python-2.7 - functools.wrapper - AttributeError : attribute '__doc__' of 'type' objects is not writable

Question

在执行下面的代码时，我得到 AttributeError: attribute '__doc__' of 'type' objects is not writable .

from functools import wraps

def memoize(f):
    """ Memoization decorator for functions taking one or more arguments.
        Saves repeated api calls for a given value, by caching it.
    """
    @wraps(f)
    class memodict(dict):
       """memodict"""
       def __init__(self, f):
           self.f = f
       def __call__(self, *args):
           return self[args]
       def __missing__(self, key):
           ret = self[key] = self.f(*key)
           return ret
     return memodict(f)

@memoize
def a():
    """blah"""
    pass

追溯:

AttributeError Traceback (most recent call last)
<ipython-input-37-2afb130b1dd6> in <module>()
     17             return ret
     18     return memodict(f)
---> 19 @memoize
     20 def a():
     21     """blah"""

<ipython-input-37-2afb130b1dd6> in memoize(f)
      7     """
      8     @wraps(f)
----> 9     class memodict(dict):
     10         """memodict"""
     11         def __init__(self, f):

/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/functools.pyc in update_wrapper(wrapper, wrapped, assigned, updated)
     31     """
     32     for attr in assigned:
---> 33         setattr(wrapper, attr, getattr(wrapped, attr))
     34     for attr in updated:
     35         getattr(wrapper, attr).update(getattr(wrapped, attr, {}))

AttributeError: attribute '__doc__' of 'type' objects is not writable

即使提供了文档字符串，我也不知道这有什么问题。

如果没有包装它工作正常，但我需要这样做。

Answer 1

@wraps(f)主要设计为用作函数装饰器，而不是用作类装饰器，因此将其用作后者可能会导致偶尔出现奇怪的怪癖。

您收到的特定错误消息与 Python 2 上的内置类型限制有关:

>>> class C(object): pass
... 
>>> C.__doc__ = "Not allowed"
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: attribute '__doc__' of 'type' objects is not writable

如果你使用 Python 3，切换到 Python 2 中的经典类(通过继承 UserDict.UserDict 而不是 dict 内置)，或者使用闭包来管理结果缓存而不是类实例，装饰器将能够从底层函数复制文档字符串。