Sets are collections of unordered values. If I construct a set via a set literal, e.g.
s = {'a', 'b', 'c'}
and then print it, I get the elements in some scrambled order. However, it seems that in Python 2.7, the above example always results in the same ordering:
print(s) # set(['a', 'c', 'b']) in Python 2.7
How does Python 2.7 decide on this ordering? Even the hashes of 'a', 'b' and 'c' are not in the order produced.
In Python 3.x (including 3.6 where dict keys are ordered) the resulting order seems to be random, though always the same within a given Python process. That is, repeatedly re-building the set literal always lead to the same ordering, as long as I do not restart the Python interpreter.
To check the ordering across multiple Python processes, consider the bash code
(for _ in {1..50}; do python3 -c "s = {'a', 'b', 'c'}; print(s)"; done) | sort -u
This will (most often) show the 6 different ways the 3 elements can be arranged. Switching out python3 with python(2), we only see the ordering ['a', 'c', 'b']. What determines the ordering in Python 3?
I see that the hash value of objects are deterministic in Python 2 while random (though constant within a Python process) in Python 3. I'm sure this is key to the full explanation.
As deceze writes in his comment, I would like to know if Python explicitly does something just to achieve this randomization, or if it happens "for free".
The reason for the difference in Python 3 (from Python 3.3 onwards) is that hash randomization is enabled by default, you could turn this off by setting the PYTHONHASHSEED environmental variable to a fixed value:
$ export PYTHONHASHSEED=0
$ (for _ in {1..50}; do python3 -c "s = {'a', 'b', 'c'}; print(s)"; done) | sort -u
{'a', 'b', 'c'}
Equally you can turn hash randomization on in Python 2 with the -R flag:
$ (for _ in {1..50}; do python2 -R -c "s = {'a', 'b', 'c'}; print(s)"; done) | sort -u
set(['a', 'b', 'c'])
set(['a', 'c', 'b'])
set(['b', 'c', 'a'])
set(['c', 'b', 'a'])
Note, you don't generally want to turn it off since having hash randomization enabled helps protect against certain denial-of-service attacks.