Python私有继承？如何欺骗isinstance（）？

Question

Snippet of django core code:

class ForeignKey(RelatedField, Field):

    ...

    def db_type(self, connection):
        rel_field = self.rel.get_related_field()
        if (isinstance(rel_field, AutoField) or
                (not connection.features.related_fields_match_type and
                isinstance(rel_field, (PositiveIntegerField,
                                       PositiveSmallIntegerField)))):
            return IntegerField().db_type(connection=connection)
        return rel_field.db_type(connection=connection)

This code is very bad, because if I define a custom field which inherits from AutoField , then my db_type method will be ignored.

What I would like to do is to hide the fact that my class is an instance of AutoField . In C++, I would do that by private inheritance.

Is there any way to trick isinstance to return False or to hide inheritance?

Code of my cutom field:

class MyAutoField(models.AutoField):

    def __init__(self, length, *args, **kwargs):
        self.length = length
        super(MyAutoField, self).__init__(*args, **kwargs)

    def db_type(self, connection):
        if connection.vendor == 'oracle':
            return 'NUMBER(%s,0)' % (self.length)
        if connection.vendor == 'postgresql':
            if self.length <= 4:
                return 'smallint'
            if self.length <= 9:
                return 'integer'
            return 'bigint'
        return super(MyAutoField, self).db_type(connection)

Answer 1

So I have a way to do it, but its via "monkey patching." You can't use ABCMeta because that requires you override your base class's metaclass.

You could "change" isinstance as follows. It is important that "patches" only gets imported once, and I would only do this if is no other way.

patches.py

import django.models
import mymodel
import __builtin__
def _isinstance(instance, clz):
    if clz is models.AutoField and isinstance_orig(instance, MyAutoField):
        return False
    return isinstance_orig(instance, clz)
__builtin__.isinstance_orig = __builtin__.isinstance
__builtin__.isinstance = _isinstance

Then your test program:

class MyAutoField(models.AutoField): pass

x = MyAutoField()
print(isinstance(x, models.AutoField))
print(isinstance(x, models.MyAutoField))

---

Introducing Abstract Base Classes: PEP-3119 . Below is an abstract example...

class ABCMeta(type):

  def __instancecheck__(cls, inst):
    """Implement isinstance(inst, cls)."""
    return any(cls.__subclasscheck__(c)
               for c in {type(inst), inst.__class__})

  def __subclasscheck__(cls, sub):
    """Implement issubclass(sub, cls)."""
    candidates = cls.__dict__.get("__subclass__", set()) | {cls}
    return any(c in candidates for c in sub.mro())