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[ Python: how to check if an item was added to a set, without 2x (hash, lookup) ]

I was wondering if there was a clear/concise way to add something to a set and check if it was added without 2x hashes & lookups.

this is what you might do, but it has 2x hash's of item

if item not in some_set:  # <-- hash & lookup
    some_set.add(item)    # <-- hash & lookup, to check the item already is in the set


This works with a single hash and lookup but is a bit ugly.

some_set_len = len(some_set)
if some_set_len != len(some_set):


Is there a better way to do this using Python's set api?

Answer 1

I don't think there's a built-in way to do this. You could, of course, write your own function:

def do_add(s, x):
  l = len(s)
  return len(s) != l

s = set()
print do_add(s, 1)
print do_add(s, 2)
print do_add(s, 1)
print do_add(s, 2)
print do_add(s, 4)

Or, if you prefer cryptic one-liners:

def do_add(s, x):
  return len(s) != (s.add(x) or len(s))

(This relies on the left-to-right evaluation order and on the fact that set.add() always returns None, which is falsey.)

All this aside, I would only consider doing this if the double hashing/lookup is demonstrably a performance bottleneck and if using a function is demonstrably faster.