New submission from weapon :
In the PEP 3119(https://www.python.org/dev/peps/pep-3119/),it introduce about
Customizing instance and subclass checks.
>The primary mechanism proposed here is to allow overloading the built-in
>functions isinstance() and issubclass(). The overloading works as follows: The
>call isinstance(x, C) first checks whether C.__instancecheck__ exists, and if
>so, calls C.__instancecheck__(x) instead of its normal implementation.
but my code doesn't works.
```
class Sizeable(object):
def __instancecheck__(cls, instance):
print("__instancecheck__ call")
return hasattr(instance, "__len__")
class B(object):
pass
b = B()
print(isinstance(b, Sizeable)) # output:False
```
The __instancecheck__ function is not called. In PyObject_IsInstance,when
isinstance(x, C) the following three conditions are reached, __instancecheck__
will take effect:
1. x can not be directly instantiated by C;
2. The C class specifies the metaclass;
3. The __instancecheck__ in the specified metaclass class is defined.
This code can work:
```
class MetaSizeable(type):
def __instancecheck__(cls, instance):
print("__instancecheck__ call")
return hasattr(instance, "__len__")
class Sizeable(metaclass=MetaSizeable):
pass
class B(object):
pass
b = B()
print(isinstance(b, Sizeable)) # output: False
print(isinstance([], Sizeable)) # output: True
```
So,the problem is that the document does not conform to the reality.Can it be
guaranteed that __instancecheck__ is always called?
If yes: PyObject_IsInstance should be tweaked.
If no: document should be tweaked.
--
assignee: docs@python
components: Documentation, Interpreter Core
messages: 316118
nosy: docs@python, hongweipeng
priority: normal
severity: normal
status: open
title: Isinstance() behavior is not consistent with the document
type: behavior
versions: Python 3.7
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<https://bugs.python.org/issue33417>
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