Question

One of the basic data structures in Python is the dictionary, which allows one to record ‘keys’ for looking up ‘values’ of any type. Is this implemented internally as a hash table? If not, what is it?

Answer 1

Yes, it is a hash mapping or hash table. You can read a description of python’s dict implementation, as written by Tim Peters, here.

That’s why you can’t use something ‘not hashable’ as a dict key, like a list:

>>> a = {} >>> b = ['some', 'list'] >>> hash(b) Traceback (most recent call last):   File '<stdin>', line 1, in <module> TypeError: list objects are unhashable >>> a[b] = 'some' Traceback (most recent call last):   File '<stdin>', line 1, in <module> TypeError: list objects are unhashable 

You can read more about hash tables or check how it has been implemented in python and why it is implemented that way.