unpacking with default values

[Terry Reedy]

McA wrote:

> Do you know the "protocol" used by python while unpacking?
> Is it a direct assingnment? Or iterating?

In CPython, at least, both, just as with normal unpack and multiple 
assignment.  The iterable is unpacked into pieces by iterating (with 
knowledge of the number of targets and which is the catchall).  The 
targets are then directly bound.

 >>> from dis import dis
# first standard assignment
 >>> dis(compile("a,b,c = range(3)", '','single'))
   1           0 LOAD_NAME                0 (range)
               3 LOAD_CONST               0 (3)
               6 CALL_FUNCTION            1
               9 UNPACK_SEQUENCE          3
              12 STORE_NAME               1 (a)
              15 STORE_NAME               2 (b)
              18 STORE_NAME               3 (c)
              21 LOAD_CONST               1 (None)
              24 RETURN_VALUE
# now starred assignment
 >>> dis(compile("a,b,*c = range(3)", '','single'))
   1           0 LOAD_NAME                0 (range)
               3 LOAD_CONST               0 (3)
               6 CALL_FUNCTION            1
               9 UNPACK_EX                2
              12 STORE_NAME               1 (a)
              15 STORE_NAME               2 (b)
              18 STORE_NAME               3 (c)
              21 LOAD_CONST               1 (None)
              24 RETURN_VALUE

The only difference is UNPACK_EX (tended) instead of UNPACK_SEQUENCE.
Tne UNPACK_EX code is not yet in the dis module documentation.
But a little reverse engineering reveals the parameter meaning:
a,*b,c and *a,b,c give parameters 257 and 512 instead of 2.
Separating 2,257,512 into bytes gives 0,2; 1,1; 2,0.
a,*b and *a,b give 1, 256 or 0,1; 1,0.  The two bytes
are the number of targets after and before the starred target.

Terry Jan Reedy