How to check for multiple values return in Python?

Question

I need to do some introspection in Numpy/Scipy. While it is relatively easy to find info on how to get the help docstring and the arguments, I was not able to get anything concerning how to get info on the returned values. More specifically, I just would like to find which functions return multiple values, or equivalently (more or less) tuples. Any way to do it?

As far as I know, there is no way from Python. Could you just check the documentation? — birdoftheday, Jan 07 '16 at 14:28
Not sure what you need... May http://stackoverflow.com/questions/9752958/how-can-i-return-two-values-from-a-function-in-python help? — BAE, Jan 07 '16 at 14:34
also http://stackoverflow.com/questions/919680/can-a-variable-number-of-arguments-be-passed-to-a-function — BAE, Jan 07 '16 at 14:36
In dynamic languages like Python you cannot infer type of the return value without running the code. The only thing you can try and pray is `help(function_name)`. Otherwise you have to run the fucntion and see what it returns — vrs, Jan 07 '16 at 14:39

score 0 · Answer 1 · answered Jan 07 '16 at 14:57

There is no way you can find this out from Python in general. The answer is not a constant.

def complicated(i):
    if i == 1:
        return 0
    elif i == 2:
        return (0,1)
    elif i == 3:
        return [0,1,2]
    elif program_halts(i):
        return {}
    else
        return "Nope"

What is worse, even if you know the inputs, you can't tell the result without solving the halting problem.

Your only chance is to read the documentation.

score 0 · Answer 2 · answered Jan 07 '16 at 14:58

The documentation ought to tell you, but you can test at runtime if you really need to:

 r = something()

 if type(r)==tuple or type(r)==list:
    for rn in r:
       # do something with each returned value
       # (which might include further checking if it's a list or a tuple)

 elif type(r)==int or type(r)==float:
    # it's a scalar numeric value

 else:
     raise ValueError( "Can't handle {}".format(r) )

In passing a function that returns a list or a tuple should return a list or a tuple of length 0 or 1 to indicate that there was nothing or only one value to return. I have seen codes where you get None, a scalar, or a tuple/list, which makes using the result unnecessarily complex!

How to check for multiple values return in Python?

2 Answers2