Appending new key to a dictionary

Question

I am trying to add elements to a dictionary in the following way:

a = {}
a['b'] = 1
a['a'] = 2

Finally a looks like:

{'a': 2, 'b': 1}

But actually I wanted the dictionary to contain keys in the order:

{'b': 1, 'a': 2}

Can anyone explain me this? Why are the keys getting sorted alphabetically when actually dictionaries (hashmaps) don't have any order?

They're not being sorted, that just happens to be how it's being shown. It's arbitrary. — Morgan Thrapp, Jun 16 '16 at 15:01
When I iterate through the dictionary, first 'a' gets printed and then 'b'. Can you explain me this? — Savinay Narendra, Jun 16 '16 at 15:02
Everyone has already explained it to you. Dictionaries are unordered. — Daniel Roseman, Jun 16 '16 at 15:02
Depends of the hash function used, and the implementation of a dictionary. With a different one, it could very well be 'b' then 'a' — T. Claverie, Jun 16 '16 at 15:04

score 1 · Answer 1 · answered Jun 16 '16 at 15:03

You are correct in that dictionaries are not ordered, but hashed. As a result, the order of a dictionary should not be relied on.

You can use the OrderedDict to help you achieve your goal:

from collections import OrderedDict
a = OrderedDict()
a['b'] = 1
a['a'] = 2

 > a
 > OrderedDict([('b', 1), ('a', 2)])

1 Answers1