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.