python: careful of swaping value

gistfile1.py

>>> class A():
...     val = 0
...
>>> a = A()
>>> a.val, a = 1, a.val                 # right
>>> a = A()
>>> a, a.val = a.val, 1                 # wrong
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'int' object has no attribute 'val'
>>> from dis import dis
>>> def swap():                         # swap internal
...     a, b = c, d
... 
>>> dis(swap)
  2           0 LOAD_GLOBAL              0 (c)
              3 LOAD_GLOBAL              1 (d)
              6 ROT_TWO             
              7 STORE_FAST               0 (a)
             10 STORE_FAST               1 (b)
             13 LOAD_CONST               0 (None)
             16 RETURN_VALUE  

What does a, b = c, d do?

stack[top] = c
stack[top+1] = d
a = stack[top]
b = stack[top+1]

what does wrong swap a, a.val = a.val, 1 do?

stack[top] = a.val
stack[top+1] = 1
a = stack[top] <-- Note: a is integer value 1
a.val = stack[top+1] <-- Error: a.val raise "AttributeError: 'int' object has no attribute 'val'"

opcode ROT_TWO will swap stack[top] with stack[top+1]