Thread-safety of dict

[Raymond Hettinger]

On May 31, 9:12 pm, "Adam Olsen" <rha... at gmail.com> wrote:
> It seems to be a commonly held belief that basic dict operations (get,
> set, del) are atomic.  

They are atomic so long as the key does not have a custom __hash__,
__eq__, or __cmp__ method which can trigger arbitrary Python code.
With strings, ints, floats, or tuples of those, the get/set/del step
is atomic (i.e. executed in a single Python opcode).

>>> from dis import dis
>>> dis(compile('del d[k]', 'example', 'exec'))
  1           0 LOAD_NAME                0 (d)
              3 LOAD_NAME                1 (k)
              6 DELETE_SUBSCR
              7 LOAD_CONST               0 (None)
             10 RETURN_VALUE

The DELETE_SUBSCR step completes the whole action in a single opcode
(unless the object has a custom hash or equality test).  Of course,
there is a possibility of a thread switch between the LOAD_NAME and
the DELETE_SUBSCR (in which case another thread could have added or
removed that key).


Raymond