[Python-Dev] Please reject or postpone PEP 526 (original) (raw)

Mark Shannon mark at hotpy.org
Sun Sep 4 07:30:18 EDT 2016

On 04/09/16 11:57, Chris Angelico wrote:

On Sun, Sep 4, 2016 at 8:52 PM, Mark Shannon <mark at hotpy.org> wrote:

The key difference is in placement. PEP 484 style variable = value # annotation

Which reads to me as if the annotation refers to the value. PEP 526 variable: annotation = value Which reads very much as if the annotation refers to the variable. That is a change in terms of semantics and a change for the worse, in terms of expressibility. So what you have is actually a change in implication (since the PEP doesn't stipulate semantics); and the old way (the comment) implies something contrary to the semantics of at least one of the type checkers that uses it (MyPy). Do we really want to make a major, irrevocable change to the language just because MyPy does something? MyPy is very far from complete (it doesn't even support Optional types yet).

Are there any current type checkers that

actually do associate that with the value? That is, to have "variable = func() # str" indicate the same type check as "def func() -> str"? If not, this is a strong argument in favour of the PEP, since it would synchronize the syntax with the current best-of-breed checkers. I believe pytype uses value, rather the variable, tracking. It is thus more precise. Of course, it is more of an inferencer than a checker. We (semmle.com) do precise value tracking to infer a lot of "type" errors in unannotated code (as the vast majority of Python code is).

It would be a real shame if PEP 526 mandates against checkers doing as good as job as possible. Forcing all uses of a variable to have the same type is a major and, IMO crippling, limitation.

E.g. def foo(x:Optional[int])->int: if x is None: return -1 return x + 1

If the type of the variable 'x' is Optional[int] then 'return x + 1' doesn't type check. If the type of the parameter 'x' is Optional[int] then a checker can readily verify the above code.

I want a checker to check my code and, with minimal annotations, give me confidence that my code is correct.

Cheers, Mark.

ChrisA

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