Use TypeGuard for has in Python 3.10 and above

[layday]

Summary

Conditionally defines has using TypeGuard in Python 3.10+ in the stub file.

xref: #987

Pull Request Check List

• Added tests for changed code.
  Our CI fails if coverage is not 100%.
• New features have been added to our Hypothesis testing strategy.
• Changes or additions to public APIs are reflected in our type stubs (files ending in .pyi).
  • ...and used in the stub test file tests/typing_example.py.
  • If they've been added to attr/__init__.pyi, they've also been re-imported in attrs/__init__.pyi.
• Updated documentation for changed code.
  • New functions/classes have to be added to docs/api.rst by hand.
  • Changes to the signature of @attr.s() have to be added by hand too.
  • Changed/added classes/methods/functions have appropriate versionadded, versionchanged, or deprecated directives.
    Find the appropriate next version in our __init__.py file.
• Documentation in .rst files is written using semantic newlines.
• Changes (and possible deprecations) have news fragments in changelog.d.
• Consider granting push permissions to the PR branch, so maintainers can fix minor issues themselves without pestering you.

[Tinche]

I don't see a direct use-case for this (since attrs classes should be statically known now) but I don't see the harm either.

[layday]

The class might not be statically known to be an attrs class (#987, #996) or it can appear in a union with other types.

[hynek]

So what exactly does this do?

[Tinche]

It essentially makes has work at compile time too.

Let's say your function takes a class (not an instance, but an actual class). You will type your argument as type (or type[C]). You can check if it's an attrs class with has, and if that's true you can pass it to a different function that takes type[AttrsInstance]. But Mypy will still yell at you, since it doesn't know that has means it's an attrs class.

If we annotate has with TypeGuard, Mypy will know.

Code example:

def needs_attrs_class(cls: type[AttrsInstance]) -> None:
    pass

def my_function(cls: type) -> None:
    if has(cls):
        needs_attrs_class(cls)  # Mypy's gonna be unhappy, unless it knows `has` narrows the type of is argument

[layday]

It tells the type checker that if the condition is true then cls is of type type[AttrsInstance]. Simplified example:

def is_dict_no_typeguard(value: object) -> bool:
    return isinstance(value, dict)

def is_dict_typeguard(value: object) -> TypeGuard[dict]:
    return isinstance(value, dict)

might_be_a_dict = ...

if is_dict_no_typeguard(might_be_a_dict):
    "type of might_be_a_dict is object"

if is_dict_typeguard(might_be_a_dict):
    "type of might_be_a_dict is dict"

[hynek]

Sounds cool! Any reason against it? If not: make it happen Tin. ;)

[hynek]

Would it be feasible to add a conditional import from typing-extensions?

[layday]

You can only use one of a small number of constants like TYPE_CHECKING and sys.version_info to define types conditionally, try/except won't work, so I don't think so.

[Tinche]

In my very limited testing this seems to work, both in Mypy and Pyright:

[layday]

Right, but typing_extensions is not a dependency of attrs, so that'll error if typing_extensions is not installed.

[layday]

Okay, it seems typeshed ships typing_extensions as part of the stdlib, so I guess we can actually do that.

[layday]

Done in fd2c819.

[hynek]

Would you mind coming up with a way to verify this works in https://github.com/python-attrs/attrs/blob/main/tests/typing_example.py ?

[layday]

There's a test in test_mypy.yml. Should I move it over to typing_example.py?

[hynek]

No, both would be great, since they work somewhat differently. Maybe if has(c): print(c.__attrs_attrs__) or something like that? That should pass, right?

[hynek]

Just so I understand this correctly: the import from typing_extensions only works, because we have the stubs in pyi files that are only touched by mypy which in turn brings in typing_extensions? IOW: if we'd use inline type hints, this would fall apart?

[layday]

That's right. The typeshed has typing_extensions in the stdlib. I assume they did this so that they can use new typing constructs in the typeshed without tripping MyPy. At runtime, you'd have to do something like:

import typing

if typing.TYPE_CHECKING:
    from typing_extensions import TypeGuard

# Use TypeGuard either in quotes or with `from __future__ import annotations`

[hynek]

Thanks!