Protocol checking against typing_extenstion.Protocol fails

[iliapolo]

Things to check first

• I have searched the existing issues and didn't find my bug already reported there

• I have checked that my bug is still present in the latest release

Typeguard version

4.3.0

Python version

Python 3.10.4

What happened?

Expected typeguard to ignore private attributes of the typing_extension.Protocol so that type checking would pass.

Note: Honestly this was my first intuition, but I realize it might be out of scope as I'm not sure what your stance is with respect to supporting the typing_extensions package. Perhaps this issue is more a feature request to allow configuring custom attributes that typeguard will ignore when typechecking protocols.

How can we reproduce the bug?

import typing_extensions
import builtins
import typeguard

class IDo(typing_extensions.Protocol):
    def do_something(self) -> builtins.str:
        ...

def doer(do: IDo) -> builtins.str:
    typeguard.check_type(value=do, expected_type=IDo)
    return do.do_something()

class DefaultDo:
    def do_something(self) -> builtins.str:
        return "did something"
    
print(doer(DefaultDo()))

Running the above program will result in:

typeguard.TypeCheckError: __main__.DefaultDo is not compatible with the IDo protocol because it has no attribute named '__protocol_attrs__'

Indeed, extending the typing_extensions.Protocol adds the __protocol_attrs__ attribute and causes the mismatch. I did notice however that typeguard has provisions to ignore private attributes of the typing.Protocol class:

typeguard/src/typeguard/_checkers.py

Lines 668 to 671 in 016f813

	ignored_attrs = set(dir(typing.Protocol)) | {
	"__annotations__",
	"__non_callable_proto_members__",
	}

So I was thinking this might be extended to support typing_extension.Protocol? If not, it would be great if we had some way to inject custom attributes we'd like typeguard to ignore.

[agronholm]

Does Mypy pass your code in strict mode?

[iliapolo]

Looks like it:

❯ mypy --version                                                                                                                                                                                                                                    [12:51:41]
mypy 1.11.2 (compiled: yes)

❯ mypy --strict tests/exp.py                                                                                                                                                                                                                        [12:51:45]
Success: no issues found in 1 source file

[agronholm]

Yeah, I recall something like this was reported before. I'll get it fixed for the next release, but I'm currently busy trying to make new releases in two other projects. I'll tackle typeguard after that.

[iliapolo]

@agronholm If we agree on an approach here i'm happy to contribute a PR. Would the solution be using dir(typing_extensions.Protocol) instead of dir(typing.Protocol)? maybe a union of both?

[thetorpedodog]

This is not specific to typing_extensions.Protocol. Take, for instance, these two completely empty protocols, which should accept all objects:

# typedmod.py

import typing as t

T_co = t.TypeVar("T_co", covariant=True)

class Empty(t.Protocol):
    pass

class GenericEmpty(t.Protocol[T_co]):
    pass


def takes_empty(it: Empty) -> None:
    pass

def takes_generic_empty(it: GenericEmpty[T_co]) -> None:
    pass

When using them in a terminal session:

>>> import typeguard
>>> typeguard.install_import_hook()
<typeguard.ImportHookManager object at 0x7fc334c03d10>
>>> import typedmod
>>> typedmod.takes_empty("")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File ".../typedmod.py", line 12, in takes_empty
    def takes_empty(it: Empty) -> None:
  File ".../venv/tg-playground/lib/python3.12/site-packages/typeguard/_functions.py", line 136, in check_argument_types
    check_type_internal(value, annotation, memo)
  File ".../venv/tg-playground/lib/python3.12/site-packages/typeguard/_checkers.py", line 861, in check_type_internal
    checker(value, origin_type, args, memo)
  File ".../venv/tg-playground/lib/python3.12/site-packages/typeguard/_checkers.py", line 738, in check_protocol
    raise TypeCheckError(
typeguard.TypeCheckError: argument "it" (str) is not compatible with the Empty protocol because it has no attribute named '__weakref__'
>>> typedmod.takes_generic_empty("")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File ".../typedmod.py", line 15, in takes_generic_empty
    def takes_generic_empty(it: GenericEmpty[T_co]) -> None:
  File ".../venv/tg-playground/lib/python3.12/site-packages/typeguard/_functions.py", line 136, in check_argument_types
    check_type_internal(value, annotation, memo)
  File ".../venv/tg-playground/lib/python3.12/site-packages/typeguard/_checkers.py", line 861, in check_type_internal
    checker(value, origin_type, args, memo)
  File ".../venv/tg-playground/lib/python3.12/site-packages/typeguard/_checkers.py", line 738, in check_protocol
    raise TypeCheckError(
typeguard.TypeCheckError: argument "it" (str) is not compatible with the GenericEmpty protocol because it has no attribute named '__orig_bases__'

What this suggests is that you could use the attributes of an empty protocol (like GenericEmpty above) to generate the list of things you need to ignore when checking a protocol.

[thetorpedodog]

What this suggests is that you could use the attributes of an empty protocol (like GenericEmpty above) to generate the list of things you need to ignore when checking a protocol.

Actually, upon further research, the solution might be simpler: use typing_extensions.get_protocol_members (to become typing.get_protocol_members in 3.13). If you don’t want to have a hard dependency on typing_extensions, this might require you to vendor the relevant functions.

[agronholm]

Actually, upon further research, the solution might be simpler: use typing_extensions.get_protocol_members (to become typing.get_protocol_members in 3.13). If you don’t want to have a hard dependency on typing_extensions, this might require you to vendor the relevant functions.

Typeguard already has a hard dependency on typing_extensions, so I'll just use that function then. Thanks!

[agronholm]

This will be fixed with #490.

[agronholm]

This was fixed in v4.4.0.