Steven D'Aprano <steve+pyt...@pearwood.info> added the comment:
The plot thickens.
I was wrong to say that type() always and only looks at the "real" underlying
type of the instance. type() does look at __class__ as well. But only sometimes.
>>> class A:
... __class__ = int
...
>>> type(A())
<class '__main__.A'>
>>> a = A()
>>> a.__class__ = int
>>> type(a)
<class '__main__.A'>
So from A, we might think that type() ignores __class__
>>> class B:
... pass
...
>>> type(B()) # no __class__ to inspect
<class '__main__.B'>
>>> b = B()
>>> b.__class__ = int
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: __class__ assignment only supported for mutable types or ModuleType
subclasses
How very odd. But okay, let's try something else:
>>> b.__class__ = A # lie that this is an A not a B
>>> type(b)
<class '__main__.A'>
So now type() *does* inspect __class__, and believes it.
If we generate the __class__ attribute dynamically with __getattr__, the method
doesn't even get called. But if we use __getattribute__:
>>> class C:
... def __getattribute__(self, name):
... print('C getattribute:', name)
... if name == '__class__':
... return int
... raise AttributeError
...
>>> C().__class__
C getattribute: __class__
<class 'int'>
>>> type(C())
<class '__main__.C'>
type() ignores the dynamically generated __class__. But isinstance does not:
>>> isinstance(C(), int)
C getattribute: __class__
True
The same applies if we use property:
>>> class D:
... @property
... def __class__(self):
... return int
...
>>> type(D())
<class '__main__.D'>
>>> isinstance(D(), int)
True
So the rules appear to be:
- you can set __class__, but only sometimes;
- type() will believe __class__, but only sometimes;
- you can generate __class__ dynamically, but not with __getattr__;
- isinstance() will believe __class__ (always?);
- and there is no indication of how much of this is deliberate language feature
and how much is an accident of implementation.
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Python tracker <rep...@bugs.python.org>
<https://bugs.python.org/issue32683>
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