装饰方法上的 getattr 会生成 TypeError

Question

我需要在实例级别应用 memoization，所以我使用了以下装饰器：

from functools import partial, update_wrapper

class memoize(object):
    def __init__(self, func):
        self.func = func
        update_wrapper(self, func)

    def __get__(self, obj):
        if obj is None:
            return self.func
        return partial(self, obj)

    def __call__(self, *args, **kwargs):
        obj = args[0]
        try:
            cache = obj.__cache__
        except AttributeError:
            cache = obj.__cache__ = {}
        key = (self.func, args[1:], frozenset(kwargs.items()))
        try:
            res = cache[key]
        except KeyError:
            res = cache[key] = self.func(*args, **kwargs)
        return res

应用：

class A(object):
    def __init__(self, parent):
        self.parent = parent

    def undecorated_method(self, pose, frame):
        pass

    @memoize
    def decorated_method(self, pose, frame):
        pass

我发现可以访问它的唯一方法是执行A.__dict__["decorated_method"]。尝试 getattr(A, "decorated_method") 或 getattr(A(5), "decorated_method") 或 A.decorated_method 等都失败了：

TypeError: __get__() takes exactly 2 arguments (3 given)

真实代码的实际回溯是：

Traceback (most recent call last):
  File "./regenerate_launch_files.py", line 145, in <module>
    main()
  File "./regenerate_launch_files.py", line 130, in main
    verify_coeffs(method, past_image_keys)
  File "./regenerate_launch_files.py", line 117, in verify_coeffs
    if not (inspect.ismethod(getattr(evaluator, component))
TypeError: __get__() takes exactly 2 arguments (3 given)

调用未修饰的方法没有问题。

>>> getattr(A, "undecorated_method")
<unbound method __main__.A.undecorated_method>

（在 Python 3 中，“未修饰的方法”会给出 <function __main__.A.undecorated_method>，但 getattr(A, "decorated_method") 仍然会失败并返回 TypeError: __get__() takes 2 positional arguments but 3 were given。）

可能是什么原因造成的？我怎样才能找到给出的论点是什么？如何调试和/或修复它？

Answer 1

getattr 将以下参数传递给memoize 的__get__：

* `self`
* `None`
* `<class '__main__.A'>`

这就是导致错误的原因。修复它：

def __get__(self, instance, owner):
    if instance is None:
        return self.func
    return partial(self, instance)