Because you're inserting items into your existing list instead of a new list. What you probably mean is:
b = []
for x in a:
b.append(x)
which creates a new list b that contains all elements whose length is
greater than four.
A better way to write this would be:
b = [x for x in a if len(x) > 4]
--
http://mail.python.org/mailman/listinfo/python-list
