python - Understanding the set() function

Question

In python, set() is an unordered collection with no duplicate elements. However, I am not able to understand how it generates the output.

For example, consider the following:

>>> x = [1, 1, 2, 2, 2, 2, 2, 3, 3]
>>> set(x)
set([1, 2, 3])

>>> y = [1, 1, 6, 6, 6, 6, 6, 8, 8]
>>> set(y)
set([8, 1, 6])

>>> z = [1, 1, 6, 6, 6, 6, 6, 7, 7]
>>> set(z)
set([1, 6, 7])

Shouldn't the output of set(y) be: set([1, 6, 8])? I tried the above two in Python 2.6.

Answer 1

Sets are unordered, as you say. Even though one way to implement sets is using a tree, they can also be implemented using a hash table (meaning getting the keys in sorted order may not be that trivial).

If you'd like to sort them, you can simply perform:

sorted(set(y))

which will produce a sorted list containing the set's elements. (Not a set. Again, sets are unordered.)

Otherwise, the only thing guaranteed by set is that it makes the elements unique (nothing will be there more than once).

Hope this helps!