Use curly braces to initialize a Set in Python

Question

I'm learning python, and I have a novice question about initializing sets. Through testing, I've discovered that a set can be initialized like so:

my_set = {'foo', 'bar', 'baz'}

Are there any disadvantages of doing it this way, as opposed to the standard way of:

my_set = set(['foo', 'bar', 'baz'])

or is it just a question of style?

The documentation does mention it, just not there. Note that that's the documentation for an deprecated module, the *real* sets are now builtin. It's in the "What’s New in Python 2.7" document, and the language reference briefly describes it: http://docs.python.org/2/reference/expressions.html#set-displays — , Jun 28 '13 at 20:38
@delnan I use python 2.7, so I didn't think to look in the python 3 docs. The link I posted is for 2.7, but it strangely doesn't mention this. — fvrghl, Jun 28 '13 at 20:44
I since edited my comment, the 2.7 docs also mention this. The link you posted is outdated, a relic of the past, wrong, deprecated. Forget that it exists and use what it itself tells you to use instead: http://docs.python.org/2/library/stdtypes.html#set and http://docs.python.org/2/library/stdtypes.html#frozenset — , Jun 28 '13 at 20:46
@delnan Thanks for helping me out. I edited the question so that I no longer say there is no documentation for it (although there are few examples mentioning this online). — fvrghl, Jun 28 '13 at 20:52
For passer by: Pycharm warns against using a function call when one can use a literal - there may be performance reasons - so do prefer the set literal way in new code — Mr_and_Mrs_D, Nov 11 '14 at 12:57

score 124 · Accepted Answer · answered Jun 28 '13 at 20:36

There are two obvious issues with the set literal syntax:

my_set = {'foo', 'bar', 'baz'}

It's not available before Python 2.7
There's no way to express an empty set using that syntax (using {} creates an empty dict)

Those may or may not be important to you.

The section of the docs outlining this syntax is here.

score 56 · Answer 2 · answered Mar 16 '14 at 17:17

56

Compare also the difference between {} and set() with a single word argument.

>>> a = set('aardvark')
>>> a
{'d', 'v', 'a', 'r', 'k'} 
>>> b = {'aardvark'}
>>> b
{'aardvark'}

but both a and b are sets of course.

answered Mar 16 '14 at 17:17

Thruston

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that's why I like to remind myself the set constructor as `set([])` rather than just `set()`. – Michael Ekoka Sep 16 '16 at 16:12
4

`set()` always take a single argument. A single iterable. – George Kettleborough Feb 26 '18 at 10:28
1

@GeorgeKettleborough a single *or no* arguments, the latter to create an empty set. – gertvdijk Mar 11 '19 at 17:29

0x90 · Answer 3 · 2018-07-23T13:06:33.857

From Python 3 documentation (the same holds for python 2.7):

Curly braces or the set() function can be used to create sets. Note: to create an empty set you have to use set(), not {}; the latter creates an empty dictionary, a data structure that we discuss in the next section.

in python 2.7:

>>> my_set = {'foo', 'bar', 'baz', 'baz', 'foo'}
>>> my_set
set(['bar', 'foo', 'baz'])

Be aware that {} is also used for map/dict:

>>> m = {'a':2,3:'d'}
>>> m[3]
'd'
>>> m={}
>>> type(m)
<type 'dict'>

One can also use comprehensive syntax to initialize sets:

>>> a = {x for x in """didn't know about {} and sets """ if x not in 'set' }
>>> a
set(['a', ' ', 'b', 'd', "'", 'i', 'k', 'o', 'n', 'u', 'w', '{', '}'])

score 9 · Answer 4 · edited Dec 11 '20 at 09:57

9

You need to do empty_set = set() to initialize an empty set. {} is an empty dict.

edited Dec 11 '20 at 09:57

S3DEV

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answered Mar 18 '20 at 21:23

Arjjun

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Use curly braces to initialize a Set in Python

4 Answers4

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