What is the order of elements in a Python set?

Question

I do not understand the ordering that Python applies to sets.

For example:

visited = set()
visited.add('C')
visited.add('A')
visited.add('B')
print(set)

The ordering is 'A', 'C', 'B'.

Why 'A' is before 'C' (maybe alphabetical order)?

What I have to do in order to preserve the adding ordering, i.e. 'C', 'A', 'B'?

https://docs.python.org/2/library/stdtypes.html#set-types-set-frozenset "A set object is an unordered collection of distinct hashable objects." — fredtantini, Oct 17 '14 at 08:30
possible duplicated http://stackoverflow.com/questions/1653970/does-python-have-an-ordered-set — Mazdak, Oct 17 '14 at 08:32
It is unordered but not random tho, I think there are still some restrictions for the ordering, aren't there any ? — BoJack Horseman, Oct 17 '14 at 09:24

score 7 · Accepted Answer · edited May 23 '17 at 12:11

7

You cannot have order in sets. and there is no way to tell how Python orders it. Check this answer for alternatives.

edited May 23 '17 at 12:11

Community

answered Oct 17 '14 at 08:32

avi

score 0 · Answer 2 · answered Oct 17 '14 at 08:30

0

Sets are different than lists. If you want to preserve an order, use a list. For example :

a = []
a.append('C')
a.append('A')
a.append('B')
print a # ['C', 'A', 'B']

answered Oct 17 '14 at 08:30

FunkySayu

6

But now you can put one element in list twice (which is impossible in sets) – Darth Kotik Oct 17 '14 at 08:36

2 Answers2