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          


        

Answer 1

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', '{', '}'])

Answer 2

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.

Answer 3

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.

Answer 4

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