Python function that returns the value at index 0?

You need to use operator.itemgetter

>>> import operator
>>> pairs = [(0,1), (5,3)]
>>> xcoords = map(operator.itemgetter(0), pairs)
>>> xcoords
[0, 5]

In Python3, map returns a map object, hence you need a list call over it.

>>> list(map(operator.itemgetter(0), pairs))
[0, 5]

The most Pythonic approach would probably to use operator.itemgetter(0). It returns just such a function.

Another approach would be to call obj.__getitem__ directly. It's less Pythonic because it explicitly calls special method names, instead of allowing Python to infer what to call internally.