Python 函数组合（最大递归深度错误，作用域？）

Question

这个函数有什么问题呢？看起来像是作用域错误（虽然我以为通过把每个可调用的函数放在列表里，而不是直接使用它，已经修复了这个问题）。错误信息是达到最大递归深度（当调用comp(inv, dbl, inc)时）...

注意：问题是：为什么它会递归，而不是为什么会达到最大深度...

def comp(*funcs):
    if len(funcs) in (0,1):
        raise ValueError('need at least two functions to compose')
    # get most inner function
    composed = []
    print("appending func 1")
    composed.append(funcs[-1])
    # pop last and reverse
    funcs = funcs[:-1][::-1]
    i = 1
    for func in funcs:
        i += 1
        print("appending func %s" % i)
        composed.append(lambda *args, **kwargs: func(composed[-1](*args,**kwargs)))
    return composed[-1]

def inc(x):
    print("inc called with %s" % x)
    return x+1
def dbl(x):
    print("dbl called with %s" % x)
    return x*2
def inv(x):
    print("inv called with %s" % x)
    return x*(-1)

if __name__ == '__main__':
    comp(inv,dbl,inc)(2)

错误追踪（如果有帮助的话）：

appending func 1
appending func 2
appending func 3
Traceback (most recent call last):
  File "comp.py", line 31, in <module>
    comp(inv,dbl,inc)(2)
  File "comp.py", line 17, in <lambda>
    composed.append(lambda *args, **kwargs: func(composed[-1](*args,**kwargs)))
  File "comp.py", line 17, in <lambda>
    composed.append(lambda *args, **kwargs: func(composed[-1](*args,**kwargs)))
  File "comp.py", line 17, in <lambda>
    composed.append(lambda *args, **kwargs: func(composed[-1](*args,**kwargs)))
  (...)
  File "comp.py", line 17, in <lambda>
    composed.append(lambda *args, **kwargs: func(composed[-1](*args,**kwargs)))
RuntimeError: maximum recursion depth exceeded while calling a Python object

Answer 1

我不太明白你为什么一开始要生成这么多函数。其实你的代码可以简化成一个简单的版本：

def compose(*funcs):
    if len(funcs) in (0,1):
        raise ValueError('need at least two functions to compose')

    # accepting *args, **kwargs in a composed function doesn't quite work
    # because you can only pass them to the first function.
    def composed(arg):
        for func in reversed(funcs):
            arg = func(arg)
        return arg

    return composed

# what's with the lambdas? These are functions already ...
def inc(x):
    print("inc called with %s" % x)
    return x+1
def dbl(x):
    print("dbl called with %s" % x)
    return x*2
def inv(x):
    print("inv called with %s" % x)
    return -x

if __name__ == '__main__':
    f = compose(inv,dbl,inc)
    print f(2)
    print f(3)

Answer 2

你创建的这个lambda函数会对composed变量形成一个闭包：

composed.append(lambda *args, **kwargs: func(composed[-1](*args,**kwargs)))

这意味着当你创建这个lambda函数时，composed[-1]并不会被计算，而是在你调用它的时候才会被计算。这样一来，composed[-1]就会不断地自我调用，形成递归。

你可以通过使用一个辅助函数（它有自己的作用域）来解决这个问题，从而创建lambda函数：

def comp2(f1, f2):
    return lambda *args, **kwargs: f1(f2(*args, **kwargs))

...
for func in funcs:
     composed.append(comp2(func, composed[-1]))