Use curly braces to initialize a Set in Python

120
votes

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?

4
pythonpython-2.7set
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: docs.python.org/2/reference/expressions.html#set-displays – user395760
@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
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 The link you posted is outdated, a relic of the past, wrong, deprecated.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: docs.python.org/2/library/stdtypes.html#set and docs.python.org/2/library/stdtypes.html#frozenset – user395760
@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
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

4 Answers

116
votes

There are two obvious issues with the set literal syntax:

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

  1. It's not available before Python 2.7

  2. 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.

53
votes

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.

30
votes

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', '{', '}'])
5
votes

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