用另一个类装饰一个python类方法

Posted 2023-02-23

【中文标题】用另一个类装饰一个python类方法

【英文标题】：decorate a python class method with another class

【发布时间】：2021-12-26 13:02:55

【问题描述】：

我目前正在研究一个 Python 类，用作类方法的装饰器。 在这种情况下，我遇到了一个我很难理解的问题。 举个例子：

from functools import partial

class Decorator:
    def __init__(self, func = None, *args):
        self.uses_init = func is None
        self.func = func
        self.instance = None
        self.args = args
    
    def __call__(self, *args, **kwargs):
        self.func = args[0]
        
        def wrapper(cls, *args, **kwargs):
            print('before')
            parsed_func = self.func(cls, *args, **kwargs)
            print('after')
            return parsed_func
        
        return wrapper
    
    def call(self, cls, *args, **kwargs):
        print('before')
        parsed_func = self.func(cls, *args, **kwargs)
        print('after')
        return parsed_func       

    def __get__(self, instance, owner):
        # This is only used when uses_init == False
        return partial(self.call, instance)

可以不带参数使用@Decorator或带参数@Decorator(*args)

使用此代码按预期工作：

class HelloWorld:
    @Decorator()
    def print(self, name):
        print(name)
hello_world = HelloWorld()
hello_world.print("Max Musterman")
print('----------------------')
class HelloWorld:
    @Decorator
    def print(self, name):
        print(name)
hello_world = HelloWorld()
hello_world.print("Max Musterman")

我想使用self.call 方法代替wrapper 以避免重复代码。当我尝试这个时：

class Decorator:
    def __init__(self, func = None, *args):
        self.uses_init = func is None
        self.func = func
        self.instance = None
        self.args = args
    
    def __call__(self, *args, **kwargs):
        self.func = args[0]        
        return self.call
    
    def call(self, cls, *args, **kwargs):
        print('before')
        parsed_func = self.func(cls, *args, **kwargs)
        print('after')
        return parsed_func       

    def __get__(self, instance, owner):
        # This is only used when uses_init == False
        return partial(self.call, instance)

尽管检查了inspect.signature(Decorator().call) 和inspect.signature(Decorator()(lambda: None)) 方法的签名，但我遇到了TypeError: print() missing 1 required positional argument: 'name' 给出了相同的结果。

【参考方案1】：

装饰器无法获取参数。

>>> class MyClass:
...     def __init__(self, func, *args):
...         print(args)
...
>>> @MyClass
... def my_func():
...     pass
... 
()
>>> @MyClass('args1', 'args2', 'args3')
... def my_func():
...     pass
... 
('args2', 'args3')
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
TypeError: MyClass object is not callable
>>>

您可以使用函数或方法来制作装饰器。 在它之前“如何解决TypeError: MyClass object is not callable？”：

>>> def my_decorator(*args **kwargs):
...     def func_getter(func):
...         return func(*args, **kwargs)
...     return func_getter  # mine decorator
... 
>>> @my_decorator('arg1', 'arg2', 'arg3', kwarg1=1, kwarg2=2) # -> func_getter
... def print_row(*args, **kwargs):
...     print(args, kwargs)
... 
('arg1', 'arg2', 'arg3') 'kwarg1': 1, 'kwarg2': 2

解决方案一：使用函数制作装饰器：

>>> class MyClass:
...     def __init__(self, func = None, *args): # copied form your code
...         ... # your code here
... 
>>> def decorator(*args):
...     def func_getter(func):
...         return MyClass(func, *args)
...     return func_getter
... 
>>> @decorator
... def test(): ...
... 
>>> test
<MyClass object at 0x0000000000000000>

解决方案一：使用方法制作装饰器：

>>> class MyClass:
...     def __init__(self, func = None, *args): # copied form your code
...         ... # your code here
...     @classmethod # romove this line in python3.10
...     def create(cls, *args):
...         def func_getter(func):
...             return cls(func, *args) # "cls" is "MyClass" instance
... 
>>> @MyClass.create('args1', 'arg2', ...)
... def test(): ...
... 
>>> test
<MyClass object at 0x0000000000000000>

【参考方案2】：

感谢@Delta 的回答，我能够编写一个装饰器（-maker）来满足我的需求

class Decorator:
    def __init__(self, func = None, text_before = "start", text_after = "end"):
        self.func = func
        self.text_before = text_before
        self.text_after = text_after
    
    def __call__(self, func):
        return self.create(func, text_before=self.text_before, text_after=self.text_after)
    
    @classmethod
    def create(cls, func, *args, **kwargs):
        return cls(func, *args, **kwargs)
    
    def call(self, cls, *args, **kwargs):
        print(f'--- self.text_before ---')
        parsed_func = self.func(cls, *args, **kwargs)
        print(f'--- self.text_after ---')
        return parsed_func       

    def __get__(self, instance, owner):
        return partial(self.call, instance)