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

Steven D'Aprano steve at pearwood.info
Fri Sep 2 14:19:13 EDT 2016

On Fri, Sep 02, 2016 at 10:47:41AM -0700, Steve Dower wrote:

"I'm not seeing what distinction you think you are making here. What distinction do you see between:

x: int = func(value) and x = func(value) #type: int" Not sure whether I agree with Mark on this particular point, but the difference I see here is that the first describes what types x may ever contain, while the latter describes what type of being assigned to x right here. So one is a variable annotation while the other is an expression annotation.

Ultimately Python is a dynamically typed language, and that's not changing. This means types are fundamentally associated with values, not variables (names). But in practice, you can go a long way by pretending that it is the variable that carries the type. That's the point of the static type checker: if you see that x holds an int here, then assume (unless told differently) that x should always be an int. Because in practice, most exceptions to that are due to bugs, or at least sloppy code.

Of course, it is up to the type checker to decide how strict it wants to be, whether to treat violations as a warning or a error, whether to offer the user a flag to set the behaviour, etc. None of this is relevant to the PEP. The PEP only specifies the syntax, leaving enforcement or non-enforcement to the checker, and it says:

PEP 484 introduced type hints, a.k.a. type annotations. While its 
main focus was function annotations, it also introduced the notion
of type comments to annotate VARIABLES [emphasis added]

not expressions. And:

This PEP aims at adding syntax to Python for annotating the types
of variables and attributes, instead of expressing them through
comments

which to me obviously implies that the two ways (type comment, and variable type hint) are intended to be absolutely identical in semantics, at least as far as the type-checker is concerned.

(They will have different semantics at runtime: the comment is just a comment, while the type hint will set an annotations mapping.)

But perhaps the PEP needs to make it explicit that they are to be treated exactly the same.

-- Steve

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