Question about PEP 484

[Adam Cataldo]

Hi Folks,

Cc: Rebecca, pytype

This is Adam Cataldo; I’m the engineering manager for the Python team at Google. Rebecca Chen, our lead 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.

• The relationship between unicode and typing.Text in Python 2 has been a recurring source of confusion for our users. Especially since we contributed to the current state of affairs, we’d like to figure out how to establish clarity here.

Thoughts?

Thanks,

Adam

[Brett Cannon]

On Mon, 16 Jul 2018 at 10:32 Adam Cataldo via Python-Dev <pytho...@python.org> wrote:

Hi Folks,

Cc: Rebecca, pytype

This is Adam Cataldo; I’m the engineering manager for the Python team at Google. Rebecca Chen, our lead 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 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. ;)

[Teddy Sudol]

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

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[Adam Cataldo]

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.

[Guido van Rossum]

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.)

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[Jelle Zijlstra]

2018-07-16 10:21 GMT-07:00 Adam Cataldo via Python-Dev <pytho...@python.org>:

Hi Folks,

Cc: Rebecca, pytype

This is Adam Cataldo; I’m the engineering manager for the Python team at Google. Rebecca Chen, our lead 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.

 

I would be happy to contribute to this if that would be useful for type checkers like pytype, although like Guido I don't think the current text of the PEP is ambiguous. Typeshed already has a set of lint rules that limit what can be put in stub files, but it could be useful to communicate the exact set of allowed constructs.

• The relationship between unicode and typing.Text in Python 2 has been a recurring source of confusion for our users. Especially since we contributed to the current state of affairs, we’d like to figure out how to establish clarity here.

There has already been a long discussion on the typing issue tracker that resulted in no consensus for any change to the current recommendation. Perhaps there is something we can do, but it's not clear to me what that would be. 

Thoughts?

Thanks,

Adam

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[Gregory P. Smith]

On Mon, Jul 16, 2018 at 1:44 PM Guido van Rossum <gu...@python.org> wrote:

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.

I feel like the "same syntax as regular Python" is too broad a statement.  That effectively requires a version specific Python interpreter to execute the files.  With at least four different Python static analyzers in existence today, keeping the behavior of all of them consistent is important.  Otherwise pyi files will be (are being) created that are analyzer specific and break other type checkers when distributed.

ex: We're encountering pyi files with conditional logic in them.  I believe we've encountered pyi files with del statements in them?  Both of these are a slippery slope towards being turing complete in something that isn't supposed to be code.  I don't like this.  Interface declarations should not contain logic.  If we allow conditions, we need to explicitly define what we do allow in the PEP.  (if+else and del?  what inputs are allowed for the expression in if statements?).  Otherwise at some point someone is going to create a pyi file containing loops, function calls, and generator expressions and expect it to _do_ something.  The horror!  Lets avoid that.

PEP-484 does contain the text, "This also reinforces the notion that no runtime behavior should be expected of stub files." But reinforcing a notion is not what I read as a concrete statement.

I'd rather see that say something like, "There must not be any runtime behavior from a stub file. They will be parsed for information, not executed."  Wordsmith that all you want, I'm not pedantic enough. :)

I expect someone pedantic to happily point out that a def or class or assignment to ... with an annotation is runtime behavior... technically correct, but that isn't how people like me think of them in this context.

We use a Pythonic syntax for stubs to be consistent with the language, that doesn't mean they are code.

I wrote more than I thought I would here, I'll stop now. :)

-gps

 

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 <pytho...@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 <tsu...@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 <br...@python.org> wrote:

On Mon, 16 Jul 2018 at 10:32 Adam Cataldo via Python-Dev <pytho...@python.org> wrote:

Hi Folks,

Cc: Rebecca, pytype

This is Adam Cataldo; I’m the engineering manager for the Python team at Google. Rebecca Chen, our lead 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 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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[Guido van Rossum]

On Mon, Jul 16, 2018 at 2:56 PM, Gregory P. Smith <gr...@krypto.org> wrote:

On Mon, Jul 16, 2018 at 1:44 PM Guido van Rossum <gu...@python.org> wrote:

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.

I feel like the "same syntax as regular Python" is too broad a statement.  That effectively requires a version specific Python interpreter to execute the files.  With at least four different Python static analyzers in existence today, keeping the behavior of all of them consistent is important.  Otherwise pyi files will be (are being) created that are analyzer specific and break other type checkers when distributed.

it doesn't require an interpreter, just a parser. Is saying it should be syntactically valid Python 3.6 (though emphatically not executable!) still too much?

