[Python-Dev] PEP 572 semantics

Guido van Rossum guido at python.org
Wed Jul 4 11:23:39 EDT 2018

Thanks for thinking about the details! I want to answer all of these but
right now I have some social obligations so it may be a few days. I expect
the outcome of this investigation to result in an improved draft for PEP

On Wed, Jul 4, 2018 at 7:29 AM Steve Dower <steve.dower at python.org> wrote:

> Now that it's a done deal, I am closely reviewing the semantics section
> of PEP 572. (I had expected one more posting of the final PEP, but it
> seems the acceptance came somewhere in a thread that was already muted.)
> Since there has been no final posting that I'm aware of, I'm referring
> to https://www.python.org/dev/peps/pep-0572/ as of about an hour before
> posting this (hopefully it doesn't take me that long).
> To be clear, I am *only* looking at the "Syntax and semantics" section.
> So if something has been written down elsewhere in the PEP, please take
> my questions as a request to have it referenced from this section. I
> also gave up on the discussion by the third python-dev thread - if there
> were things decided that you think I'm stupid for not knowing, it
> probably means they never made it into the PEP.
> = Syntax and Semantics
> Could we include the changes necessary to
> https://docs.python.org/3/reference/grammar.html in order to specify
> where these expressions are valid? And ideally check that they work.
> This may expose new exceptional cases, but will also clarify some of the
> existing ones, especially for those of us who write Python parsers.
> == Exceptional cases
> Are the cases in the "Exceptional cases" section supposed to raise
> SyntaxError on compilation? That seems obvious, but no harm in stating
> it. (FWIW, I'd vote to ban the "bad" cases in style guides or by forcing
> parentheses, rather than syntactically. And for anyone who wonders why
> that's different from my position on slashes in f-strings, it's because
> I don't think we can ever resolve these cases but I hope that one day we
> can fix f-string slashes :) )
> == Scope of the target
> The PEP uses the phrase "an assignment expression occurs in a
> comprehension" - what does this mean? Does it occur when/where it is
> compiled, instantiated, or executed? This is important because where it
> occurs determines which scope will be modified. For sanity sake, I want
> to assume that it means compiled, but now what happens when that scope
> is gone?
> >>> def f():
> ...     return (a := i for i in range(5))
> ...
> >>> list(f())
> [0, 1, 2, 3, 4]   # or a new error because the scope has gone?
> >>> a
> ???
> I'll push back real hard on doing the assignment in the scope where the
> generator is executed:
> >>> def do_secure_op(name, numbers):
> ...     authorised = check_authorised(name)
> ...     if not all(numbers):
> ...         raise ValueError()
> ...     if not authorised:
> ...         raise SecurityError()
> ...     print('You made it!')
> ...
> >>> do_secure_op('whatever', (authorised := i for i in [1, 2, 3]))
> You made it!
> >>> authorised
> NameError: name 'authorised' is undefined
> From the any()/all() examples, it seems clear that the target scope for
> the assignment has to be referenced from the generator scope (but not
> for other comprehension types, which can simply do one transfer of the
> assigned name after fully evaluating all the contents). Will this
> reference keep the frame object alive for as long as the generator
> exists? Can it be a weak reference? Are assignments just going to be
> silently ignored when the frame they should assign to is gone? I'd like
> to see these clarified in the main text.
> When an assignment is "expressly invalid" due to avoiding "edge cases",
> does this mean we should raise a SyntaxError? Or a runtime error? I'm
> not sure how easily these can be detected by our current compiler (or
> runtime, for that matter), but in the other tools that I work on it
> isn't going to be a trivial check.
> Also, I'm not clear at all on why [i := i+1 for i in range(5)] is a
> problem? Similarly for the other examples here. There's nothing wrong
> with `for i in range(5): i = i+1`, so why forbid this?
> == Relative precedence
> "may be used directly in a positional function call argument" - why not
> use the same syntax as generator expressions? Require parentheses unless
> it's the only argument. It seems like that's still got a TODO on it from
> one of the examples, so consider this a vote for matching
> generator-as-argument syntax.
> == Differences between assignment expressions
> I'm pretty sure the equivalent of "x = y = z = 0" would be "z := (y :=
> (x := 0))". Not that it matters when there are no side-effects of
> assignment (unless we decide to raise at runtime for invalid
> assignments), but it could become a point of confusion for people in the
> future to see it listed like this. Assignment expressions always
> evaluate from innermost to outermost.
> Gramatically, "Single assignment targets *other than* NAME are not
> supported" would be more precise. And for specification's sake, does
> "not supported" mean "is a syntax error"?
> The "equivalent needs extra parentheses" examples add two sets of extra
> parentheses. Are both required? Or just the innermost set?
> ---
> Apologies for the lack of context. I've gone back and added the section
> headings for as I read through this section.
> Cheers,
> Steve
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--Guido van Rossum (python.org/~guido)
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