Use Curly Braces to Initialize a Set in Python
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 =...
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 Answers
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.
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.
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', '{', '}'])
You need to do empty_set = set() to initialize an empty set. {} is an empty dict.
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