Python私有继承？如何欺骗isinstance()？

Question

django核心代码片段:

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)

这段代码非常糟糕,因为如果我定义了一个继承自的自定义字段AutoField,那么我的db_type方法将被忽略.

我想做的是隐藏我的类是一个实例的事实AutoField.在C++中,我会通过私有继承来实现.

有没有办法欺骗isinstance返回False或隐藏继承？

我的cutom字段代码:

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

所以我有办法做到这一点,但它通过"猴子补丁".您无法使用,ABCMeta因为这需要您覆盖基类的元类.

您可以"更改"实例如下.重要的是"补丁"只能导入一次,我只会这样做,如果没有别的办法.

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

然后你的测试程序:

class MyAutoField(models.AutoField): pass

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

介绍抽象基类: PEP-3119.以下是一个抽象的例子......

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())