Is there an atomic counter implementation in Python?

README.md

Readings

• http://www.gossamer-threads.com/lists/python/python/31447
• http://stackoverflow.com/questions/18892712/differnce-between-binary-add-and-inplace-add
• http://blog.ziade.org/2015/11/25/should-i-use-pythonoptimize/
• http://akaptur.com/blog/2013/08/14/python-bytecode-fun-with-dis/
• https://29a.ch/2009/2/20/atomic-get-and-increment-in-python

Conclusion

The GIL could not protect a += 1 and a = a + 1 both.

itertools.count() is the only way for atomic increment in Python. It benefits from the C-implemented itertools. Otherwise we need to around the inplace adding operation with threading.Lock().

inspect_iadd.py

from dis import dis


def increment1(n):
    n += 1
    return n


def increment2(n):
    n = n + 1
    return n


for label, code in [
    ('n += 1', increment1),
    ('n = n + 1', increment2),
]:
    print('-' * 50)
    print '%s' % label
    print('')
    dis(code)
    print('-' * 50)

inspect_iadd.txt

--------------------------------------------------
n += 1

  5           0 LOAD_FAST                0 (n)
              3 LOAD_CONST               1 (1)
              6 INPLACE_ADD
              7 STORE_FAST               0 (n)

  6          10 LOAD_FAST                0 (n)
             13 RETURN_VALUE
--------------------------------------------------
--------------------------------------------------
n = n + 1

 10           0 LOAD_FAST                0 (n)
              3 LOAD_CONST               1 (1)
              6 BINARY_ADD
              7 STORE_FAST               0 (n)

 11          10 LOAD_FAST                0 (n)
             13 RETURN_VALUE
--------------------------------------------------