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

Dictionaries have the nice setdefault function to avoid a whole class of problems related to the "double lookup" mentioned in the question. Since, in CPython at least, most of the set code is shared with dictionaries, I tried using that when working with a very large set (500k+ add, +/- 10% duplicates entries).

In addition, in order to reduce the overhead implied by the Python symbol name lookup, I wrapped that in a higher-order function so the compiler will build a closure and so will be able to use the index-based LOAD_FAST/LOAD_DEREF opcodes instead of the more expensive name lookup based LOAD_ATTR/LOAD_GLOBAL:

def cache():
  s = {}
  setdefault = s.setdefault
  n = 0

  def add(x):
    nonlocal n
    n+=1
    return setdefault(x,n) != n

  return add


# Usage
cached = cache()

for i in my_large_generator_with_duplicates():
  if not cached(i):
    do_something()

In my particular use case, this solution runs more than 20% faster than the one suggested in the other answer. Of course, your mileage may vary, so you should run your own tests.

---

For reference, here are the disassembled code of both solution (Python3.5 running on Linux):

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


dis.dis(do_add)

 20           0 LOAD_GLOBAL              0 (len)                                                                                                                                                                    
              3 LOAD_FAST                0 (s)                                                                                                                                                                      
              6 CALL_FUNCTION            1 (1 positional, 0 keyword pair)                                                                                                                                           
              9 STORE_FAST               2 (l)                                                                                                                                                                      

 21          12 LOAD_FAST                0 (s)                                                                                                                                                                      
             15 LOAD_ATTR                1 (add)                                                                                                                                                                    
             18 LOAD_FAST                1 (x)                                                                                                                                                                      
             21 CALL_FUNCTION            1 (1 positional, 0 keyword pair)                                                                                                                                           
             24 POP_TOP                                                                                                                                                                                             

 22          25 LOAD_GLOBAL              0 (len)                                                                                                                                                                    
             28 LOAD_FAST                0 (s)                                                                                                                                                                      
             31 CALL_FUNCTION            1 (1 positional, 0 keyword pair)
             34 LOAD_FAST                2 (l)
             37 COMPARE_OP               3 (!=)

def cache():
  s = {}
  setdefault = s.setdefault
  n = 0

  def add(x):
    nonlocal n
    n+=1
    return setdefault(x,n) != n

  return add


dis.dis(cache.__code__.co_consts[2])


 13           0 LOAD_DEREF               0 (n)
              3 LOAD_CONST               1 (1)
              6 INPLACE_ADD
              7 STORE_DEREF              0 (n)

 14          10 LOAD_DEREF               1 (setdefault)
             13 LOAD_FAST                0 (x)
             16 LOAD_DEREF               0 (n)
             19 CALL_FUNCTION            2 (2 positional, 0 keyword pair)
             22 LOAD_DEREF               0 (n)
             25 COMPARE_OP               3 (!=)

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)
  s.add(x)
  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.