Python function that returns the value at index 0?

Question

Does the Python standard library have a function that returns the value at index 0? In other words:

zeroth = lambda x: x[0]

I need to use this in a higher-order function like map(). I ask because I believe it's clearer to use a reusable function rather than define a custom one - for example:

pairs = [(0,1), (5,3), ...]

xcoords = map(funclib.zeroth, pairs)  # Reusable
vs.
xcoords = map(lambda p: p[0], pairs)  # Custom

xcoords = [0, 5, ...]  # (or iterable)

I also ask because Haskell does have such a function Data.List.head, which is useful as an argument to higher-order functions:

head :: [a] -> a
head (x:xs) = x
head xs = xs !! 0

xcoords = (map head) pairs

Answer 1

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]

Answer 2

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.