## reverse a linked list
list = {'d':20,
        'next': {'d': 10,
                 'next': {'d': 5,
                          'next': {'d':15,
                                   'next':None}}}}
def reverse(list):
    prev = None
    while list is not None:
        list['next'], list, prev = prev, list['next'], list
    return prev
import pprint
pprint.pprint (reverse(list))