[Tutor] making a typing speed tester

[Tom]

Hi, I asked this question last year but got no response.

I'm trying to write a program to test someones typing speed and show
them their mistakes. However I'm getting weird results when looking
for the differences in longer (than 100 chars) strings:

import difflib

# a tape measure string (just makes it easier to locate a given index)
a = '1-3-5-7-9-12-15-18-21-24-27-30-33-36-39-42-45-48-51-54-57-60-63-66-69-72-75-78-81-84-87-90-93-96-99-103-107-111-115-119-123-127-131-135-139-143-147-151-155-159-163-167-171-175-179-183-187-191-195--200'

# now with a few mistakes
b = '1-3-5-7-l-12-15-18-21-24-27-30-33-36-39o42-45-48-51-54-57-60-63-66-69-72-75-78-81-84-8k-90-93-96-9l-103-107-111-115-119-12b-1v7-131-135-139-143-147-151-m55-159-163-167-a71-175j179-183-187-191-195--200'

s = difflib.SequenceMatcher(None, a ,b)
ms = s.get_matching_blocks()

print ms

>>>[(0, 0, 8), (200, 200, 0)]

Have I made a mistake or is this function designed to give up when the
input strings get too long? If so what could I use instead to compute
the mistakes in a typed text?

Thanks in advance,
Thomas