Assign class boolean value in Python

Question

If statements in Python allow you to do something like:

   if not x:
       print \"X is false.\"

This works if you\'re using an empty list

Answer 1

class Foo:
     def __nonzero__(self): return False
     __bool__ = __nonzero__ # this is for python3

In [254]: if Foo():
   .....:     print 'Yeah'
   .....: else: print 'Nay'
   .....:
Nay

Or, if you want to be ultra-portable, you can define __len__ only, which will have the same effect in both languages, but that has the (potential) downside that it implies that your object has a meaningful measure of length (which it may not).

This will work for any instance, depending on the actual logic you put in the method.