Transpose/Unzip Function (inverse of zip)?

zip is its own inverse! Provided you use the special * operator.

>>> zip(*[('a', 1), ('b', 2), ('c', 3), ('d', 4)])[('a', 'b', 'c', 'd'), (1, 2, 3, 4)]

The way this works is by calling zip with the arguments:

zip(('a', 1), ('b', 2), ('c', 3), ('d', 4))

… except the arguments are passed to zip directly (after being converted to a tuple), so there's no need to worry about the number of arguments getting too big.

You could also do

result = ([ a for a,b in original ], [ b for a,b in original ])

It should scale better. Especially if Python makes good on not expanding the list comprehensions unless needed.

(Incidentally, it makes a 2-tuple (pair) of lists, rather than a list of tuples, like zip does.)

If generators instead of actual lists are ok, this would do that:

result = (( a for a,b in original ), ( b for a,b in original ))

The generators don't munch through the list until you ask for each element, but on the other hand, they do keep references to the original list.

I like to use zip(*iterable) (which is the piece of code you're looking for) in my programs as so:

def unzip(iterable):    return zip(*iterable)

I find unzip more readable.