This was intentional in PEP 572 so it is not a grammar bug fix. Put your money where your mouth is, or become another armchair language designer. Your choice.
On Sun, Feb 7, 2021 at 23:58 Paul Sokolovsky <pmis...@gmail.com> wrote: > Hello, > > On Sun, 7 Feb 2021 13:10:55 -0800 > Guido van Rossum <gu...@python.org> wrote: > > > Hi Paul, > > > > I suggest that you just go straight to the PEP phase. > > Thanks. In all fairness, I don't expect immediate resolution to this > issue. But I'm aware of out for at least a year, and keep returning to > it (yes, in context of my SSA experiments). So, I proceeded to the next > stage - bring it up on its own, then created a bug ticket: > https://bugs.python.org/issue43143 > > And my idea is to try to argue that it would be just a "grammar > bugfix", similarly to already existing grammar elaborations for walrus: > > https://bugs.python.org/issue42316 > https://bugs.python.org/issue42374 > https://bugs.python.org/issue42381 > > Granted, allowing "foo((a, b) := (b, a))" is a bit bigger change than > allowing "foo[a := b]" instead of "foo[(a := b)]". But if people > find assigning within index expression useful, up to reporting it, and > then other people ack it and fix it, then why not fix parallel > assignment case? Implementation-wise they *seem* to be of the > similar effort/complexity - just a one-term grammar change. (I still > need to run the testsuite, yeah). > > > > > > --Guido > > > > On Thu, Feb 4, 2021 at 11:54 PM Paul Sokolovsky <pmis...@gmail.com> > > wrote: > > > > > Hello, > > > > > > Everyone knows how hard to find a compelling usecase for the > > > assignment expression operator (":=", colloquially "walrus > > > operator"). https://www.python.org/dev/peps/pep-0572/ examples > > > never felt compelling and we all remember the split it caused. > > > > > > I finally found a usecase where *not* using assignment expression is > > > *much* worse than using it. I'm working on SSA (Static Single > > > Assignment, > > > https://en.wikipedia.org/wiki/Static_single_assignment_form) > > > conversion of Python programs, where there's a need to "join" > > > dataflow of values from different control flow paths using a > > > special function (Phi function). This "joining" itself creates a > > > new variable, and of course, the original variable was used in an > > > expression. We've got assignment in expression, assignment > > > expression operator to the rescue! > > > > > > With it, a simple loop like: > > > > > > ---- > > > a = 0 > > > while a < 5: > > > a += 1 > > > ---- > > > > > > becomes: > > > > > > ---- > > > a0 = 0 > > > while (a1 := phi(a0, a2)) < 5: > > > a2 = a1 + 1 > > > ---- > > > > > > So far, so good. But semantics of Phi function is parallel > > > assignment. No problem with Python either, "a, b = b, c" is exactly > > > parallel assignment. So, let's try example with 2 variables: > > > > > > ---- > > > a = 0 > > > b = 10 > > > while a < 5: > > > a += 1 > > > b += 1 > > > ---- > > > > > > becomes: > > > > > > ---- > > > a0 = 0 > > > b0 = 10 > > > while ((a1, b1) := phi([a0, a2], [b0, b2]))[0] < 5: > > > a2 = a1 + 1 > > > b2 = b1 + 1 > > > ---- > > > > > > But oops: > > > > > > > SyntaxError: cannot use assignment expressions with tuple > > > > > > To reproduce in the REPL: > > > > > > ---- > > > >>> ((a, b) := (1, 2)) > > > File "<stdin>", line 1 > > > SyntaxError: cannot use assignment expressions with tuple > > > ---- > > > > > > Why this accidental syntactic gap? Why assignment statement can do > > > parallel assignment with a tuple on LHS, and assignment operator > > > suddenly can't? > > > > > > Why the adhoc naming and conceptual shift on the AST level, when > > > PEP572 explicitly talks about *assignment operator*, but > > > corresponding node on the AST level is called NamedExpr? Why look > > > at assignment expression as "name of expression" instead of > > > assignment expression per se? > > > > > > It's of course not a problem to recast: > > > > > > NamedExpr(expr target, expr value) > > > > > > to > > > > > > NamedExpr(expr* target, expr value) > > > > > > in the ASDL (and it works out of the box), the point is that it > > > should have been ExprAssign from the start (folloing the AugAssign > > > and AnnAssign tradition). > > > > > > > > > -- > > > Best regards, > > > Paul mailto:pmis...@gmail.com > > > _______________________________________________ > > > 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/6IFDG7PAFPHVPGULANOQDAHP2X743HCE/ > > > Code of Conduct: http://python.org/psf/codeofconduct/ > > > > > > > > > -- > > --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/ > > > > > > -- > Best regards, > Paul mailto:pmis...@gmail.com > -- --Guido (mobile)
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