python order of elements in set

问题:

I do not understand the ordering what Python applies from holding 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'?

回答1:

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

回答2:

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']