If you copy and paste the block of code I suggested into the end of the Test Cases (not the Sample Test Cases), that should be enough.

You might want to include a randomized test similar to:

from random import choice,randint
import string

WEIGHTS = ((1, 15), (2, 16), (3, 17), (4, 18), (5, 19), (6, 20))
def weight():
    n = randint(1, 20)
    return next(w[0] for w in WEIGHTS if n <= w[1])

def encode_block(b):
    l = len(b)
    return b if l == 1 else "\\" + str(l) + b

test_blocks = [''.join(choice(string.ascii_lowercase) for _ in range(weight())) for _ in range(randint(200, 300))]
test_string = ''.join([encode_block(block) for block in test_blocks])

Test.assert_equals(decode(test_string), test_blocks);

I weighted the distribution of characters, but you could probably simplify that.

Awesome! Thank you Cliffstamp. Is it live or do I have to "approve" it somehow?

I don't see how that helps with a scoping issue.

"NameError: global name 'stack' is not defined" < This completely obviates doing an object-oriented solution in Python.

I finally got a moment to update with the suggestions.

Ooh. Good advice. I'll get started on a revision to take your suggestions into account. Thanks!

Got it. This should be fixed now. I think. (first kata published)