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) 

This does NOT work:

if fruit = isBig(y): return fruit 

Why doesn't the 2nd one work!? I want a 1-liner. Except, the 1st one will call the function TWICE.

How to make it 1 liner, without calling the function twice?

Answer 1

Starting Python 3.8, and the introduction of assignment expressions (PEP 572) (:= operator), it's now possible to capture the condition value (isBig(y)) as a variable (x) in order to re-use it within the body of the condition:

if x := isBig(y): return x 

Answer 2

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).