[Python-Dev] inconsistency in annotated assigned targets (original) (raw)

Joe Jevnik jjevnik at quantopian.com
Thu Jan 25 22:20:42 EST 2018

Thank you for the clarification! I should have looked through the PEPs first.

On Thu, Jan 25, 2018 at 10:14 PM, Guido van Rossum <guido at python.org> wrote:

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.zijlstra at gmail.com> wrote:

2018-01-25 15:00 GMT-08:00 Joe Jevnik via Python-Dev <_ _python-dev at 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.

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