CODEWITHSUNDEEP
pythondictionary
user1971598
user1971598

Reputation:

Dictionary order when keys are numbers

We know that regular Python dictionaries are unordered, but what about the case when the keys are the set of natural numbers? The hash function seems to be an identity function when the domain is the set of natural numbers, and iterating over something like x = {0:'a', 1:'b', 2:'c'} seems to yield the natural order of the keys, aka 0, 1, 2..

So can this behavior be relied on?

(Yes I know about OrderedDict)

EDIT:

Here's my specific usage, or at least this captures the spirit of what I'm looking at.

x = dict((a, chr(a)) for a in range(10))

for i in x: print i

This seems to preserve order.

Upvotes: 1

Views: 125

Answers (3)

nneonneo
nneonneo

Reputation: 179602

Nope. Never, ever, ever rely on the order of the dictionary keys. An illustrative example:

>>> {0: 1, 8: 2}
{0: 1, 8: 2}
>>> {8: 1, 0: 2}
{8: 1, 0: 2}

This happens because a dictionary starts off with 8 buckets. Thus, 0 and 8 end up in the same bucket, which will cause an immediate collision. The order depends on which is inserted first. (But don't rely on that either: that initial size isn't a guarantee!)

You are correct, however, that the hash of ints (in Python 2.x) is the identity function (with the sole exception of -1, which is mapped to -2 to avoid collision with the usual "error return" value). However, the distinction between int and long is quite subtle sometimes, and longs use a different hashing algorithm (because they can be arbitrarily large).

Upvotes: 6

John La Rooy
John La Rooy

Reputation: 304393

No. The order also depends on the order the keys are inserted.

Upvotes: 0

Alfe
Alfe

Reputation: 59556

No, you cannot rely on that behaviour. It is an implementation detail which can change from one version of Python to the next or even from one system to the next.

That being said, it is unlikely to change soon.

Upvotes: 0

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