Following example in Section 4.2 would run into infinite loop.

https://docs.python.org/2/tutorial/controlflow.html#for-statements

>>> 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']

Should be something like this (but still leaves duplicate elements in the list):

-----------------------------------

words_tmp = copy.deepcopy(words)

for w in words_tmp:

    if len(w)>6 :

            words.insert(0,w)

-----------------------------------

Thanks & Regards

Abhinav Tyagi