[Python-ideas] set.add() return value

[Ralf W. Grosse-Kunstleve]

Has this come up before?

Python 2.6.1 behavior:

>>> s = set()
>>> print s.add(1)
None
>>> print s.add(1)
None
>>> 

Desired behavior:

>>> s = set()
>>> print s.add(1)
True
>>> print s.add(1)
False
>>> 

Motivation:

Instead of

  if (1 not in s): # O(N log N) lookup
    s.add(1) # O(N log N) lookup again
    do_something_else()

or

  prev_len = len(s)
  s.add(1) # O(N log N) lookup
  if (len(s) != prev_len):
    do_something_else()

one could write

  if (s.add(1)):
    do_something_else()

which would be as fast as the second form and the most concise of all alternatives.