Have function reject bad argument when argument is called by another function?

[gontajones]

I'm not sure if I understood what you want, but if you want to check for a key in a dict, you can do this:

x = {"F": "Fluorine", "Cl": "Chlorine"}
y = {"O": "Oxygen"}


def check_key(d, key):
    if key in d:
        print(d[key])
        return True
    return False


def is_halogen_bond(a, b):
    return check_key(x, a) and check_key(y, b)


print(is_halogen_bond("F", "O"))