew...@canterbury.ac.nz> wrote: > > On 7/05/20 1:07 pm, David Mertz wrote: > > For *most* functions, the substitution > > principle is fine in Python. A whole lot of the time, numeric functions > > can take either an int or a float that are equal to each other and > > produce results that are equal to each other. > > It's not much use for deciding whether two things *should* be > equal, though, because whatever your opinion on the matter, > you can come up with a set of functions that satisfy it and > then say "those are the kinds of functions I mean". > > Also, as a definition of equality it seems somewhat circular, > since if you're not sure whether x == y, you may be equally > uncertain whether f(x) == f(y) for some f, x, y.
It's not so much a definition of equality as a consistency requirement. The contrapositive can be very clear: if you already know that f(x) and f(y) do different things or return unequal objects then the question of whether x == y is answered. It's important though that it's not just about equality of return types: when you carry the principle over from maths to programming then you need to consider non-pure functions, IO, exceptions being raised etc. In simple situations it is nice to be able to duck-type over lists and tuples but in practice it has to be done carefully by sticking to the sequence or iterable interfaces precisely or by coercing to a known type at the entry points of your code. Once you have a large codebase with lots of objects flying around internally and you no longer know whether anything is a list or a tuple (or a set...) any more it's just a mess. Oscar _______________________________________________ Python-ideas mailing list -- python-ideas@python.org To unsubscribe send an email to python-ideas-le...@python.org https://mail.python.org/mailman3/lists/python-ideas.python.org/ Message archived at https://mail.python.org/archives/list/python-ideas@python.org/message/S3U4VNQERUGNWNOMGA3LBKXWBNBBVUVZ/ Code of Conduct: http://python.org/psf/codeofconduct/
