Is python's “set” stable?

Question

The question arose when answering to another SO question (there).

When I iterate several times over a python set (without changing it between calls), can I assume it

Answer 1

The answer is simply a NO.

Python set operation is NOT stable.

I did a simple experiment to show this.

The code:

import random
random.seed(1)

x=[]
class aaa(object):
    def __init__(self,a,b):
        self.a=a
        self.b=b

for i in range(5):
    x.append(aaa(random.choice('asf'),random.randint(1,4000)))

for j in x:
    print(j.a,j.b)

print('====')
for j in set(x):
    print(j.a,j.b)

Run this for twice, you will get this:

First time result:

a 2332
a 1045
a 2030
s 1935
f 1555
====
a 2030
a 2332
f 1555
a 1045
s 1935

Process finished with exit code 0

Second time result:

a 2332
a 1045
a 2030
s 1935
f 1555
====
s 1935
a 2332
a 1045
f 1555
a 2030

Process finished with exit code 0

The reason is explained in comments in this answer.

However, there are some ways to make it stable:

• set PYTHONHASHSEED to 0, see details here, here and here.

• Use OrderedDict instead.