[docs] [issue33417] Isinstance() behavior is not consistent with the document

[weapon]

New submission from weapon <hongweichen8888 at sina.com>:

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 at python
components: Documentation, Interpreter Core
messages: 316118
nosy: docs at 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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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue33417>
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