老版functional.curry有没有精确替代品？

Question

我正在尝试在 Python 2.6 环境中运行这个代码片段，来源于 http://www.ibm.com/developerworks/linux/library/l-prog3.html。

from functional import *

taxcalc = lambda income,rate,deduct: (income-(deduct))*rate

taxCurry = curry(taxcalc)
taxCurry = taxCurry(50000)
taxCurry = taxCurry(0.30)
taxCurry = taxCurry(10000)
print "Curried taxes due =",taxCurry

print "Curried expression taxes due =", \
      curry(taxcalc)(50000)(0.30)(10000)

好的，我从 http://www.python.org/dev/peps/pep-0309/ 了解到，functional 被改名为 functools，而 curry 被改名为 partial，但仅仅改名字并没有解决问题。我遇到了这个错误：

taxCurry = taxCurry(50000)
TypeError: <lambda>() takes exactly 3 arguments (1 given)

下面的代码可以正常工作，但我真的需要改得这么多吗？

from functools import partial

taxcalc = lambda income,rate,deduct: (income-(deduct))*rate

taxCurry = partial(taxcalc)
taxCurry = partial(taxCurry, 50000)
taxCurry = partial(taxCurry, 0.30)
taxCurry = partial(taxCurry, 10000)
print "Curried taxes due =", taxCurry()

print "Curried expression taxes due =", \
      taxcalc(50000, 0.30, 10000)

有没有更好的方法来保留原始示例的机制？最后，原始示例真的算是柯里化（currying）吗，还是只是部分应用（partial application）？（根据 http://www.uncarved.com/blog/not_currying.mrk）

谢谢你的时间

Answer 1

我写了一个可以使用的柯里化装饰器，效果很好：

def curry(func):
    """
    Decorator to curry a function, typical usage:

    >>> @curry
    ... def foo(a, b, c):
    ...    return a + b + c

    The function still work normally:
    >>> foo(1, 2, 3)
    6

    And in various curried forms:
    >>> foo(1)(2, 3)
    6
    >>> foo(1)(2)(3)
    6

    This also work with named arguments:
    >>> foo(a=1)(b=2)(c=3)
    6
    >>> foo(b=1)(c=2)(a=3)
    6
    >>> foo(a=1, b=2)(c=3)
    6
    >>> foo(a=1)(b=2, c=3)
    6

    And you may also change your mind on named arguments,
    But I don't know why you may want to do that:
    >>> foo(a=1, b=0)(b=2, c=3)
    6

    Finally, if you give more parameters than expected, the exception
    is the expected one, not some garbage produced by the currying
    mechanism:

    >>> foo(1, 2)(3, 4)
    Traceback (most recent call last):
       ...
    TypeError: foo() takes exactly 3 arguments (4 given)
    """
    def curried(*args, **kwargs):
        if len(args) + len(kwargs) >= func.__code__.co_argcount:
            return func(*args, **kwargs)
        return (lambda *args2, **kwargs2:
                curried(*(args + args2), **dict(kwargs, **kwargs2)))
    return curried


if __name__ == "__main__":
    import doctest
    doctest.testmod()

Answer 2

toolz项目中的curry实现可以直接替换使用。

$ pip install toolz
>>> from toolz import curry

Answer 3

我想他们之所以做出这个改变，是因为Python是一种动态类型的语言。这意味着如果原来的curry代码出现问题，调试起来会非常困难——比起像Haskell这样的语言，Haskell会直接给你一个很清晰的类型错误提示。所以我觉得用更明确的partial版本来替代它是个合理的决定（看起来更符合Python的风格）。

你的例子也有点奇怪，因为你只是把部分应用的函数重新赋值给了同一个名字。通常情况下，部分应用的函数应该传递给另一个函数。至少在我能想到的Python中，这才是合理的用法。