Implementing Guards using Decorators in Python

UniformGuard.py

"""
Intended to be used as a decorator, UniformGuard
ensures that all of the arguments passed to its
decorated function `_f` satisfy the guard `g`.
"""
def UniformGuard(g, e=TypeError):
    def _g(f):
        def _f(*args, **kwargs):
            if not all(map(g, set(args).union(kwargs.values()))):
                if e:
                    raise e('All arguments must satisfy {}'.format(g))
                return None
            return f(*args, **kwargs)
        return _f
    return _g

"""
Let's use UniformGuard to define a safe division
function that doesn't blow up when the denominator
is 0. Of course, in actual applications you don't
want to silence the guard, so you'll remove `None`.
"""
def is_nonzero(n):
    return n < 0 or n > 0

@UniformGuard(is_nonzero, None)
def safe_div(a, b):
    return a / b

assert safe_div(4, 2) == 2
assert safe_div(4, 0) == None