[Python-ideas] Default values in multi-target assignment

[Serhiy Storchaka]

Yet one crazy idea. What if allow default values for targets in 
multi-target assignment?

     >>> (a, b=0) = (1, 2)
     >>> a, b
     (1, 2)
     >>> (a, b=0) = (1,)
     >>> a, b
     (1, 0)
     >>> (a, b=0) = ()
     Traceback (most recent call last):
       File "<stdin>", line 1, in <module>
     ValueError: not enough values to unpack (expected at least 1, got 0)
     >>> (a, b=0) = (1, 2, 3)
     Traceback (most recent call last):
       File "<stdin>", line 1, in <module>
     ValueError: too many values to unpack (expected at most 2)

Currently you need either explicitly check the length of the right-hand 
part (if it is a sequence and not an arbitrary iterator),

     if len(c) == 1:
         a, = c
         b = 0
     elif len(c) == 2:
         a, b = c
     else:
         raise TypeError

or use an intermediate function:

     def f(a, b=0):
         return a, b
     a, b = f(*c)

The latter can be written as an ugly one-liner:

     a, b = (lambda a, b=0: (a, b))(*c)