“add to set” returns a boolean in java - what about python?

Question

In Java I like to use the Boolean value returned by an \"add to the set\" operation to test whether the element was already present in the set:

if (set.add(\         


        

Answer 1

For the built-in set type, add() always gives back None, but there is nothing to prevent you from using a "smarter" subclass:

class sset(set):
    """return True if on add if an object was really added (and not in yet)"""
    def add(self, val):
        if val in self:
            return True
        set.add(self, val)
        return False

This allows you to write:

s = sset()
if s.add('1'):
    print('try 1')
if s.add('1'):
    print('try 2')

and get try 2 printed when run.

This prevents repeated multiple lines of code like @brandizzi's which can easily contain inefficiencies:

z = set()
was_here = y not in z
z.add(y)
if was_here: # If the object was not in the list yet...
    print something

which is inefficient as y is added even if it is already in. It should be something like:

z = set()
was_here = y not in z
if not was_here:
    z.add(y)
else: # If the object was not in the list yet...
    print something

With sset() this can be reduced to:

z = sset()
if z.add(y): # If the object was not in the list yet...
    print something