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pythonhashdictionaryhashmaphashtable
Tommy Herbert
Tommy Herbert

Reputation: 21001

Is a Python dictionary an example of a hash table?

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?

Upvotes: 270

Views: 272699

Answers (4)

Bob Stein
Bob Stein

Reputation: 17254

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).)

(I haven't found any collisions in Python 3.9.6. Since the hashes are bigger -- hash(1.1) == 230584300921369601 -- I estimate it would take my desktop a thousand years to find one. So I'll get back to you on this.)

Upvotes: 58

Ben Hoffstein
Ben Hoffstein

Reputation: 103375

Yes. Internally it is implemented as open hashing based on a primitive polynomial over Z/2 (source).

Upvotes: 23

nosklo
nosklo

Reputation: 223102

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.

Upvotes: 326

Jeremy Cantrell
Jeremy Cantrell

Reputation: 27424

To expand upon nosklo's explanation:

a = {}
b = ['some', 'list']
a[b] = 'some' # this won't work
a[tuple(b)] = 'some' # this will, same as a['some', 'list']

Upvotes: 9

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