PEP 526 has this in the "Rejected/Postponed Proposals" section: - **Allow annotations in** ``with`` **and** ``for`` **statement:** This was rejected because in ``for`` it would make it hard to spot the actual iterable, and in ``with`` it would confuse the CPython's LL(1) parser.
On Thu, Jan 25, 2018 at 3:17 PM, Jelle Zijlstra <jelle.zijls...@gmail.com> wrote: > > > 2018-01-25 15:00 GMT-08:00 Joe Jevnik via Python-Dev < > python-dev@python.org>: > >> Currently there are many ways to introduce variables in Python; however, >> only a few allow annotations. I was working on a toy language and chose to >> base my syntax on Python's when I noticed that I could not annotate a loop >> iteration variable. For example: >> >> for x: int in range(5): >> ... >> >> This led me to search for other places where new variables are introduced >> and I noticed that the `as` target of a context manager cannot have an >> annotation. In the case of a context manager, it would probably need >> parenthesis to avoid ambiguity with a single-line with statement, for >> example: >> >> with ctx as (variable: annotation): body >> >> Finally, you cannot annotate individual members of a destructuring >> assignment like: >> >> a: int, b: int, c: int = 1, 2, 3 >> >> Looking at the grammar, these appear to be `expr` or `exprlist` targets. >> One change may be to allow arbitrary expressions to have an annotation . >> This would be a small change to the grammar but would potentially have a >> large effect on the language or static analysis tools. >> >> I am posting on the mailing list to see if this is a real problem, and if >> so, is it worth investing any time to address it. I would be happy to >> attempt to fix this, but I don't want to start if people don't want the >> change. Also, I apologize if this should have gone to python-idea; this >> feels somewhere between a bug report and implementation question more than >> a new feature so I wasn't sure which list would be more appropriate. >> > I have written a fair amount of code with variable annotations, and I > don't remember ever wanting to add annotations in any of the three contexts > you mention. In practice, variable annotations are usually needed for > class/instance variables and for variables whose type the type checker > can't infer. The types of loop iteration variables and context manager > assignment targets can almost always be inferred trivially. > > >> >> _______________________________________________ >> Python-Dev mailing list >> Python-Dev@python.org >> https://mail.python.org/mailman/listinfo/python-dev >> Unsubscribe: https://mail.python.org/mailman/options/python-dev/jelle. >> zijlstra%40gmail.com >> >> > > _______________________________________________ > Python-Dev mailing list > Python-Dev@python.org > https://mail.python.org/mailman/listinfo/python-dev > Unsubscribe: https://mail.python.org/mailman/options/python-dev/ > guido%40python.org > > -- --Guido van Rossum (python.org/~guido)
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