[Python-ideas] Re: [Python-Dev] What's up with assignment expression and tuples?

Hello, On Sun, 7 Feb 2021 13:10:55 -0800 Guido van Rossum 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 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:pmiscml@gmail.com _______________________________________________ Python-Dev mailing list -- python-dev@python.org To unsubscribe send an email to python-dev-leave@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/6IFDG7PA... 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-c...

-- Best regards, Paul mailto:pmiscml@gmail.com