10000 bpo-44975: [typing] Support issubclass for ClassVar data members by Fidget-Spinner · Pull Request #27883 · python/cpython · GitHub
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bpo-44975: [typing] Support issubclass for ClassVar data members #27883

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bpo-44975: [typing] Support issubclass for ClassVar data members #27883

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a613af6
Support issubclass for ClassVar data members
Fidget-Spinner Aug 22, 2021
f045d2e
make it work with string annotations and PEP 563
Fidget-Spinner Aug 23, 2021
58c69e1
Apply Lukasz' suggestions
Fidget-Spinner Aug 24, 2021
2ed2ee5
fix test
Fidget-Spinner Aug 24, 2021
c842370
Use identity check instead of equality
Fidget-Spinner Aug 26, 2021
f44d3b6
Merge remote-tracking branch 'upstream/main' into issubclass_data_pro…
Fidget-Spinner Dec 5, 2021
9278a34
fix merge issues
Fidget-Spinner Dec 5, 2021
ba57bb1
Add more tests and comments
Fidget-Spinner Dec 5, 2021
43ec2ea
support value checking classvars
Fidget-Spinner Dec 5, 2021
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Add more tests and comments
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Fidget-Spinner committed Dec 5, 2021
commit ba57bb1c2da82fcce41f029205455f5f20d34739
6 changes: 6 additions & 0 deletions Lib/test/test_typing.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1631,8 +1631,13 @@ class C: pass

class D:
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Let's also add a case with the same class prop, but different value. Example:

class E:
    x = 2

Because right now all names / values always match. It is not clear whether value is a part of the protocol or not.

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@Fidget-Spinner Fidget-Spinner Dec 5, 2021 •
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Agreed.

It is not clear whether value is a part of the protocol or not.

It shouldn't be. IMO, we should only care about the "shape". Trying to runtime-check the value is difficult and should be left to third-party libs to do.

Nevermind, I changed my mind. Let's do it!

x = 1

class E:
x = 2

self.assertNotIsSubclass(C, P)
self.assertIsSubclass(D, P)
self.assertIsSubclass(E, P)

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Let's also test these classes:

class G:
   x: ClassVar[int] = 1
class H:
   x: 'ClassVar[int]' = 1

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These aren't testing more code paths, the code doesn't look at the annotations of the first class argument in issubclass.

# String annotations (forward references).
@runtime_checkable
Expand All @@ -1645,6 +1650,7 @@ class D:
x = 1
y = 2
z = 3

self.assertNotIsSubclass(C, P)
self.assertIsSubclass(D, P)

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13 changes: 9 additions & 4 deletions Lib/typing.py
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Expand Up @@ -1404,21 +1404,26 @@ def _get_protocol_attrs(cls):
attrs.add(attr)
return attrs

_classvar_prefixes = ("typing.ClassVar[", "t.ClassVar[", "ClassVar[")
_CLASSVAR_PREFIXES = ("typing.ClassVar", "t.ClassVar", "ClassVar")

def _is_callable_or_classvar_members_only(cls):
def _is_callable_or_classvar_members_only(cls, instance=None, verify_classvar_values=False):
attr_names = _get_protocol_attrs(cls)
annotations = getattr(cls, '__annotations__', {})
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What about using typing.get_type_hints to resolve annotations for a class?

It will allow having ClassVar annotated as str:

@runtime_checkable
class P(Protocol):
    x: 'ClassVar[int]' = 1
Suggested change
annotations = getattr(cls, '__annotations__', {})
annotations = get_type_hints(cls)

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Good question! I'd considered that too but decided against it for two main reasons:

  1. get_type_hints is slow
  2. get_type_hints won't work with forward references/ PEP 563 if ClassVar is not defined in the caller's namespace

Consider the following:

# foo.py
from __future__ import annotations
from typing import *

@runtime_checkable
class X(Protocol):
 x: ClassVar[int] = 1
 y: SomeUndeclaredType = None
# bar.py
from .foo import X
class Y: ...

# Error! get_type_hints cannot resolve 'ClassVar' and 'SomeUndeclaredType'
issubclass(X, Y)

Nonetheless, your suggestion has reminded me of a basic workaround for string annotations. Thanks.

# PEP 544 prohibits using issubclass() with protocols that have non-method members.
# bpo-44975: Relaxing that restriction to allow for runtime-checkable
# protocols with class variables since those should be available at class
# definition time.
for attr_name in attr_names:
attr = getattr(cls, attr_name, None)
# Method-like.
if callable(attr):
continue
annotation = annotations.get(attr_name)
# ClassVar member
if getattr(annotation, '__origin__', None) is ClassVar:
continue
# String annotations (forward references).
if isinstance(annotation, str) and annotation.startswith(_classvar_prefixes):
# ClassVar string annotations (forward references).
if isinstance(annotation, str) and annotation.startswith(_CLASSVAR_PREFIXES):
continue
return False
return True
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0