Iteration order of sets in Python

Question

If I have two identical sets, meaning a == b gives me True, will they have the same iteration order? I tried it, and it works:

>>> foo = set("abc")
>>> bar = set("abc")
>>> zip(foo, bar)
[('a', 'a'), ('c', 'c'), ('b', 'b')]

My question is, was I lucky, or is this behavior guaranteed?

Answer 1

You were lucky, the order is not guaranteed. The only thing that's guaranteed is that the sets will have the same elements.

If you need some sort of predictability, you could sort them like this: zip(sorted(foo), sorted(bar)).

Answer 2

I'd say you got lucky. Though, it might also be that, since the elements in the set were the same, they were stored in the same order. This behavior is not something you'd want to rely on.

Answer 3

No.:

>>> class MyStr( str ):
...     def __hash__( self ):
...             return 0
...
>>> a = MyStr( "a" )
>>> b = MyStr( "b" )
>>> c = MyStr( "c" )
>>> foo = { a, b, c }
>>> foo
{'c', 'b', 'a'}
>>> bar = { b, a, c }
>>> foo is bar
False
>>> foo ==  bar
True
>>> list( zip( foo, bar ) )
[('c', 'c'), ('b', 'a'), ('a', 'b')]

P.S. I have no idea if the __hash__ override is necessary. I just tried something I thought would break this, and it did.

Answer 4

It wasn't just a coincidence that they came out the same: the implementation happens to be deterministic, so creating the same set twice produces the same ordering. But Python does not guarantee that.

If you create the same set in two different ways:

n = set("abc")
print n

m = set("kabc")
m.remove("k")
print m

...you can get different ordering:

set(['a', 'c', 'b'])
set(['a', 'b', 'c'])

Answer 5

Yes, you were lucky. See for example:

import random
r = [random.randint(1,10000) for i in range(20)]
foo = set(r)
r.sort(key=lambda _: random.randint(1,10000))
bar = set(r)
print foo==bar
print zip(foo, bar)

Which gave me the result:

True
[(3234, 3234), (9393, 9393), (9361, 1097), (1097, 5994), (5994, 2044), (1614, 1614), (6074, 4377), (4377, 9361), (5202, 5202), (2355, 2355), (1012, 1012), (7349, 7349), (6198, 6198), (8489, 8489), (7929, 7929), (6556, 6074), (6971, 6971), (2044, 6556), (7133, 7133), (383, 383)]