How to assign a variable in an IF condition, and then return it?

Question

def isBig(x):
   if x > 4: 
       return \'apple\'
   else: 
       return \'orange\'

This works:

if isBig(y): return isBig(y)
         


        

Answer 1

I see somebody else has already pointed to my old "assign and set" Cookbook recipe, which boils down in its simplest version to:

class Holder(object):
   def set(self, value):
     self.value = value
     return value
   def get(self):
     return self.value

h = Holder()

...

if h.set(isBig(y)): return h.get()

However, this was intended mostly to ease transliteration between Python and languages where assignment is directly supported in if or while. If you have "hundreds" of such check-and-return in a cascade, it's much better to do something completely different:

hundreds = isBig, isSmall, isJuicy, isBlah, ...

for predicate in hundreds:
  result = predicate(y)
  if result: return result

or even something like

return next(x for x in (f(y) for f in hundreds) if x)

if it's OK to get a StopIteration exception if no predicate is satisfied, or

return next((x for x in (f(y) for f in hundreds) if x)), None)

if None is the proper return value when no predicate is satisfied, etc.

Almost invariably, using (or even wishing for;-) the Holder trick/non-idiom is a "design smell" which suggests looking for a different and more Pythonic approach -- the one case where Holder is justified is exactly the special case for which I designed it, i.e., the case where you want to keep close correspondence between the Python code and some non-Python (you're transliterating a reference algorithm in Python and want it working first before refactoring it into a more Pythonic form, or you're writing Python as a prototype that will be transliterated into C++, C#, Java, etc, once it's working effectively).