[Tutor] why can you swap an immutable tuple?

[Steven D'Aprano]

On 26/05/13 05:23, Mark Lawrence wrote:
> On 25/05/2013 19:56, Jim Mooney wrote:
>> I thought tuples were immutable but it seems you can swap them, so I'm
>> confused:
>>
>> a,b = 5,8
>
> You've defined two names a and b so where's the tuple?

On the right hand side, 5,8 creates a tuple, which is then immediately unpacked to two individual values. You can see this by disassembling the code. In 2.7, you get this:

py> from dis import dis
py> code = compile("a, b = 5, 8", "", "exec")
py> dis(code)
   1           0 LOAD_CONST               3 ((5, 8))
               3 UNPACK_SEQUENCE          2
               6 STORE_NAME               0 (a)
               9 STORE_NAME               1 (b)
              12 LOAD_CONST               2 (None)
              15 RETURN_VALUE


Other versions may be slightly different, for example in Python 1.5 (ancient history!) the constant tuple (5, 8) is not created at compile-time, but at run-time:

# output of dis from Python 1.5:

           0 SET_LINENO          0
           3 SET_LINENO          1
           6 LOAD_CONST          0 (5)
           9 LOAD_CONST          1 (8)
          12 BUILD_TUPLE         2
          15 UNPACK_TUPLE        2
          18 STORE_NAME          0 (a)
          21 STORE_NAME          1 (b)
          24 LOAD_CONST          2 (None)
          27 RETURN_VALUE


But whenever it is created, the right hand side creates a tuple.

An interesting fact: on the left hand side, Python is very flexible with its sequence unpacking syntax. All of these are equivalent, where RHS (Right Hand Side) evaluates to exactly two items:

a, b = RHS
(a, b) = RHS
[a, b] = RHS


Note also that it is *sequence* unpacking, not *tuple* unpacking. Any sequence will work. Actually, any iterable object will work, not just sequences. But the name comes from way back in early Python days when it only worked on sequences.



-- 
Steven