Abstract class inheritance regression in Python 3.13

[txgk]

Bug report

Bug description:

Good day to you, dear Python maintainers! I have a little issue with the recent Python release, 3.13. Here's a snippet:

import ctypes

class EnumMetaclass(type(ctypes.c_uint)):
  def __new__(metaclass, name, bases, cls_dict):
    cls = type(ctypes.c_uint).__new__(metaclass, name, bases, cls_dict)
    if name == 'Enum':
      return cls
    for i, value in enumerate(cls_dict['_values_']):
      setattr(cls, value, cls.from_param(i))
    return cls

class Enum(ctypes.c_uint, metaclass=EnumMetaclass):
  @classmethod
  def from_param(cls, param):
    print("Calling method 'from_param' of", cls)
    return cls(param)
  def __repr__(self):
    return self._values_[self.value]

class NodeType(Enum):
  _values_ = ['DOCUMENT', 'ELEMENT', 'TEXT']

def main():
  print(NodeType(1))

if __name__ == "__main__":
  main()

When run with Python 3.12.7, I get the following output:

Calling method 'from_param' of <class '__main__.NodeType'>
Calling method 'from_param' of <class '__main__.NodeType'>
Calling method 'from_param' of <class '__main__.NodeType'>
ELEMENT

When run with Python 3.13.1, I get the following output:

Calling method 'from_param' of <class '__main__.NodeType'>
Traceback (most recent call last):
  File "/home/grisha/test.py", line 20, in <module>
    class NodeType(Enum):
      _values_ = ['DOCUMENT', 'ELEMENT', 'TEXT']
  File "/home/grisha/test.py", line 9, in __new__
    setattr(cls, value, cls.from_param(i))
                        ~~~~~~~~~~~~~~^^^
  File "/home/grisha/test.py", line 16, in from_param
    return cls(param)
TypeError: abstract class

As you can see, new TypeError detection triggers where it presumably mustn't have, because it's perfectly correct to call from_param from NodeType object.

I checked that this problem persists in both 3.13.0 and 3.13.1.

CPython versions tested on:

3.13

Operating systems tested on:

Linux

[ZeroIntensity]

Is this related to #124520?

[txgk]

Is this related to #124520?

Hello, @ZeroIntensity! Seems like it is related. I've read through the discussion at #124520 and modified the snippet according to the new requirements of metaclasses initialization, i.e. moved custom logic to the __init__ method and made sure my metaclass is called explicitly (return cls(param)), as per this piece of documentation:

• As a consequence of necessary internal refactoring, initialization of
  internal metaclasses now happens in __init__ rather
  than in __new__. This affects projects that subclass these internal
  metaclasses to provide custom initialization.
  Generally:

  • Custom logic that was done in __new__ after calling super().__new__
    should be moved to __init__.
  • To create a class, call the metaclass, not only the metaclass's
    __new__ method.

  See :gh:124520 for discussion and links to changes in some affected
  projects.

Here's a modified snippet:

import ctypes

class EnumMetaclass(type(ctypes.c_uint)):
  def __new__(metaclass, name, bases, cls_dict):
    cls = super().__new__(metaclass, name, bases, cls_dict)
    return cls
  def __init__(self, name, bases, cls_dict):
    if name != 'Enum':
      for i, value in enumerate(cls_dict['_values_']):
        setattr(self, value, self.from_param(i))

class Enum(ctypes.c_uint, metaclass=EnumMetaclass):
  @classmethod
  def from_param(cls, param):
    print("Calling method 'from_param' of", cls)
    return cls(param)
  def __repr__(self):
    return self._values_[self.value]

class NodeType(Enum):
  _values_ = ['DOCUMENT', 'ELEMENT', 'TEXT']

def main():
  print(NodeType(1))

if __name__ == "__main__":
  main()

Unfortunately, this doesn't fix the problem when it's run with Python 3.13.1:

Calling method 'from_param' of <class '__main__.NodeType'>
Traceback (most recent call last):
  File "/home/grisha/test.py", line 20, in <module>
    class NodeType(Enum):
      _values_ = ['DOCUMENT', 'ELEMENT', 'TEXT']
  File "/home/grisha/test.py", line 10, in __init__
    setattr(self, value, self.from_param(i))
                         ~~~~~~~~~~~~~~~^^^
  File "/home/grisha/test.py", line 16, in from_param
    return cls(param)
TypeError: abstract class

[ZeroIntensity]

You need to call __init__ of ctypes.c_uint in EnumMetaclass.__init__. As the documentation says:

Custom logic that was done in new after calling super().new
should be moved to init.

ctypes.c_uint isn't ready until it's __init__ has been called.

[txgk]

You need to call __init__ of ctypes.c_uint in EnumMetaclass.__init__. As the documentation says:

Custom logic that was done in new after calling super().new
should be moved to init.

ctypes.c_uint isn't ready until it's __init__ has been called.

You're right. Adding super().__init__(name, bases, cls_dict) in the beginning of the EnumMetaclass's __init__ did help. Now code works the same for both 3.12.7 and 3.13.1.

In case someone stumbles upon this discussion in the future, here's a patch against original snippet which has fixed the erroneous behavior:

--- a/test.py   2024-12-15 21:59:57.248825568 +0300
+++ b/test.py   2024-12-15 21:59:34.655857763 +0300
@@ -2,12 +2,13 @@
 
 class EnumMetaclass(type(ctypes.c_uint)):
   def __new__(metaclass, name, bases, cls_dict):
-    cls = type(ctypes.c_uint).__new__(metaclass, name, bases, cls_dict)
-    if name == 'Enum':
-      return cls
-    for i, value in enumerate(cls_dict['_values_']):
-      setattr(cls, value, cls.from_param(i))
+    cls = super().__new__(metaclass, name, bases, cls_dict)
     return cls
+  def __init__(self, name, bases, cls_dict):
+    super().__init__(name, bases, cls_dict)
+    if name != 'Enum':
+      for i, value in enumerate(cls_dict['_values_']):
+        setattr(self, value, self.from_param(i))
 
 class Enum(ctypes.c_uint, metaclass=EnumMetaclass):
   @classmethod

@ZeroIntensity, thank you very much for your help and your fast replies!

Best regards, Grigory