Is Python unpacking thread safe?

Question

Is an operation like x,y = self.mytuple atomic in Python ?

Or could self.mytuple be modified by another thread during the assignment, leading to x refering to the first version of self.mytuple, and y to the second version?

I'm using Python 3.

Answer 1

It is thread-safe only if item access is not handled by Python code.

The unpacking is handled by one bytecode:

>>> def f():
...     a, b = self.mytuple
... 
>>> import dis
>>> dis.dis(f)
  2           0 LOAD_GLOBAL              0 (self) 
              3 LOAD_ATTR                1 (mytuple) 
              6 UNPACK_SEQUENCE          2 
              9 STORE_FAST               0 (a) 
             12 STORE_FAST               1 (b) 
             15 LOAD_CONST               0 (None) 
             18 RETURN_VALUE         

Provided self.mytuple is really a standard Python tuple, that is threadsafe.

As soon as accessing items from mytuple triggers a custom __getitem__, all bets are off.