Python函数组合(最大递归深度错误,范围？)

Question

这个功能有什么问题？这似乎是一个范围错误(虽然我认为我通过将每个callable放在列表中而不是直接使用它来修复它).错误是达到最大递归深度(调用comp(inv,dbl,inc)时)...

注意:问题是:为什么它甚至会递归,而不是它达到最大深度的原因......

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composed[-1]在创建 lambda函数时,但在调用它时,不会对其进行求值.效果是,这composed[-1]将一次又一次递归地调用自己.

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]))