Track value changes in a repetitive list in Python

Question

I have a list with repeating values as shown below:

x = [1, 1, 1, 2, 2, 2, 1, 1, 1]

This list is generated from a pattern matching regular expression (not shown here). The list is guaranteed to have repeating values (many, many repeats - hundreds, if not thousands), and is never randomly arranged because that's what the regex is matching each time.

What I want is to track the list indices at which the entries change from the previous value. So for the above list x, I want to obtain a change-tracking list [3, 6] indicating that x[3] and x[6] are different from their previous entries in the list.

I managed to do this, but I was wondering if there was a cleaner way. Here's my code:

x = [1, 1, 1, 2, 2, 2, 1, 1, 1]

flag = []
for index, item in enumerate(x):
    if index != 0:
        if x[index] != x[index-1]:
            flag.append(index)

print flag

Output: [3, 6]

Question: Is there a cleaner way to do what I want, in fewer lines of code?

Answer 1

It can be done using a list comprehension, with a range function

>>> x = [1, 1, 1, 2, 2, 2, 3, 3, 3]
>>> [i for i in range(1,len(x)) if x[i]!=x[i-1] ]
[3, 6]
>>> x = [1, 1, 1, 2, 2, 2, 1, 1, 1]
>>> [i for i in range(1,len(x)) if x[i]!=x[i-1] ]
[3, 6]