[Python-Dev] Question about PEP 484

Mon Jul 16 16:40:51 EDT 2018

As one of the authors of PEP 484, *I* never thought there was an ambiguity
here. The intention was for stub files to conform to the same grammar as
regular .py files, but with a different interpretation.

> "Have the same syntax as regular Python modules" and "are files
containing type hints" are at odds with each other.

That depends. *same syntax as regular Python* is normative while
*containing type hints* is an informal description of intent.

I happen to be somewhat familiar with the situation that lead to this
question -- pytype has its own parser for stub files that cannot parse all
Python constructs. But claiming that PEP 484 is ambiguous feels wrong, and
if we really need to clarify it the only way to go is to make "same syntax
as regular Python" more clearly normative. Type checkers should of course
feel free ignore everything they don't care about.

Regarding the unicode issue, that is indeed unfortunate, and there's a long
but inconclusive discussion at https://github.com/python/typing/issues/208.
(If you want a longer discussion here please start a new subject.)

On Mon, Jul 16, 2018 at 1:02 PM, Adam Cataldo via Python-Dev <
python-dev at python.org> wrote:

> Thanks Brett and Teddy,
>
> Just so it doesn't get lost in the shuffle as folks dive into details,
> I'll re-ask my earlier question about stub files. Assuming there is
> consensus that there is ambiguity to resolve in the current definition, is
> updating the section on stub files the preferred option? The only
> alternative I can think of is to pull this out into a separate PEP. I
> frankly have no opinion on what the best way to capture this is. We're
> happy to help out either way.
>
> On Mon, Jul 16, 2018 at 12:48 PM Teddy Sudol <tsudol at google.com> wrote:
>
>> Hi, my name is Teddy Sudol. I work with Adam and Rebecca on pytype.
>>
>> The explanation of stub files is unclear. The section you linked starts
>> with, "Stub files are files containing type hints that are only for use by
>> the type checker, not at runtime." According to
>> https://www.python.org/dev/peps/pep-0484/#acceptable-type-hints, type
>> hints may be classes, abstract base classes, types defined in the `types`
>> and `typing` modules, type variables, type aliases and None. Further in the
>> section you linked, PEP 484 also states, "Stub files have the same syntax
>> as regular Python modules," and, "no runtime behavior should be expected of
>> stub files."
>>
>> "Have the same syntax as regular Python modules" and "are files
>> containing type hints" are at odds with each other. This has led to
>> compatibility issues between Mypy and pytype. For example, `b''` is not a
>> valid type annotation, but until a month ago, `codecs.pyi` in typeshed used
>> exactly that: https://github.com/python/typeshed/commit/
>> 6bbf3d89eb9b6c3fd5b0c0f632b2ad9258cecf15#diff-
>> 5f6f48c425bc0c283784cf5277880c0cL95. If statements can be useful for
>> things like version checks, but on the other hand, pyi files aren't
>> supposed to have any runtime behavior. Additionally, codifying the syntax
>> for pyi files would settle questions like whether constants should be typed
>> using "x: <type hint>" or "x = ...  # type: <type hint>".
>>
>> We would like to see a clear statement about the syntax of stub files.
>> Personally, I think they should be a subset of Python, but I'd also be
>> happy with an EBNF grammar for them.
>>
>> -- Teddy
>>
>>
>> On Mon, Jul 16, 2018 at 11:05 AM Brett Cannon <brett at python.org> wrote:
>>
>>>
>>>
>>> On Mon, 16 Jul 2018 at 10:32 Adam Cataldo via Python-Dev <
>>> python-dev at python.org> wrote:
>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>> *Hi Folks,Cc: Rebecca, pytypeThis is Adam Cataldo; I’m the engineering
>>>> manager for the Python team at Google. Rebecca Chen, our lead pytype
>>>> <https://github.com/google/pytype> contributor, and I are interested in
>>>> helping finalize PEP 484 if possible. To that end, we wanted to find out
>>>> what technical issues the PEP 484 authors feel they still need to finalize.
>>>> We also wanted to know how we can help.We have a large Python code base at
>>>> Google, and may be able to use this to help resolve current incomplete
>>>> definitions, by collecting data on how types are used. We also have a
>>>> couple ambiguities that we’d love to get closure on: - One thing we care
>>>> about in particular, given the implementation of pytype, is the detailed
>>>> definition of what goes in a .pyi file. Do folks think this makes sense to
>>>> include as part of PEP 484, or would this be better in a separate PEP? We’d
>>>> love to get your thoughts.*
>>>>
>>>
>>> What specifically do you want beyond https://www.python.org/dev/
>>> peps/pep-0484/#stub-files?
>>>
>>>
>>>>
>>>>
>>>>
>>>> * - The relationship between unicode and typing.Text in Python 2 has
>>>> been a recurring source of confusion for our users. Especially since we
>>>> contributed <https://github.com/python/peps/pull/302> to the current state
>>>> of affairs, we’d like to figure out how to establish clarity here.Thoughts?*
>>>>
>>>
>>> Do be aware, Adam, that due to Guido's retirement last week people might
>>> be a bit preoccupied and so a little slow in responding. But then again
>>> Guido just got a bit more free time so he might chime in on this one. ;)
>>>
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>>
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-- 
--Guido van Rossum (python.org/~guido)
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