Python函数组合（最大递归深度错误，范围？）

Question

这个函数有什么问题吗？这似乎是一个范围错误（尽管我认为我已经通过将每个可调用对象放在列表中而不是直接使用它来修复了这个问题）。错误是达到最大递归深度（调用 comp(inv,dbl,inc) 时）...

注意：问题是：为什么它甚至递归，而不是为什么它达到最大深度...

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)

回溯（如果有帮助）：

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

您创建的 lambda 函数在 composed 变量上构建一个闭包：

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

这意味着当您创建时，不会评估 composed[-1] lambda 函数，但是当你调用它时。结果是，composed[-1] 将一次又一次地递归调用自身。

您可以通过使用辅助函数（具有自己的作用域）来创建 lambda 函数来解决此问题：

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

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

Answer 2

我不知道为什么你一开始会生成很多函数。您的代码有一个简单版本：

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)