Question

如何优雅地定义一个只需要一个且只有一个命名参数的函数？

def foo (a=None, b=None, c=None):
    if a and b in None and c is None:
        return something for a
    if a in None and b and c is None:
        return something for b
    if a is None and b in None and c:
        return something for c
    raise ValueError("foo requires one and only one of a, b and c",a,b,c)

比

更好的东西

def foo (**args):
    if len(args) != 1:
        raise ValueError("foo: too many or too few args",args)
    arg,val = args.iteritems().next()
    if arg == "a":
        return something for a
    if arg == "b":
        return something for b
    if arg == "c":
        return something for c
    raise ValueError("foo: unknown argument",arg,val)

另请注意，不仅第二个版本不短于第一个版本，它还可以防止像pylint这样的静态检查程序检测到可接受的参数列表。

Answer 1

非常简单，您只需计算None s：

if [a,b,c].count(None) != 2:
    raise ValueError("foo requires exactly one argument among a,b and c.")

Answer 2

此代码可让您轻松判断哪个参数不是无。

def foo(a=None, b=None, c=None):
    paramDict = {'a': a, 'b' : b, 'c' : c}
    matches = [x for x in paramDict if paramDict[x] != None]
    if len(matches) != 1:
        raise ValueError("foo requires one and only one of a, b and c")
    else:
        print("parameter %s == %s" % (matches[0], paramDict[matches[0]]))

如何强制论证互斥？

2 个答案: