I have a list of integers:

x = [3, 5, 2, 7]

And I want to create a new list where the nth element is the sum of elements in x from 0 to n-1.

This would result in:

y = [0, 3, 8, 10]

How can I do this with list comprehension, without running a loop?

How about without a loop and without a list comprehension? Sadly only Python 3:

>>> x = [3, 5, 2, 7]
>>> from itertools import accumulate
>>> [0] + list(accumulate(x[:-1]))
[0, 3, 8, 10]

Update: Here's an O(n) list comprehension solution:

>>> s = [0]
>>> [s.append(s[0]+n) or s.pop(0) for n in x]
[0, 3, 8, 10]

But I only wanted to show that that's possible without too much effort. I think accumulate or a for loop are much better.