Is sequence unpacking atomic?

Question

Is sequence unpacking atomic? e.g.:

(a, b) = (c, d)

I'm under the impression it is not.

Edit: I meant atomicity in the context of multi-threading, i.e. whether the entire statement is indivisible, as atoms used to be.

Answer 1

It is one operation; the right-hand expression is evaluated before the left-hand assignment is applied:

>>> a, b = 10, 20
>>> a, b
(10, 20)
>>> b, a = a, b
>>> a, b
(20, 10)
>>> a, b = a*b, a/b
>>> a, b
(200, 2)

Or, if you are talking about multi-threaded environments, then the assignment is not atomic; the interpreter evaluates a tuple assignment with a single opcode, but uses separate opcodes to then store the results into each affected variable:

>>> def t(self): a,b=20,20
... 
>>> dis.dis(t)
  1           0 LOAD_CONST               2 ((20, 20))
              3 UNPACK_SEQUENCE          2
              6 STORE_FAST               1 (a)
              9 STORE_FAST               2 (b)
             12 LOAD_CONST               0 (None)
             15 RETURN_VALUE        

However, normal assigment is always going to be at least two opcodes (one for the right-hand expression, one for storing the result), so in python in general assigment is not atomic. Sequence unpacking is no different.

Answer 2

Definitely not atomic in a multi-threaded environment, tested using the following script:

import threading

a, b = 10, 10
finished = False
def thr():
    global finished
    while True:
        # if sequence unpacking and assignment is atomic then (a, b) will always
        # be either (10, 10) or (20, 20).  Could also just check for a != b
        if (a, b) in [(10, 20), (20, 10)]:
            print('Not atomic')
            finished = True
            break

t = threading.Thread(target=thr)
t.start()

while True:
    for i in range(1000000):
        a, b = 20, 20
        a, b = 10, 10
    if finished:
        t.join()
        break

Tested using CPython 2.6, 2.7, and 3.2. On each version this script printed 'Not atomic' and exited in well under a second.