Python/Django: Are there any decorators to tell that input is not none?

Question

• I am new to Django and come from Java/Spring background.
• I am wondering if there are decorators something like following that can be done in Django or Python?

Want

def addToList(@not_none a, @not_none b):
    # so that I do not check for nullity explicitly  
    do_things_with_a(a)
    do_things_with_b(b)

• Since this is something which is pretty easy to get in Java, just looking if Python/Django has it

Answer 1

One doesn't typically constraint data-types in Python. Also, decorators can only be applied to classes and to methods/functions.

Although, you shouldn't really be doing this, this is how you would.

(You could amend this to accept argument names to enforce constraints on with a little work).

def not_none(f):
    def func(*args, **kwargs):
        if any(arg is None for arg in args):
            raise ValueError('function {}: does not take arguments of None'.format(f.__name__))
        return f(*args, **kwargs)
    return func

@not_none
def test(a, b):
    print a, b

Answer 2

You can write a decorator rejectNone as follows:

def rejectNone(f):
    def myF(*args, **kwargs):
        if None in args or None in kwargs.values():
            raise Exception('One of the arguments passed to {0} is None.'.format(f.__name__)
        return f(*args, **kwargs)
    return myF

@rejectNone
def f(a, b, k=3):
   print a * b

You will now get an Exception if you try to call f with a None argument. Note that decorators can be applied to functions or class methods but you can't put them in front of function parameters.