I had seen this test question on Pluralsight:
Given these sets:
x = {'a', 'b', 'c', 'd'}
y = {'c', 'e', 'f'}
z = {'a', 'g', 'h', 'i'}
What is the value of x | y ^ z?
The expected answer is:
{'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i'}
Combines the sets (automatically discarding duplicates), and orders them from lowest to greatest.
My questions are:
- What is this expression called?
- Why do I get 3 different results from 3 different Python versions?
Result on Python 3.7.5 on Ubuntu 18.04:
{'c', 'h', 'f', 'd', 'b', 'i', 'g', 'a', 'e'}
Result on Python 2.17.17rc1 on Ubuntu 18.04:
set(['a', 'c', 'b', 'e', 'd', 'g', 'f', 'i', 'h'])
Result on Python 3.7.2 on Windows 10:
{'a', 'd', 'h', 'f', 'b', 'g', 'e', 'c', 'i'}
Here is a repl of the same code I'm using for this:
https://repl.it/repls/RudeMoralWorkplace
I'd like to understand what happens behind the scenes with these expressions so I can debunk why I get different results.
The set operations you have mentioned are:
^ - symmetric difference (XOR):
Return a new set with elements in either the set or other but not both.
Example: {'1', '2', '3'} ^ {'2', '3', '4'} = {'1', '4'}
| - union (OR):
Return a new set with elements from the set and all others.
Example: {'1', '2', '3'} | {'2', '3', '4'} = {'1', '2', '3', '4'}
There are also other set operations in python:
& - intersection (AND):
Return a new set with elements common to the set and all others.
Example: {'1', '2', '3'} & {'2', '3', '4'} = {'2', '3'}
- - difference:
Return a new set with elements in the set that are not in the others.
Example: {'1', '2', '3'} - {'2', '3', '4'} = {'1'}
The order of precedence for these operations is -, &, ^, |, so in your example, we first apply ^:
>>> y^z
{'a', 'c', 'e', 'f', 'g', 'h', 'i'}
And then |:
>>> x|{'a', 'c', 'e', 'f', 'g', 'h', 'i'}
{'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i'}
The different outputs you describe are actually the same set, as sets are not ordered.
>>> {'c', 'h', 'f', 'd', 'b', 'i', 'g', 'a', 'e'} == {'a', 'd', 'h', 'f', 'b', 'g', 'e', 'c', 'i'}
True
Any order shown in the string representation of a set is an implementation detail and should not be relied upon as it will vary unpredictably, as you have found.
