如何修补 `call` 方法？

Question

我似乎无法修补 class 实例的 __call__ 方法（是的，我只想修补单个实例，而不是所有实例）。

以下代码：

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())

输出这个：

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

如何使用 monkeypatch __call__()？为什么我不能像修补其他方法一样修补它？

虽然this answer告诉了怎么做（据说，我还没有测试过），它没有解释问题的为什么部分。

Answer 1

For custom classes, implicit invocations of special methods are only guaranteed to work correctly if defined on an object’s type, not in the object’s instance dictionary. That behaviour is the reason why the following code raises an exception:
>>> class C:
...     pass
...
>>> c = C()
>>> c.__len__ = lambda: 5
>>> len(c)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: object of type 'C' has no len()

来源：https://docs.python.org/3/reference/datamodel.html#special-lookup

Answer 2

因此，正如 J.J. Hakala 评论的那样，Python 真正做的是调用：

type(a).__call__(a)

因此，如果我想覆盖 __call__ 方法，我必须覆盖 class 的 __call__，但是如果我不想影响相同 class 的其他实例的行为，我需要使用覆盖的 __call__ 方法创建一个新的 class。

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

如何修补 `call` 方法？

How to monkey patch a `call` method?

python

monkeypatching

如何修补 `__call__` 方法？

How to monkey patch a `__call__` method?

python

monkeypatching

如何修补 `call` 方法？

How to monkey patch a `call` method?