python - 如何猴子修补 `__call__`方法？

我似乎无法猴子修补类实例的__call__方法(是的，我只想修补单个实例，而不是全部实例)。

如下代码:

class A(object):
    def test(self):
        return "TEST"

    def __call__(self):
        return "EXAMPLE"

a = A()
print("call method: {0}".format(a.__call__))
print("test method: {0}".format(a.test))
a.__call__ = lambda : "example"
a.test = lambda : "test"
print("call method: {0}".format(a.__call__))
print("test method: {0}".format(a.test))

print(a())
print("Explicit call: {0}".format(a.__call__()))
print(a.test())

输出此:

call method: <bound method A.__call__ of <__main__.A object at 0x7f3f2d60b6a0>>
test method: <bound method A.test of <__main__.A object at 0x7f3f2d60b6a0>>
call method: <function <lambda> at 0x7f3f2ef4ef28>
test method: <function <lambda> at 0x7f3f2d5f8f28>
EXAMPLE
Explicit call: example
test

虽然我希望它输出:

...
example
Explicit call: example
test

如何猴子补丁__call__()？为什么我不能像修补其他方法一样修补它？

尽管this answer告诉了如何做(据说，我还没有测试过)，但是它没有解释问题的原因。

最佳答案

因此，正如J.J. Hakala所评论的那样，Python的真正作用是调用:

type(a).__call__(a)

这样，如果我想覆盖__call__方法，我必须覆盖类的__call__，但是如果我不想影响同一类的其他实例的行为，则需要使用覆盖的__call__方法。

因此，如何覆盖__call__的示例如下所示:

class A(object):
    def test(self):
        return "TEST"

    def __call__(self):
        return "EXAMPLE"

def patch_call(instance, func):
    class _(type(instance)):
        def __call__(self, *arg, **kwarg):
           return func(*arg, **kwarg)
    instance.__class__ = _

a = A()
print("call method: {0}".format(a.__call__))
print("test method: {0}".format(a.test))
patch_call(a, lambda : "example")
a.test = lambda : "test"
print("call method: {0}".format(a.__call__))
print("test method: {0}".format(a.test))

print("{0}".format(a()))
print("Explicit a.__call__: {0}".format(a.__call__()))
print("{0}".format(a.test()))

print("Check instance of a: {0}".format(isinstance(a, A)))

运行它会产生以下输出:

call method: <bound method A.__call__ of <__main__.A object at 0x7f404217a5f8>>
test method: <bound method A.test of <__main__.A object at 0x7f404217a5f8>>
call method: <bound method patch_call.<locals>._.__call__ of <__main__.patch_call.<locals>._ object at 0x7f404217a5f8>>
test method: <function <lambda> at 0x7f404216d048>
example
Explicit a.__call__: example
test
Check instance of a: True

关于python - 如何猴子修补 `__call__`方法？，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/38541015/