如何在python中乘法函数？

Question

def sub3(n):
    return n - 3

def square(n):
    return n * n

在python中编写函数非常简单：

>>> my_list
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> [square(sub3(n)) for n in my_list]
[9, 4, 1, 0, 1, 4, 9, 16, 25, 36]

不幸的是，当想要使用组合作为 键时 ，这有点of脚：

>>> sorted(my_list, key=lambda n: square(sub3(n)))
[3, 2, 4, 1, 5, 0, 6, 7, 8, 9]

这实际上应该只是sorted(my_list, key=square*sub3)，因为该函数__mul__无论如何都不会用于其他任何事情：

>>> square * sub3
TypeError: unsupported operand type(s) for *: 'function' and 'function'

好吧，那就定义它吧！

>>> type(sub3).__mul__ = 'something'
TypeError: can't set attributes of built-in/extension type 'function'

天哪！

>>> class CoolerFunction(types.FunctionType):
...     pass
...
TypeError: Error when calling the metaclass bases
    type 'function' is not an acceptable base type

天哪！

class Hack(object):
    def __init__(self, function):
        self.function = function
    def __call__(self, *args, **kwargs):
        return self.function(*args, **kwargs)
    def __mul__(self, other):
        def hack(*args, **kwargs):
            return self.function(other(*args, **kwargs))
        return Hack(hack)

嘿，现在我们到了某个地方。

>>> square = Hack(square)
>>> sub3 = Hack(sub3)
>>> [square(sub3(n)) for n in my_list]
[9, 4, 1, 0, 1, 4, 9, 16, 25, 36]
>>> [(square*sub3)(n) for n in my_list]
[9, 4, 1, 0, 1, 4, 9, 16, 25, 36]
>>> sorted(my_list, key=square*sub3)
[3, 2, 4, 1, 5, 0, 6, 7, 8, 9]

但是我不想Hack上课！在我不完全了解的范围内，作用域规则完全不同，这甚至比“ lameda”还要丑。我想对这些 功能
进行修补。我怎样才能做到这一点？

Answer 1

尽管您可能希望为该类选择一个更合适的名称，但是您可以将hack类几乎用作编写器的装饰器。

像这样：

class Composable(object):
    def __init__(self, function):
        self.function = function
    def __call__(self, *args, **kwargs):
        return self.function(*args, **kwargs)
    def __mul__(self, other):
        @Composable
        def composed(*args, **kwargs):
            return self.function(other(*args, **kwargs))
        return composed
    def __rmul__(self, other):
        @Composable
        def composed(*args, **kwargs):
            return other(self.function(*args, **kwargs))
        return composed

然后，您可以像这样装饰功能：

@Composable
def sub3(n):
    return n - 3

@Composable
def square(n):
    return n * n

然后像这样组成它们：

(square * sub3)(n)

基本上，这是使用hack类完成的事情，只是将其用作装饰器。