Python检查调用函数时使用的输出参数的数量

Question

在python中有没有一种方法可以从被调用函数内部检查调用函数的输出参数的数量？在

例如：

a,b = Fun() #-> number of output arguments would be 2
a,b,c = Fun() #-> number of output arguments would be 3

在matlab中，这将使用nargout 我知道这样做的"normal way"是将不需要的值解压到变量中：

^{pr2}$

我要做的很简单。我有一个函数，如果用某些参数调用，它将返回一个对象，否则将返回两个对象：

def f(a,b=None):
    if b is None:
        return 1
    else:
        return 1,2

但我想强制元组解包不要发生，并强制错误，例如：

x = f(a) #-> Fine
x,y = f(a,b) #-> Fine
x,y = f(a) #-> Will throw native error: ValueError: need more than Foo values to unpack
x = f(a,b) #-> Want to force this to throw an error and not default to the situation where x will be a tuple.

Answer 1

正如阿什维尼指出的那样，这似乎奏效了：

import inspect,dis

def expecting():
    """Return how many values the caller is expecting"""
    f = inspect.currentframe()
    f = f.f_back.f_back
    c = f.f_code
    i = f.f_lasti
    bytecode = c.co_code
    instruction = bytecode[i+3]
    if instruction == dis.opmap['UNPACK_SEQUENCE']:
        howmany = bytecode[i+4]
        return howmany
    elif instruction == dis.opmap['POP_TOP']:
        return 0
    return 1

def cleverfunc():
    howmany = expecting()
    if howmany == 0:
        print("return value discarded")
    if howmany == 2:
        return 1,2
    elif howmany == 3:
        return 1,2,3
    return 1

def test():
    cleverfunc()
    x = cleverfunc()
    print(x)
    x,y = cleverfunc()
    print(x,y)
    x,y,z = cleverfunc()
    print(x,y,z)

test()