[Python] When making a set out of sorted list, the output set is not sorted [duplicate]

Could anyone please explain to me why making a set out of a sorted list results in a set which is NOT sorted?

list.sort() should sort the list in place, as it does. set(list) should return unique values of the list, as it does.

But why is the resulting set not sorted? Do I need to make a set, turn it back into list2 and then sort list2?

(Using Python 2.7.6)

Code:

def longest(s1, s2):
list = []
s = s1 + s2
for letter in s:
list.append(letter)
print list
list.sort()
print list
unique = set(list)
print unique
s4 = ""
for item in unique:
s4 = s4 + item
print s4
return s4


Console output:

['a', 'r', 'e', 't', 'h', 'e', 'y', 'h', 'e', 'r', 'e', 'y', 'e', 's', 't', 'h', 'e', 'y', 'a', 'r', 'e', 'h', 'e', 'r', 'e']
['a', 'a', 'e', 'e', 'e', 'e', 'e', 'e', 'e', 'e', 'e', 'h', 'h', 'h', 'h', 'r', 'r', 'r', 'r', 's', 't', 't', 'y', 'y', 'y']
set(['a', 'e', 'h', 's', 'r', 't', 'y'])
aehsrty

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