How does a set in python store the elements?

[idknuttin]

I am coming learning python after working with c++ for a few years. In c++ when you insert into a set it orders the elements, I can't figure out why in python sometimes the set will order the elements and sometimes it does not. Here are a few examples:

a = set([3, 2, 1])
print(a)

prints: {1, 2, 3}

a = set([1, 2, 3])
print(a)

prints: {1, 2, 3}

a = set([5, 7, 9])
print(a)

prints: {9, 5, 7}

a = set([7, 9, 5])
print(a)

prints: {9, 5, 7}

a = set([5, 3, 7])
print(a)

prints: {3, 5, 7}

I don't know what's going on here, I know you can sort the set using sorted() but the way it originally stores the information doesn't make any sense to me.

[menator01]

That's interesting. Test that I've tried:
using 3 digits return is in order if you do not use a digit higher than 7.
using 5 digits it returns correct order.

a = set([4,2,7])
print(a)

Output:
{2, 4, 7}

a = set([4,6,8,10,7])
print(a)

Output:
{4, 6, 7, 8, 10}

[Vimart]

Like other collections, sets support x in set, len(set), and for x in set. Being an unordered collection, sets do not record element position or order of insertion. Accordingly, sets do not support indexing, slicing, or other sequence-like behavior
https://docs.python.org/3.8/library/stdt...-frozenset

[bowlofred]

It hashes the elements, then uses the last N bits of the hash, where N depends on the size of the set. For the small set given, N is probably 3.

>>> format(hash(9) & 0x7, "b")
'1'
>>> format(hash(5) & 0x7, "b")
'101'
>>> format(hash(7) & 0x7, "b")
'111'

So the hash of 9 is smallest, 5, is next, then 7.

If the hash is identical, then order of insertion matters.

7 and 15 have the same hash value (in small sets).

>>> print(set((7, 15)))
{15, 7}
>>> print(set((15, 7)))
{7, 15}

[snippsat]

There where discussion also about sets order,when in Python 3.7 modern dict where implement that are guarantee to keep order.

In short, the implementation of modern dicts that preserves insertion order is unique and not considered appropriate with sets.
In particular,dicts are used everywhere to run Python (e.g. __dict__ in namespaces of objects).
A major motivation behind the modern dict was to reduce size, making Python more memory-efficient overall.
In contrast,sets are less prevalent than dicts within Python's core and thus dissuade such a refactoring

Having unordered sets are consistent with a very generic and ubiquitous data structure that unpins most modern math, i.e. Set Theory.
I submit,unordered sets in Python are good to have.

[Gribouillis]

For a set that preserve the insertion order, you could try sortedcollections.OrderedSet from pypi.