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?
4 Answers
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.
There must be more to a Python dictionary than a table lookup on hash(). By brute experimentation I found this hash collision:
>>> hash(1.1)
2040142438
>>> hash(4504.1)
2040142438
Yet it doesn't break the dictionary:
>>> d = { 1.1: 'a', 4504.1: 'b' }
>>> d[1.1]
'a'
>>> d[4504.1]
'b'
Sanity check:
>>> for k,v in d.items(): print(hash(k))
2040142438
2040142438
Possibly there's another lookup level beyond hash() that avoids collisions between dictionary keys. Or maybe dict() uses a different hash.
(By the way, this in Python 2.7.10. Same story in Python 3.4.3 and 3.5.0 with a collision at hash(1.1) == hash(214748749.8)
.)
Yes. Internally it is implemented as open hashing based on a primitive polynomial over Z/2 (source).
dict
implementation. – Torsten Marek