Best way to ensure user to specify one and only one of the two function parameters in Python

Question

In some_function, I have two parameters freq and frac, I don't want users to specify both of them, or none of them. I want them to specify just one of them.

Here is the working code:

def some_function(freq=False, frac=False):
    if (freq is False) & (frac is False):
        return (str(ValueError)+': Both freq and frac are not specified')
    elif (freq is not False) & (frac is not False):
        return (str(ValueError)+': Both freq and frac are specified')
    elif (freq is not False) & (frac is False):
        try:
            print ('Do something')
        except Exception as e:
            print (e)
    elif (freq is False) & (frac is not False):
        try: 
            print ('Do something else')
        except Exception as e:
            print (e)
    else: return (str(ValueError)+': Undetermined error')

Are there better and less verbose practices to express this in Python?

Python [needs a logical xor](https://stackoverflow.com/q/432842/2564301) ... — Jongware, Nov 09 '18 at 15:53

score 2 · Accepted Answer · answered Nov 09 '18 at 15:34

2

You can use assert before your if statement. The type of your inputs is unclear; in general, I would use None if I know this isn't a valid input.

def some_function(freq=None, frac=None):

    freq_flag = freq is not None
    frac_flag = frac is not None

    assert freq_flag + frac_flag == 1, "Specify exactly one of freq or frac"

    if freq_flag:
        print('Do something')

    elif frac_flag:
        print('Do something else')

answered Nov 09 '18 at 15:34

jpp

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Doesn't save many lines, but makes a lot more readable coding. Thanks very much! – KubiK888 Nov 09 '18 at 17:16

score 2 · Answer 2 · answered Nov 09 '18 at 15:37

There's a lot you're doing wrong here. You can test not frac instead of frac is False, you should be using logical and instead of bitwise &, and you should be raising those ValueErrors, not returning them:

def some_function(freq=False, frac=False):
    if not freq and not frac:
        raise ValueError('Both freq and frac are not specified')
    elif freq and frac:
       raise ValueError('Both freq and frac are specified')
    elif freq:      
        print ('Do something')
    else:
        print ('Do something else')

Generally though, you're looking for one of two options. Why not require that the user pass a single boolean, then represents freq if True and frac if False?

def some_function(freq):
    if freq: 
        print ('Do something')
    else:
        print ('Do something else')

If the same argument doubles for `freq` OR `fraq`, you still need an argument `flag` to indicate whether that first argument is `freq` or `fraq`. I think the use case is not with Booleans but with an actual value (number, string, or list) instead, for only one of the two. — Jongware, Nov 09 '18 at 15:55
True, the real user case is to have None or actual float value. But thanks very much for pointing out my errors in coding (never cease to learn the nuances). — KubiK888, Nov 09 '18 at 16:00

score 0 · Answer 3 · answered Nov 09 '18 at 16:01

Dead simple pythonic solution: use two distinct functions (they can eventually just be facades for the real one):

__all__ = ["freqfunc", "fracfunc"]

# private implementation
def _somefunc(freq=False, frac=False):
   # your code here

def freqfunc(freq):
    return _somefunc(freq=freq)

def fraqfunc(frac):
    return _somefunc(frac=frac)

Now there might be better solutions but it's impossible to tell without more details...

Best way to ensure user to specify one and only one of the two function parameters in Python

3 Answers3