I know that Python supports variable arguments *args and keyword arguments **kwargs but is there a way to have a default for these fields? If not, why?
*args = (1,'v')) , **kwargs = {'a':20}
I am not saying that I have a use case for this but it seems there is also no reason to disallow this. I do understand that it is trivial to get around this by checking for empty args.
No, *args and **kwargs are the catch-all arguments that receive any arguments for which there are no explicit arguments instead. Specifying defaults defeats their purpose. Instead, your example defaults can be handled with existing syntax, there is no need to complicate the function definition syntax here.
You can cover your defaults by specifying normal keyword arguments:
def foo(pos0, pos1='v', a=20, *args, **kwargs):
pass
I replaced your (1, 'v') 'default' with another keyword argument, because you can provide values for keyword arguments with a positional parameter; foo(42, 'spam') will set pos1 to 'spam'.
In addition, you can always do further processing inside the function you define; the args value is a tuple, just test for length to check if you need to use a default for missing values. kwargs is a dictionary, just use kwargs.get(key, default) or kwargs.pop(key, default) or if key not in kwargs: or any of the other dict methods to handle missing keys.
You can have default kwargs and still unpack with splat a la **kwargs
def john(name, dog, *args, bob="bob", **kwargs):
print(name, dog, bob)
# "ream" is an unpacked arg
john("john", 1, "ream") # john 1 bob
# jane is an unpacked kwarg
john("john", 1, "ream", jane="jane") # john 1 bob
john("john", 1, "ream", bob="jane") # john 1 jane
Having a default for an *arg is pretty difficult because the idea is to make the function require that input. You could look at some tricks in this vein a la the implementation for the range builtin. I would just make it a kwarg though.
