Using .join() function on a set incorrectly reorders it

Question

I have a set of characters (x) that is ordered as I need it:

{'a',
 'b',
 'c',
 'd',
 'e',
 'f',
 'g',
 'h',
 'i',
 'j',
 'k',
 'l',
 'm',
 'n',
 'o',
 'p',
 'q',
 'r',
 's',
 't',
 'u',
 'v',
 'w',
 'x',
 'y',
 'z'}

However, when I attempt to convert these back to a string using the .join() function:

return ' '.join(x)

The characters are being randomly reordered:

'c g e w i z n t l a q h p d f v m k b x u r j o y'

Any ideas as to what's going on here?

Prior to Python-3.7, sets were not ordered, so it can return the elements in *any* possible order. — Willem Van Onsem, Aug 14 '19 at 19:49
You can use an `OrderedSet` if you want to ensure that set retains its insertion order. — Willem Van Onsem, Aug 14 '19 at 19:50
if you really want sorted order you can use `return ''.join(sorted(x))` — Poojan, Aug 14 '19 at 19:51
@juanpa.arrivillaga: ah yes, only dictionaries. It's a bit a pitty. Since a `set` can be seen as a dictionary, but where the values are not that relevant. So one could probably with some templates "reuse" the C++ code of a `PyDict`. — Willem Van Onsem, Aug 14 '19 at 20:04

score 2 · Answer 1 · answered Aug 14 '19 at 19:52

Sets don't "promise" to maintain order, sometimes they do, but they shouldn't be used with a dependency on it. Furthermore, consider using the following:

alpha = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z']

Then:

return " ".join(alpha)

However, if you only care about it being in alphabetical and want to use a set you can force it to be sorted before using the join function...

return " ".join(sorted(x))

Good luck!

Michael Bianconi · Answer 2 · 2019-08-14T20:26:15.507

-1

Sets and dictionaries are unordered (pre Python 3.7). Their exact implementation involves hashtables and can be a little complicated. However, suffice it to say that the order you put elements into the set does not determine the order they are stored.

You can use OrderedDict or you can convert the set to a list, sort, and go from there.

edited Aug 14 '19 at 20:26

answered Aug 14 '19 at 19:51

Michael Bianconi

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`OrderedSet`. There is no such thing built in to python. Similarly a `set` is still not in insertion order in Python3.7 (though dictionaries are). – Error - Syntactical Remorse Aug 14 '19 at 20:19
I meant OrderedDict. – Michael Bianconi Aug 14 '19 at 20:26

Using .join() function on a set incorrectly reorders it

2 Answers2