nested function returning outer function

Question

I have a common pattern that goes like

def f(x):
  if x.type == 'Failure':
     # return `x` immediately without doing work
     return x
  else:
  # do stuff with x...
  return x

I would like to abstract the if/else pattern into a stand alone function. However I want that function, when called from inside f, to return from f immediately. Else it should just return x to a value inside f for further processing. Something like

def g(x):
  if x.type == 'Failure':
    global return x
  else:
    return x.value

def f(x):
  x_prime = g(x) # will return from f
                 # if x.type == 'Failure'
  # do some processing...
  return x_prime

Is this possible in Python?

Are you asking if it's possible at all, or if there's a quick shortcut for doing it in Python? It's certainly quite possible and relatively easy to put in the logic yourself, such as by returning a tuple from g instead of a single value, and using one of the values in the tuple to tell f to return immediately or not. That's just one example of how you could do it, but it's not necessarily something built-in in Python. — Random Davis
– Random Davis, Commented Nov 8, 2018 at 0:45
@RandomDavis I want to abstract that, so I can use g everywhere without changing code that only handles x.value. I'm thinking a decorator function could be a neat solution, but I welcome other alternatives. Yes, an actual solution would be better than a plain yes or no! — dmvianna
– dmvianna, Commented Nov 8, 2018 at 0:48
I doubt you can do this, and it seems like a bad idea in the first place. A function should not make assumptions about how the caller will use its result. — Barmar
– Barmar, Commented Nov 8, 2018 at 1:10
The only language I can think of where this could be done is Common Lisp, using a macro that expands into a RETURN expression. And even then, its behavior would depend on where it's used, because RETURN will exit the closest enclosing loop or function. — Barmar
– Barmar, Commented Nov 8, 2018 at 1:15
In Scheme you could do it with a function that takes a second argument, a continuation to call in the case where you want to do the global return. — Barmar
– Barmar, Commented Nov 8, 2018 at 1:16

dmvianna · Accepted Answer · 2018-11-08 01:19:15Z

2

I'm using Validation from my branch of pycategories:

def fromSuccess(fn):
    """
    Decorator function. If the decorated function
    receives Success as input, it uses its value.
    However if it receives Failure, it returns
    the Failure without any processing.
    Arguments:
        fn :: Function
    Returns:
        Function
    """
    def wrapped(*args, **kwargs):
        d = kwargs.pop('d')
        if d.type == 'Failure':
            return d
        else:
            kwargs['d'] = d.value
        return fn(*args, **kwargs)
    return wrapped

@fromSuccess
def return_if_failure(d):
    return d * 10

return_if_failure(d = Failure(2)), return_if_failure(d = Success(2))

>>> (Failure(2), 20)

answered Nov 8, 2018 at 1:19

dmvianna

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