Set changes element order?

Question

Recently I noticed that when I am converting list to set the order or elements is changed and is sorted by character.

Consider this example:

x=[1,2,20,6,210]
print x 
# [1, 2, 20, 6, 210] # the order is same as initial order

set(x)
# set([1, 2, 20, 210, 6]) # in the set(x) output order is sorted

My questions are -

1. Why is this happening?
2. How can I do set operations (especially Set Difference) without losing the initial order?

Answer 1

In Python 3.6, set() now should keep the order, but there is another solution for Python 2 and 3:

>>> x = [1, 2, 20, 6, 210]
>>> sorted(set(x), key=x.index)
[1, 2, 20, 6, 210]

Answer 2

As denoted in other answers, sets are data structures (and mathematical concepts) that do not preserve the element order -

However, by using a combination of sets and dictionaries, it is possible that you can achieve wathever you want - try using these snippets:

# save the element order in a dict:
x_dict = dict(x,y for y, x in enumerate(my_list) )
x_set = set(my_list)
#perform desired set operations
...
#retrieve ordered list from the set:
new_list = [None] * len(new_set)
for element in new_set:
   new_list[x_dict[element]] = element

Answer 3

Answering your first question, a set is a data structure optimized for set operations. Like a mathematical set, it does not enforce or maintain any particular order of the elements. The abstract concept of a set does not enforce order, so the implementation is not required to. When you create a set from a list, Python has the liberty to change the order of the elements for the needs of the internal implementation it uses for a set, which is able to perform set operations efficiently.

Answer 4

An implementation of the highest score concept above that brings it back to a list:

def SetOfListInOrder(incominglist):
    from collections import OrderedDict
    outtemp = OrderedDict()
    for item in incominglist:
        outtemp[item] = None
    return(list(outtemp))

Tested (briefly) on Python 3.6 and Python 2.7.

Answer 5

Here's an easy way to do it:

x=[1,2,20,6,210]
print sorted(set(x))

Answer 6

Building on Sven's answer, I found using collections.OrderedDict like so helped me accomplish what you want plus allow me to add more items to the dict:

import collections

x=[1,2,20,6,210]
z=collections.OrderedDict.fromkeys(x)
z
OrderedDict([(1, None), (2, None), (20, None), (6, None), (210, None)])

If you want to add items but still treat it like a set you can just do:

z['nextitem']=None

And you can perform an operation like z.keys() on the dict and get the set:

z.keys()
[1, 2, 20, 6, 210]