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

Jelle Zijlstra jelle.zijlstra at gmail.com
Thu Jan 25 18:17:37 EST 2018

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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