How to check in python that at least one of the default parameters of the function specified

Question

What is the best practice in python to check if at least one of the default parameters of the function is specified?

Let's suppose we have some function:

def some_function(arg_a=None, arg_b=None, arg_c=False)

with some default parameters. In my case, I need to check if either arg_a or arg_b is specified. So I thought of implementing something like this:

def some_function(arg_a=None, arg_b=None, arg_c=False):
    ...
    if arg_a is not None:
        ...
    elif arg_b is not None:
        ...
    else:
        raise ValueError('Expected either arg_a or arg_b args')
    ...
    ...

So, what is more pythonic way to implement such kind of functionality?

Answer 1

You could use all to check if they all equal None and raise the ValueError:

if all(v is None for v in {arg_a, arg_b}):
    raise ValueError('Expected either arg_a or arg_b args')

this gets rid of those if-elif clauses and groups all checks in the same place:

f(arg_a=0) # ok    
f(arg_b=0) # ok
f()        # Value Error  

Alternatively, with any():

if not any(v is not None for v in {arg_a, arg_b}):
    raise ValueError('Expected either arg_a or arg_b args')

but this is definitely more obfuscated.

In the end, it really depends on what the interpretation of pythonic actually is.