Python's Equivalent Of Java's Set.add()?

Java's Set.add function will return a boolean value, which is true if the set did not already contain the specified element. Python's Set.add does not have a return value. Seems li

Solution 1:

If you want a one-liner, you could use or to add the element only if it is not already in the set (short-circuit evaluation), and not to inverse the returned value and at the same time coerce to bool, turning the None returned by add, if it is called, into True, and otherwise just inverting the result of the in check:

>>>s = set()>>>not(42in s or s.add(42))
True
>>>not(42in s or s.add(42))
False
>>>s
set([42])

However, since that one-liner might not be very easy to grasp, and you have to write the value to be inserted twice, you should probably make it a function, and then it does not matter much how many lines it uses.

defin_or_add(s, x):
    returnnot(x in s or s.add(x))

Solution 2:

No, Python's set implementation has no such method; as you noted you'll have to test for presence separately:

if obj notin setobj:
    setobj.add(obj)

or, what I usually do:

if obj in setobj:
    return  # or break out of a loop, etc.

# handle the casewhere the set doesn't have the object yet.

You can always subclass the set type:

classSetWithPresenceCheck(set):defadd(self, value):
        not_present = value notinselfsuper(SetWithPresenceCheck, self).add(value)
        return not_present

Note that the real reason Set.add() returns a boolean is to make adding and testing an atomic operation; implementations of the interface can (optionally) make the method synchronised and let callers avoid race conditions. Python's built-in set doesn't make any thread-safety promises anyway.

Solution 3:

The union operator is much faster than add anyway.

>>> set_a = set('pqrs')
>>> set_b = ['t', 'u', 'v']
>>> set_a |= set(set_b)
>>> set_a
set(['p','q','r','s','t','u','v'])

Solution 4:

maybe with ternary conditional operator :

the_set.add(what_ever) orTrueif what_ever not in the_set elseFalse

This will return False if what_ever was in the set, True if it wasn't