[每日一氵] Python Function __get__ 方法在做什么

群里的大佬同学讨论了一个这样的问题：

源码取自：
https://github.com/open-mmlab/mmcv/blob/master/mmcv/cnn/utils/flops_counter.py

以下这个 函数.__get__ 的用法是啥?

def add_flops_counting_methods(net_main_module):
    # adding additional methods to the existing module object,
    # this is done this way so that each function has access to self object
    net_main_module.start_flops_count = start_flops_count.__get__(   # <---- 以下几处 __get__ 什么意思
        net_main_module)
    net_main_module.stop_flops_count = stop_flops_count.__get__(
        net_main_module)
    net_main_module.reset_flops_count = reset_flops_count.__get__(
        net_main_module)
    net_main_module.compute_average_flops_cost = compute_average_flops_cost.__get__(  # noqa: E501
        net_main_module)

    net_main_module.reset_flops_count()

    return net_main_module

而 start_flops_count 函数是这样定义的

def start_flops_count(self):
    """Activate the computation of mean flops consumption per image.

    A method to activate the computation of mean flops consumption per image.
    which will be available after `add_flops_counting_methods()` is called on a
    desired net object. It should be called before running the network.
    """
    add_batch_counter_hook_function(self)

    def add_flops_counter_hook_function(module):
        if is_supported_instance(module):
            if hasattr(module, '__flops_handle__'):
                return

            else:
                handle = module.register_forward_hook(
                    MODULES_MAPPING[type(module)])

            module.__flops_handle__ = handle

    self.apply(partial(add_flops_counter_hook_function))

凭感觉，这样做的作用是 相当于在外部写了一个类内的方法，注释也说了：

adding additional methods to the existing module object,
this is done this way so that each function has access to self object

这个 __get__ 到底做了什么呢?

参考官方文档：
https://docs.python.org/zh-cn/3/howto/descriptor.html
(我竟然没看到后面部分，然后去翻了CPython源码… 我同学找到的)

class Function:
    ...

    def __get__(self, obj, objtype=None):
        "Simulate func_descr_get() in Objects/funcobject.c"
        if obj is None:
            return self
        return MethodType(self, obj)

而 MethodType 是什么：

class MethodType:
    "Emulate PyMethod_Type in Objects/classobject.c"

    def __init__(self, func, obj):
        self.__func__ = func
        self.__self__ = obj

    def __call__(self, *args, **kwargs):
        func = self.__func__
        obj = self.__self__
        return func(obj, *args, **kwargs)

这里相当于把参数传入函数中，将该函数返回，但未执行，感觉像是：

from functools import partial
partial(func, arg)

在举个例子：

def print_char(char):
	print(char)
	return char

print_A = print_char.__get__("A")
print_A()

A