装饰可调用类的属性访问答案

【问题标题】：Attribute access on a decorated callable class装饰可调用类的属性访问
【发布时间】：2018-10-06 09:51:29
【问题描述】：

我有一个可调用的类：

class CallMeMaybe:

    __name__ = 'maybe'

    def __init__(self):
        self.n_calls = 0

    def __call__(self):
        self.n_calls += 1
        raise Exception

这似乎像宣传的那样工作：

>>> f = CallMeMaybe()
>>> f.n_calls
0
>>> for i in range(7):
...     try:
...         f()
...     except Exception:
...         pass
...     
>>> f.n_calls
7

我想用指数装饰它backoff：

from backoff import on_exception, expo
dec = on_exception(expo, Exception, max_tries=3, on_backoff=print)
f = CallMeMaybe()
f2 = dec(f)

现在看起来属性访问停止工作了：

>>> f2.n_calls
0
>>> f2()
{'target': <__main__.CallMeMaybe object at 0xcafef00d>, 'args': (), 'kwargs': {}, 'tries': 1, 'elapsed': 2.1e-05, 'wait': 0.4843249208229148}
{'target': <__main__.CallMeMaybe object at 0xcafef00d>, 'args': (), 'kwargs': {}, 'tries': 2, 'elapsed': 0.484935, 'wait': 1.6524016553598126}
---------------------------------------------------------------------------
Exception                                 Traceback (most recent call last)
... blah blah blah
>>> f2.n_calls
0

我的问题：谁将n_calls 名称复制到f2 的命名空间中，为什么？现在它拥有一个陈旧的值 - 正确的值应该是 3：

>>> f2.__wrapped__.n_calls
3

【问题讨论】：

标签： python decorator python-decorators exponential-backoff

【解决方案1】：

backoff 模块在其实现 uses functools.wraps 中调用 functools.update_wrapper，您可以看到 from the source code 默认情况下它会更新包装器的 __dict__：

WRAPPER_ASSIGNMENTS = ('__module__', '__name__', '__qualname__', '__doc__',
                       '__annotations__')
WRAPPER_UPDATES = ('__dict__',)
def update_wrapper(wrapper,
                   wrapped,
                   assigned = WRAPPER_ASSIGNMENTS,
                   updated = WRAPPER_UPDATES):
    """Update a wrapper function to look like the wrapped function

       wrapper is the function to be updated
       wrapped is the original function
       assigned is a tuple naming the attributes assigned directly
       from the wrapped function to the wrapper function (defaults to
       functools.WRAPPER_ASSIGNMENTS)
       updated is a tuple naming the attributes of the wrapper that
       are updated with the corresponding attribute from the wrapped
       function (defaults to functools.WRAPPER_UPDATES)
    """
    for attr in assigned:
        try:
            value = getattr(wrapped, attr)
        except AttributeError:
            pass
        else:
            setattr(wrapper, attr, value)
    for attr in updated:
        getattr(wrapper, attr).update(getattr(wrapped, attr, {}))
    +
−# Issue #17482: set __wrapped__ last so we don't inadvertently copy it
    # from the wrapped function when updating __dict__
    wrapper.__wrapped__ = wrapped
    # Return the wrapper so this can be used as a decorator via partial()
    return wrapper

def wraps(wrapped,
          assigned = WRAPPER_ASSIGNMENTS,
          updated = WRAPPER_UPDATES):
    """Decorator factory to apply update_wrapper() to a wrapper function

       Returns a decorator that invokes update_wrapper() with the decorated
       function as the wrapper argument and the arguments to wraps() as the
       remaining arguments. Default arguments are as for update_wrapper().
       This is a convenience function to simplify applying partial() to
       update_wrapper().
    """
    return partial(update_wrapper, wrapped=wrapped,
                   assigned=assigned, updated=updated)

不幸的是，似乎不可能实现您想要的。 backoff 模块可以允许将可选的assigned/updated 属性列表传递给wraps，以避免复制属性。但是，这确实可以解决问题，因为那时您将无法访问n_calls。

您可能需要使用可变对象而不是普通的int。

【讨论】：