I have a list of 2-item tuples and I'd like to convert them to 2 lists where the first contains the first item in each tuple and the second list holds the second item.
For example:
original = [('a', 1), ('b', 2), ('c', 3), ('d', 4)]
# and I want to become...
result = (['a', 'b', 'c', 'd'], [1, 2, 3, 4])
Is there a builtin function that does that?
See also: Transpose list of lists if the specific choice of lists vs. tuples in the result matters. Most answers here assume it doesn't.
In 2.x, 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)]
This is equivalent to calling zip with each element of the list as a separate argument:
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.
In 3.x, zip returns a lazy iterator, but this is trivially converted:
>>> list(zip(*[('a', 1), ('b', 2), ('c', 3), ('d', 4)]))
[('a', 'b', 'c', 'd'), (1, 2, 3, 4)]