正确执行特殊方法查找

Question

如果我想查找对象的特殊方法，我可以在对象或其类型上进行。不过，这两个选项似乎都不正确。

def lookup_add(o):
    return o.__add__
def lookup_sub(o):
    return type(o).__sub__

class Foo(type):
    def __add__(self, other):
        return NotImplemented
    def __sub__(self, other):
        return NotImplemented

class Bar(object):
    __metaclass__ = Foo

    def __add__(self, other):
        return NotImplemented

baz = Bar()

lookup_add(Bar) # Produces wrong method
lookup_sub(baz) # Should raise an error. Instead, produces wrong method.

lookup_add 在对象上查找它。它对lookup_add(baz) 正常工作，为baz 的方法返回一个绑定的方法对象，但它对lookup_add(Bar) 产生错误的结果。

lookup_sub 在类型上查找它。它对lookup_sub(Bar) 正常工作，为Bar 的方法返回一个未绑定的方法对象，但它对lookup_sub(baz) 产生错误的结果。

我可以尝试functools.partial(operator.add, o)，但这并不能真正查找o 的方法。如果o 没有真正实现__add__，这个尝试不会产生我想要的错误。

有没有复制解释器特殊方法查找行为的好方法？

Answer 1

我想您可以使用 isinstance 将两者结合起来：

def lookup_method(obj, method):
    if not isinstance(obj, type):
        return getattr(obj, method, None)
    return getattr(type(obj), method, None)

>>> print(lookup_method(Bar, '__add__'))
<unbound method Foo.__add__>
>>> print(lookup_method(baz, '__add__'))
<bound method Bar.__add__ of <__main__.Bar object at 0x23f27d0>>
>>> print(lookup_method(baz, '__sub__'))
None

Answer 2

好的，我想我明白了。诀窍是始终获取类型的（未绑定）方法并绑定它：

import types

def special_lookup_mimic(obj, name):
    if not hasattr(obj, name):
        raise TypeError("No method of that name")

    meth = getattr(obj, name)
    if not isinstance(meth, types.MethodType):
        raise TypeError("Expected method")

    #always look-up the type's method
    cls = obj.__class__
    return getattr(cls, name).__get__(obj, cls)  

演示：

class Foo(type):
    def __add__(cls, other):
        print 'Foo().__add__'
        return 999

class Bar(object):
    __metaclass__ = Foo

    def __init__(self, id):
        self.id = id

    def __add__(self, other):
        print 'Bar(%d).__add__' % (self.id,)
        return self.id

b1 = Bar(1)
b2 = Bar(2)

b1 + 10; special_lookup_mimic(b1, '__add__')(10)
b2 + 10; special_lookup_mimic(b2, '__add__')(10)

b1.__add__ = b2.__add__

b1 + 10; special_lookup_mimic(b1, '__add__')(10)
b2 + 10; special_lookup_mimic(b2, '__add__')(10)

Bar + 10; special_lookup_mimic(Bar, '__add__')(10)

def patched_add(num):
    def patch_add(cls, other):
        print "Patched add '%d'" % (num,)
        return num
    return patch_add

print "Patching Bar.__add__..."
Bar.__add__ = patched_add(1337)

b1 + 10; special_lookup_mimic(b1, '__add__')(10)
b2 + 10; special_lookup_mimic(b2, '__add__')(10)
Bar + 10; special_lookup_mimic(Bar, '__add__')(10)

print "Patching Foo.__add__..."
Foo.__add__ = patched_add(10000)

b1 + 10; special_lookup_mimic(b1, '__add__')(10)
b2 + 10; special_lookup_mimic(b2, '__add__')(10)
Bar + 10; special_lookup_mimic(Bar, '__add__')(10)

输出：

Bar(1).__add__
Bar(1).__add__
Bar(2).__add__
Bar(2).__add__
Bar(1).__add__
Bar(1).__add__
Bar(2).__add__
Bar(2).__add__
Foo().__add__
Foo().__add__
Patching Bar.__add__...
Patched add '1337'
Patched add '1337'
Patched add '1337'
Patched add '1337'
Foo().__add__
Foo().__add__
Patching Foo.__add__...
Patched add '1337'
Patched add '1337'
Patched add '1337'
Patched add '1337'
Patched add '10000'
Patched add '10000'