If you need to modify the sequence you are iterating over while inside the loop (for example to duplicate selected items), it is recommended that you first make a copy. Iterating over a sequence does not implicitly make a copy. The slice notation makes this especially convenient:
for w in words[:]: # Loop over a slice copy of the entire list.
if len(w) > 6:
words.insert(0, w)
#words: ['defenestrate', 'cat', 'window', 'defenestrate']
