Efficient way to find missing elements in an integer sequence

Question

Suppose we have two items missing in a sequence of consecutive integers and the missing elements lie between the first and last elements. I did write a code that does accomp

Answer 1

We found a missing value if the difference between two consecutive numbers is greater than 1:

>>> L = [10,11,13,14,15,16,17,18,20]
>>> [x + 1 for x, y in zip(L[:-1], L[1:]) if y - x > 1]
[12, 19]

Note: Python 3. In Python 2 use itertools.izip.

Improved version for more than one value missing in a row:

>>> import itertools as it
>>> L = [10,11,14,15,16,17,18,20] # 12, 13 and 19 missing
>>> [x + diff for x, y in zip(it.islice(L, None, len(L) - 1),
                              it.islice(L, 1, None)) 
     for diff in range(1, y - x) if diff]
[12, 13, 19]