Below you will find interesting fact of how python works when collision happens, CPython ( & Pypy & Jython) behaviour in such case is called **probing** http://stackoverflow.com/questions/327311/how-are-pythons-built-in-dictionaries-implemented

repl.py

>>> dict = {1: 2}
>>> dict[1] = 3
>>> dict
{1: 3}
>>> class A:
...     def __hash__(self):
...             return 1
... 
>>> class B:
...     def __hash__(self):
...             return 1
... 
>>> dict = {A(): 2}
>>> dict
{<__main__.A instance at 0x1043508c0>: 2}
>>> dict[B()] = 3
>>> dict
{<__main__.A instance at 0x1043508c0>: 2, <__main__.B instance at 0x104350950>: 3}