为什么要使用partial给函数添加属性答案

【问题标题】：Why is partial used to add attributes to functions为什么要使用partial给函数添加属性
【发布时间】：2020-06-21 19:58:03
【问题描述】：

前几天看了this的文章，不明白为什么在“Logging Decorators”部分使用partial：

def attach_wrapper(obj, func=None):  # Helper function that attaches function as attribute of an object
    if func is None:
        return partial(attach_wrapper, obj)
    setattr(obj, func.__name__, func)
    return func

attach_wrapper 未设置用于包装另一个函数的函数的某些属性：

@attach_wrapper(wrapper)  # Attaches "set_level" to "wrapper" as attribute
def set_level(new_level):  # Function that allows us to set log level
    nonlocal level
    level = new_level

为什么不能在wrapper 和setattr 上直接设置这些属性？

【问题讨论】：

标签： python python-decorators

【解决方案1】：

它的用途是，如果在没有要包装的函数的情况下调用 attach_wrapper 函数，它将返回一些内容，然后您可以使用其他函数调用这些内容以将多个附加到目标对象。例如，如果您有三个函数f、g 和h，以及一个对象foo，您可以使用attach_to_foo = attach_wrapper(foo) 创建一个“附加器”，然后将它与attach_to_foo(f)、@987654328 一起使用@ 和 attach_to_foo(h) 以 foo.f、foo.g 和 foo.h 结尾。

【讨论】：

【解决方案2】：

如果您检测attach_wrapper，您可以看到它的func 参数是None。

def attach_wrapper(obj, func=None):  # Helper function that attaches function as attribute of an object
    if func is None:
        print(f'obj is {obj.__name__} - func is',func)
        return partial(attach_wrapper, obj)
    print(f'a_w:obj is {obj.__name__} - func is {func.__name__}')
    setattr(obj, func.__name__, func)
    return func

然后

@log(logging.WARN, "example-param")
def somefunc(args):
    return args

结果

>>>
obj is somefunc - func is None
a_w:obj is somefunc - func is set_level
obj is somefunc - func is None
a_w:obj is somefunc - func is set_message
>>>

attach_wrapper 被调用两次。第一次它的func 是None，它返回一个带有第一个参数obj 的部分函数attach_wrapper 设置为 some_func。然后调用部分函数（装饰器机制的所有部分）并装饰函数。

attach_wrapper's obj 看起来像 some_func 但实际上是 some_func 包裹 wrapper - 看起来和感觉都一样，因为这就是 @wraps 的用途.

这里是更多细节，将inspect.stack 添加到instrumentation。

import inspect

def attach_wrapper(obj, func=None):  # Helper function that attaches function as attribute of an object
    print("called from: ", inspect.stack()[1].code_context[0].strip())
    print("\tby function ", inspect.stack()[1].function)
    if func is None:
        print(f'\tobj is {obj.__name__}, func is {func}\n')
        return partial(attach_wrapper, obj)
    print(f'\tobj is {obj.__name__}, func is {func.__name__}\n')
    setattr(obj, func.__name__, func)
    return func

哪个产生

>>>
called from:  @attach_wrapper(wrapper)  # Attaches "set_level" to "wrapper" as attribute
    by function  decorate
    obj is somefunc, func is None

called from:  def set_level(new_level):  # Function that allows us to set log level
    by function  decorate
    obj is somefunc, func is set_level

called from:  @attach_wrapper(wrapper)  # Attaches "set_message" to "wrapper" as attribute
    by function  decorate
    obj is somefunc, func is None

called from:  def set_message(new_message):  # Function that allows us to set message
    by function  decorate
    obj is somefunc, func is set_message

>>>

周围有一些更好的详细解释。如果我找到了，我会更新。

【讨论】：