8000 gh-125783: Add more tests to prevent regressions with the combination of ctypes and metaclasses. by junkmd · Pull Request #126126 · python/cpython · GitHub
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gh-125783: Add more tests to prevent regressions with the combination of ctypes and metaclasses. #126126

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Merged
merged 5 commits into from
Nov 1, 2024
Merged

gh-125783: Add more tests to prevent regressions with the combination of ctypes and metaclasses. #126126

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011c978
Add `test_creating_pointer_in_dunder_init_...`.
junkmd Oct 29, 2024
7a67476
Add comments.
junkmd Oct 29, 2024
5996870
Add `super().__init__` calls and early returns to limit recursion.
junkmd Oct 30, 2024
e1831ac
Fix an early return in `__init__`.
junkmd Oct 30, 2024
0b67450
Merge branch 'main' into test_metacls_PyCSimpleType_dunder_init
encukou Oct 31, 2024
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56 changes: 56 additions & 0 deletions Lib/test/test_ctypes/test_c_simple_type_meta.py
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Original file line number Diff line number Diff line change
Expand Up @@ -85,3 +85,59 @@ class Sub(CtBase):

self.assertIsInstance(POINTER(Sub), p_meta)
self.assertTrue(issubclass(POINTER(Sub), Sub))

def test_creating_pointer_in_dunder_init_1(self):
# Test for modern metaclass initialization that does not
# require recursion avoidance.

class ct_meta(type):
def __init__(self, name, bases, namespace):
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  • Please don't leave out the super().__init__ call when subclassing.
  • Please do limit recursion, as with __new__.
Suggested change
# Test for modern metaclass initialization that does not
# require recursion avoidance.
class ct_meta(type):
def __init__(self, name, bases, namespace):
class ct_meta(type):
def __init__(self, name, bases, namespace):
super().__init__(name, bases, namespace)
# Avoid recursion.
# (See test_creating_pointer_in_dunder_new_1)
if bases == (c_void_p,):
return
if issubclass(self, PtrBase):
return

(And similar in test_creating_pointer_in_dunder_init_2.)

The reason you don't need recursion avoidance here is that the internal PyCSimpleType is not set up for diamond inheritance patterns, and doesn't call super().__init__ (that is, ct_meta.__init__ here). That's arguably a bug. (It definitely would be one if we wanted to support such subclassing.)
If you switch the metaclass bases, writing class p_meta(ct_meta, PyCSimpleType), recursion avoidance becomes necessary again.

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Good catch! thank you.

if bases == (object,):
ptr_bases = (self, PtrBase)
else:
ptr_bases = (self, POINTER(bases[0]))
p = p_meta(f"POINTER({self.__name__})", ptr_bases, {})
ctypes._pointer_type_cache[self] = p

class p_meta(PyCSimpleType, ct_meta):
pass

class PtrBase(c_void_p, metaclass=p_meta):
pass

class CtBase(object, metaclass=ct_meta):
pass

class Sub(CtBase):
pass

class Sub2(Sub):
pass

self.assertIsInstance(POINTER(Sub2), p_meta)
self.assertTrue(issubclass(POINTER(Sub2), Sub2))
self.assertTrue(issubclass(POINTER(Sub2), POINTER(Sub)))
self.assertTrue(issubclass(POINTER(Sub), POINTER(CtBase)))

def test_creating_pointer_in_dunder_init_2(self):
# A simpler variant of the above.

class ct_meta(type):
def __init__(self, name, bases, namespace):
p = p_meta(f"POINTER({self.__name__})", (self, c_void_p), {})
ctypes._pointer_type_cache[self] = p

class p_meta(PyCSimpleType, ct_meta):
pass

class Core(object):
pass

class CtBase(Core, metaclass=ct_meta):
pass

class Sub(CtBase):
pass

self.assertIsInstance(POINTER(Sub), p_meta)
self.assertTrue(issubclass(POINTER(Sub), Sub))
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