装饰器参数中无法识别的类

Posted 2023-02-23

【中文标题】装饰器参数中无法识别的类

【英文标题】：Class not recognized in decorator arguments

【发布时间】：2022-01-16 00:51:06

【问题描述】：

将类变量作为装饰器函数的参数传递会导致类名称为NameError。运行这个：

def log(prefix=None):

    def decorator(function):
        """Decorates the function"""

        def wrapper(*args, **params):
            """Wraps the function"""
            name = "-".join([prefix, function.__name__])

            result = function(*args, **params)
            print(f"completed the execution of 'name'")
            return result

        return wrapper

    return decorator
    
class ExampleClass:

    _CONSTANT = "test"

    def __init__(self, x):
        self._x = x

    @log(prefix=ExampleClass._CONSTANT)
    def product_of_number(self, y):
        return self._x * y

if __name__ == "__main__":
    x = ExampleClass(3)
    x.product_of_number(4)

导致错误

Traceback (most recent call last):
  File "/home/developer/reproduce_decorator_name_space.py", line 23, in <module>
    class ExampleClass:
  File "/home/developer/reproduce_decorator_name_space.py", line 31, in ExampleClass
    @log(prefix=ExampleClass._CONSTANT)
NameError: name 'ExampleClass' is not defined

但是，运行这个

def log(prefix=None):

    def decorator(function):
        """Decorates the function"""

        def wrapper(*args, **params):
            """Wraps the function"""
            name = "-".join([prefix, function.__name__])

            result = function(*args, **params)
            print(f"completed the execution of 'name'")
            return result

        return wrapper

    return decorator
    
_CONSTANT = "test"
class ExampleClass:

    def __init__(self, x):
        self._x = x

    @log(prefix=_CONSTANT)
    def product_of_number(self, y):
        return self._x * y

if __name__ == "__main__":
    x = ExampleClass(3)
    x.product_of_number(4)

给出输出

completed the execution of 'test-product_of_number'

为什么ExampleClass 无法识别？装饰的方法在__init__ 之后的类中并引用了self。错误消息本身是指模块ExampleClass。类名怎么不存在于命名空间中？

【问题讨论】：

这能回答你的问题吗？ Python - Referencing class name from inside class body。 TL;DR，名称 ExampleClass 直到类块执行后才分配给。

你应该也可以在第一个sn-p中直接使用@log(prefix=_CONSTANT)。

也相关***.com/q/19622550/11082165

【参考方案1】：

装饰器可以访问ExampleClass，可能不需要向装饰器传递参数：

def log(prefix=''):
    def decorator(function):
        """Decorates the function"""
        def wrapper(*args, **params):
            """Wraps the function"""
            # args[0] references the decorated class

            if prefix:
                # Option2: Prefix passed as string
                name = "-".join(
                    [getattr(args[0], prefix, ''), function.__name__])
            else:
                # Option1: No prefix
                name = "-".join([args[0]._CONSTANT, function.__name__])

            result = function(*args, **params)
            print(f"completed the execution of 'name'")
            return result

        return wrapper

    return decorator


class ExampleClass:

    _CONSTANT = "test"
    _ANOTHER_CONSTANT = "test2"

    def __init__(self, x):
        self._x = x

    @log()
    def product_of_number(self, y):
        return self._x * y

    @log('_ANOTHER_CONSTANT')
    def product_of_anothernumber(self, y):
        return self._x * y


if __name__ == "__main__":
    x = ExampleClass(3)
    x.product_of_number(4)
    x.product_of_anothernumber(4)

输出：

completed the execution of 'test-product_of_number'
completed the execution of 'test2-product_of_anothernumber'