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gh-105144: abc: Suppress errors raised by unrelated other subclasses by JelleZijlstra · Pull Request #105159 · python/cpython · GitHub

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gh-105144: abc: Suppress errors raised by unrelated other subclasses #105159

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gh-105144: abc: Suppress errors raised by unrelated other subclasses #105159

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18 changes: 12 additions & 6 deletions Lib/_py_abc.py
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Original file line number Diff line number Diff line change
Expand Up @@ -134,14 +134,20 @@ def __subclasscheck__(cls, subclass):
return True
# Check if it's a subclass of a registered class (recursive)
for rcls in cls._abc_registry:
if issubclass(subclass, rcls):
cls._abc_cache.add(subclass)
return True
try:
if issubclass(subclass, rcls):
cls._abc_cache.add(subclass)
return True
except Exception:

JelleZijlstra May 31, 2023

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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

In the C code I actually suppress all exceptions, not just Exception. Not sure what the right thing to do is.

AlexWaygood May 31, 2023

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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

I'd say suppressing all exceptions would definitely be bad -- that would mean that we'd be suppressing e.g. KeyboardInterrupt, and SystemExit.

Honestly, I feel pretty uncomfortable about suppressing all subclasses of Exception -- could we just do TypeError? If I had a typo in a custom __subclasscheck__ method, I'm not sure I'd want e.g. the resulting AttributeError to be silenced because of the fact that ABCMeta was being used somewhere

JelleZijlstra May 31, 2023

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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

You'd still see the error if you use your broken class directly. What this PR changes is that you'll no longer see the error if you use issubclass() checks on a separate class that inherits from the same ABC.
pass
# Check if it's a subclass of a subclass (recursive)
for scls in cls.__subclasses__():
if issubclass(subclass, scls):
cls._abc_cache.add(subclass)
return True
try:
if issubclass(subclass, scls):
cls._abc_cache.add(subclass)
return True
except Exception:
pass
# No dice; update negative cache
cls._abc_negative_cache.add(subclass)
return False
38 changes: 36 additions & 2 deletions Lib/test/test_abc.py
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Original file line number Diff line number Diff line change
Expand Up @@ -435,17 +435,22 @@ class C:
None,
lambda x: [],
lambda: 42,
lambda: [42],
]

for i, func in enumerate(bogus_subclasses):
class S(metaclass=abc_ABCMeta):
__subclasses__ = func

with self.subTest(i=i):
with self.subTest(i=i, func=func):
with self.assertRaises(TypeError):
issubclass(int, S)

# If __subclasses__ contains non-classes, we suppress the error instead.
class S(metaclass=abc_ABCMeta):
__subclasses__ = lambda: [42]

self.assertIs(issubclass(int, S), False)

# Also check that issubclass() propagates exceptions raised by
# __subclasses__.
exc_msg = "exception from __subclasses__"
Expand All @@ -459,6 +464,35 @@ class S(metaclass=abc_ABCMeta):
with self.assertRaisesRegex(Exception, exc_msg):
issubclass(int, S)

def test_subclass_with_broken_subclasscheck(self):
class A(metaclass=abc_ABCMeta):
pass

class Unrelated: pass

self.assertIs(issubclass(Unrelated, A), False)

class BrokenMeta(abc_ABCMeta):
is_broken = True
def __subclasscheck__(cls, subclass):
if not BrokenMeta.is_broken:
return super().__subclasscheck__(subclass)
raise Exception("broken")

class Broken(A, metaclass=BrokenMeta):
pass

self.assertIs(issubclass(Unrelated, A), False)

class RegisteredBroken(metaclass=BrokenMeta):
pass

BrokenMeta.is_broken = False
A.register(RegisteredBroken)
BrokenMeta.is_broken = True

self.assertIs(issubclass(Unrelated, A), False)

def test_subclasshook(self):
class A(metaclass=abc.ABCMeta):
@classmethod
Expand Down
3 changes: 3 additions & 0 deletions Misc/NEWS.d/next/Library/2023-05-31-09-55-16.gh-issue-105144.fTqfGv.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,3 @@
Subclass checks on :class:`abc.ABC` subclasses no longer propagate errors
raised by subclass checks on unrelated base classes of the ABC. Patch by
Jelle Zijlstra.
6 changes: 3 additions & 3 deletions Modules/_abc.c
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Original file line number Diff line number Diff line change
Expand Up @@ -786,7 +786,8 @@ _abc__abc_subclasscheck_impl(PyObject *module, PyObject *self,
goto end;
}
if (r < 0) {
goto end;
// Ignore subclasses that throw on issubclass().
PyErr_Clear();
}
}

Expand Down Expand Up @@ -856,8 +857,7 @@ subclasscheck_check_registry(_abc_data *impl, PyObject *subclass,
int r = PyObject_IsSubclass(subclass, rkey);
Py_DECREF(rkey);
if (r < 0) {
ret = -1;
break;
PyErr_Clear();
}
if (r > 0) {
if (_add_to_weak_set(&impl->_abc_cache, subclass) < 0) {
Expand Down
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