Python 为什么使用partial向函数添加属性

前几天我读了这篇文章，我不明白为什么在“日志装饰器”一节中使用了

partial

：

attach\u wrapper

未设置，用于在包装另一个函数的函数上设置某些属性：

@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

---

为什么不能用

setattr

直接在

wrapper

上设置这些属性？

如果插入

attach\u wrapper

，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

导致

>>>
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\u wrapper

被调用两次。它的

func

第一次是

None

，它返回一个分部函数

attach\u wrapper

，其第一个参数

obj

设置为

some\u func

。然后调用分部（装饰器机制的所有部分），并对函数进行装饰

attach\u wrapper的

obj

看起来像

some\u func

，但实际上它是

some\u func

用

wrapper

包装的-它的外观和感觉都是一样的，因为这就是它的用途

---

下面是将

inspect.stack

添加到仪器的更多细节

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

>>>

---

---

这里有一些更详细的解释。如果我找到一个，我将进行更新。

它的使用方式是，如果调用

attach\u wrapper

函数时没有要包装的函数，它将返回一些东西，然后您可以使用其他函数调用这些东西，以便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\u to\u foo=attach\u wrapper（foo）

创建一个“attacher”，然后将其与

attach\u to\u foo（f）

、

attach\u to\u foo（g）

和

attach\u to\u foo（h）一起使用

以

foo.f

、

foo.g

和

foo.h

结束

>>>
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
>>>

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

>>>
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

>>>