Word ladders program in Python
def spacing(a, b):
    score = 0
    for x, y in zip(a, b):
        if x != y:
            score += 1
    return score - 1


can be simplified to:

def spacing(a, b):
    """Returns numerical spacing between a and b on the word ladder"""
    return sum(1 for x, y in zip(a, b) if x != y) - 1


I suggest writing the comments as a string inside the function (a docstring) like above, then you can later write 

help(spacing) 


and get the documentation.  It also helps your IDE to find the documentation.