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

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?

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