Atomic get and increment in python

By Jonas Wagner, 2009-02-20

For generating continuous unique id's in python I needed a thread safe way to do this:

x = counter
counter += 1

When disassembling this code we will get this:

  2           0 LOAD_FAST                0 (counter)
              3 STORE_FAST               1 (x)

  3           6 LOAD_FAST                0 (counter)
              9 LOAD_CONST               1 (1)
             12 INPLACE_ADD         
             13 STORE_FAST               0 (counter)

As you can see not even counter += 1 is atomic. Now the obvious solution would be to use a lock. The not so obvious solution is to use a itertools.count(). The counter is implemented in C and doesn't release the GIL so it is atomic. The code would the look like this:

x = counter.next()

which is more pretty anyway.