Extract array from list in python

Question

If I have a list like this:

>>> data = [(1,2),(40,2),(9,80)]

how can I extract the the two lists [1,40,9] and [2,2,80] ? Of course I can iterate and extract the numbers myself but I guess there is a better way ?

Answer 1

List comprehensions save the day:

first = [x for (x,y) in data]
second = [y for (x,y) in data]

Answer 2

The unzip operation is:

In [1]: data = [(1,2),(40,2),(9,80)]
In [2]: zip(*data)
Out[2]: [(1, 40, 9), (2, 2, 80)]

Edit: You can decompose the resulting list on assignment:

In [3]: first_elements, second_elements = zip(*data)

And if you really need lists as results:

In [4]: first_elements, second_elements = map(list, zip(*data))

To better understand why this works:

zip(*data)

is equivalent to

zip((1,2), (40,2), (9,80))

The two tuples in the result list are built from the first elements of zip()'s arguments and from the second elements of zip()'s arguments.

Answer 3

There is also

In [1]: data = [(1,2),(40,2),(9,80)]
In [2]: x=map(None, *data)
Out[2]: [(1, 40, 9), (2, 2, 80)]
In [3]: map(None,*x)
Out[3]: [(1, 2), (40, 2), (9, 80)]