正确执行特殊方法查找

Question

如果我想查找一个对象的特殊方法，我可以在对象或其类型上执行此操作。虽然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查看对象。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，但它产生了lookup_add(Bar)的错误结果。

lookup_sub查看它的类型。它适用于lookup_sub(Bar)，为Bar的方法返回一个未绑定的方法对象，但它产生了lookup_sub(baz)的错误结果。

我可以尝试functools.partial(operator.add, o)，但这并不真正查找o的方法。如果o实际上没有实现__add__，则此尝试不会产生我想要的错误。

是否有复制解释器的特殊方法查找行为的好方法？

Answer 1

我想你可以你的两个使用isinstance结合：

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

好吧，我想我得到了它。关键是要始终获得该类型的（未绑定）方法，并将其绑定：

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'