 

ex: We're encountering pyi files with conditional logic in them.  I believe we've encountered pyi files with del statements in them?  Both of these are a slippery slope towards being turing complete in something that isn't supposed to be code.  I don't like this.  Interface declarations should not contain logic.  If we allow conditions, we need to explicitly define what we do allow in the PEP.  (if+else and del?  what inputs are allowed for the expression in if statements?).  Otherwise at some point someone is going to create a pyi file containing loops, function calls, and generator expressions and expect it to _do_ something.  The horror!  Lets avoid that.

Syntactically, conditional logic *is* part of the spec because of `sys.version_info` and `sys.platform` checks. These are mentioned in PEP 484 (though IIRC they were added at some point).

Feel free to ignore `del` statements, but they are valid syntax.

 

PEP-484 does contain the text, "This also reinforces the notion that no runtime behavior should be expected of stub files." But reinforcing a notion is not what I read as a concrete statement.

I'd rather see that say something like, "There must not be any runtime behavior from a stub file. They will be parsed for information, not executed."  Wordsmith that all you want, I'm not pedantic enough. :)

I don't see that as any more precise as what the PEP says. :-)

I'd be happy to claim that if the ast module can't parse a .pyi file, it's invalid, otherwise it's valid, except that `# type:` comments are significant and the ast module doesn't preserve those.

 

I expect someone pedantic to happily point out that a def or class or assignment to ... with an annotation is runtime behavior... technically correct, but that isn't how people like me think of them in this context.

And mypy agrees. :-)

 

We use a Pythonic syntax for stubs to be consistent with the language, that doesn't mean they are code.

Define "code". To some people HTML is code.

 

I wrote more than I thought I would here, I'll stop now. :)

It would be nice if the pytype team could articulate the problems they're trying to solve, rather than offering to "help finalize PEP 484". My guess is that their parser for .pyi files only accepts a subset of Python and they're (you're? do you report to Adam?) reluctant to sink more time in that parser -- but I don't think that's my problem. :-)

My intention for stubs was exactly what I've been saying in this thread: they must be syntactically valid Python, they are not meant to be executed, and type checkers are free to ignore things they don't need. If that's not sufficiently clear in the way the PEP is currently written, I welcome a PR to the peps repo. If the pytype team wants a different interpretation, the bar is much higher.

[Rebecca Chen]

Hi Guido and all,

Let me try to clarify our (pytype team's) intentions a bit. Our
overall goal is to see PEP 484 finalized, in the interest of having a
definitive type-checking reference to point to and work off of. We're
willing and eager to help with this, if we get some guidance on what
the authors are still trying to finalize and how we can be useful. As
a conversation starter, we offered two issues in which pytype has some
personal stake.

Best,
Rebecca

P.S. Yes, Adam manages Python at Google.

[Guido van Rossum]

Thanks, Rebecca! So the status of PEP 484 is that we don't want to add new features to it -- those require their own PEP. But we do accept PRs for clarifications, assuming the clarifications are about issues where the intention is clear but the text is not (usually because we thought there was only one possible interpretation). If PEP 484 had intended to only allow a specific subset of Python *syntax* we surely would have specified that subset -- so I think the intention is clear (enough).

Unfortunately a lot of PEP 484 is written in the form of examples, and in many cases it would be possible to quibble about edge cases not covered by the examples. (An example that just came up: is `#type: foo` a valid type comment? What about `#   type:foo`? Or `# < type: foo`? The intention was `#\s*type:\s*` but all the PEP examples use exactly one space.)

[Adam Cataldo]

Hi Sebastian,

Of course, we'd be happy to work with you on this!  We just need to figure out which of us will drive this on our end (most likely Rebecca or Teddy). I'll huddle with the team and get back to you with an answer on who later today.

On Tue, Jul 17, 2018 at 9:58 AM Sebastian Rittau <sri...@rittau.biz> wrote:

On 17.07.2018 17:05, Guido van Rossum wrote:
> This is a good point. I presume specifying this unambiguously would be
> a huge amount of work, and it would mostly codify mypy's current
> behavior. I don't think that's within the scope of PEP 484, but it
> could well be done as a separate PEP (perhaps an informational one?).
> I hope you understand that I am not volunteering.
An informational PEP sounds about right to me. Such a PEP could also
include style recommendations like those from typeshed's CONTRIBUTING
file (https://github.com/python/typeshed/blob/master/CONTRIBUTING.md).

I guess I just volunteered to help with such a PEP, although I feel that
someone from mypy's core team should take the lead on that. And if I
understood this thread correctly, the pytype team is also willing to
help out?

  - Sebastian

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[Rebecca Chen]

Hi Sebastian,

Both Teddy (cc'd) and I would like to volunteer to help. We're excited about the prospect of an informational pyi PEP.

Best,

Rebecca

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