Subtracting the current and previous item in a list

Question

It is very common to write a loop and remember the previous.

I want a generator that does that for me. Something like:

import operator

def foo(it):
    it = iter(it)
    f = it.next()
    for s in it:
        yield f, s
        f = s

Now subtract pair-wise.

L = [0, 3, 4, 10, 2, 3]

print list(foo(L))
print [x[1] - x[0] for x in foo(L)]
print map(lambda x: -operator.sub(*x), foo(L)) # SAME

Outputs:

[(0, 3), (3, 4), (4, 10), (10, 2), (2, 3)]
[3, 1, 6, -8, 1]
[3, 1, 6, -8, 1]

• What is a good name for this operation?
• What is a better way to write this?
• Is there a built-in function that does something similar?
• Trying to use 'map' didn't simplify it. What does?

Answer 1

[y - x for x,y in zip(L,L[1:])]

Answer 2

l = [(0,3), (3,4), (4,10), (10,2), (2,3)]
print [(y-x) for (x,y) in l]

Outputs: [3, 1, 6, -8, 1]