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

Mark Shannon mark at hotpy.org
Sun Sep 4 07:31:26 EDT 2016

On 02/09/16 20:33, Guido van Rossum wrote:

On Fri, Sep 2, 2016 at 10:47 AM, Steve Dower <steve.dower at python.org> 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. But that's not what type comments mean! They don't annotate the expression. They annotate the variable. The text in PEP 484 that introduces them is clear about this (it never mentions expressions, only variables).

In PEP 484, the section on type comments says: (Quoting verbatim) """ No first-class syntax support for explicitly marking variables as being of a specific type is added by this PEP. To help with type inference in complex cases, a comment of the following format may be used... """

Some mentions of the type of a variable are made in other places in the PEP, but those were all added after I had approved the PEP.

In other words PEP 484 specifically states that annotations are to help with type inference. As defined in PEP 526, I think that type annotations become a hindrance to type inference.

Cheers, Mark.

Personally, I prefer expression annotations over variable annotations, as there are many other languages I'd prefer if variable have fixed types (e.g. C++, where I actually enjoy doing horrible things with implicit casting ;) ).

Variable annotations appear to be inherently restrictive, so either we need serious clarification as to why they are not, or they actually are and we ought to be more sure that it's the direction we want the language to go. At runtime the variable annotations are ignored. And a type checker will only ask for them when it cannot infer the type. So I think we'll be fine.

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