On Sat, Jul 11, 2020 at 5:58 PM Chris Angelico <ros...@gmail.com> wrote:
> On Sun, Jul 12, 2020 at 10:30 AM Greg Ewing <greg.ew...@canterbury.ac.nz> > wrote: > > > > Just had another thought about marking assignment targets. > > > > The PEP currently forbids repeating bound names in a pattern > > to avoid raising expectations that > > > > case Point(x, x): > > > > would match only if the two arguments were equal. > > > > But if assignment targets were marked, we could write this as > > > > case Point(?x, x): > > > > and it would work as expected. > > Hang on. Matching happens before assignment, so this should use the > previous value of x for the matching. At least, that's my > understanding. If you do something like: > > case Point(x, 2): > > it won't assign x unless the second coordinate is 2, right? > Good catch. That's actually undefined -- we want to let the optimizer have some leeway in how to generate the best code for matching. See https://www.python.org/dev/peps/pep-0622/#performance-considerations Currently it doesn't optimize all that much -- it just processes patterns from left to right: ``` >>> match Point(3, 3): ... case Point(x, 42): pass ... >>> print(x) 3 >>> ``` -- --Guido van Rossum (python.org/~guido) *Pronouns: he/him **(why is my pronoun here?)* <http://feministing.com/2015/02/03/how-using-they-as-a-singular-pronoun-can-change-the-world/>
_______________________________________________ Python-Dev mailing list -- python-dev@python.org To unsubscribe send an email to python-dev-le...@python.org https://mail.python.org/mailman3/lists/python-dev.python.org/ Message archived at https://mail.python.org/archives/list/python-dev@python.org/message/7CZXK6ECT6MKLEUTMRFXFCF54RZ4G72H/ Code of Conduct: http://python.org/psf/codeofconduct/
