python – Empty set literal?

The Question :

497 people think this question is useful

[] = empty list

() = empty tuple

{} = empty dict

Is there a similar notation for an empty set? Or do I have to write set()?

The Question Comments :
  • you don’t accept {i for i in []}, didn’t you?
  • Just want to show nearly anything is possible with python. If you want to create a set without using to set(), you can.
  • Yeah, you can do pretty much everything in a hundred convulted ways. I don’t add map(lambda x: x) to my code examples just to show you it works either. It’s not a set literal as well, it’s just a set comprehension.
  • A dumb way which works would be {0}-{0}. It’s not as clear as set(), but it does have the advantage of looking like a funny pair of eyes.
  • @Chris {} is a dictionary. {0} is a set. {0} - {0} is the difference between a set and itself, which is the empty set.

The Answer 1

566 people think this answer is useful

No, there’s no literal syntax for the empty set. You have to write set().

The Answer 2

65 people think this answer is useful

By all means, please use set() to create an empty set.

But, if you want to impress people, tell them that you can create an empty set using literals and * with Python >= 3.5 (see PEP 448) by doing:

>>> s = {*()}  # or {*{}} or {*[]}
>>> print(s)
set()

this is basically a more condensed way of doing {_ for _ in ()}, but, don’t do this.

The Answer 3

47 people think this answer is useful

Just to extend the accepted answer:

From version 2.7 and 3.1 python has got set literal {} in form of usage {1,2,3}, but {} itself still used for empty dict.

Python 2.7 (first line is invalid in Python <2.7)

>>> {1,2,3}.__class__
<type 'set'>
>>> {}.__class__
<type 'dict'>

Python 3.x

>>> {1,4,5}.__class__
<class 'set'>
>>> {}.__class__
<type 'dict'>

More here: https://docs.python.org/3/whatsnew/2.7.html#other-language-changes

The Answer 4

6 people think this answer is useful

It depends on if you want the literal for a comparison, or for assignment.

If you want to make an existing set empty, you can use the .clear() metod, especially if you want to avoid creating a new object. If you want to do a comparison, use set() or check if the length is 0.

example:

#create a new set    
a=set([1,2,3,'foo','bar'])
#or, using a literal:
a={1,2,3,'foo','bar'}

#create an empty set
a=set()
#or, use the clear method
a.clear()

#comparison to a new blank set
if a==set():
    #do something

#length-checking comparison
if len(a)==0:
    #do something

The Answer 5

6 people think this answer is useful

Yes. The same notation that works for non-empty dict/set works for empty ones.

Notice the difference between non-empty dict and set literals:

{1: 'a', 2: 'b', 3: 'c'} — a number of key-value pairs inside makes a dict
{'aaa', 'bbb', 'ccc'} — a tuple of values inside makes a set

So:

{} == zero number of key-value pairs == empty dict
{*()} == empty tuple of values == empty set

However the fact, that you can do it, doesn’t mean you should. Unless you have some strong reasons, it’s better to construct an empty set explicitly, like:

a = set()

Performance:

The literal is ~15% faster than the set-constructor (CPython-3.8, 2019 PC, Intel(R) Core(TM) i7-8550U CPU @ 1.80GHz):

>>> %timeit ({*()} &amp; {*()}) | {*()}
214 ns ± 1.26 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)

>>> %timeit (set() &amp; set()) | set()
252 ns ± 0.566 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)

… and for completeness, Renato Garcia’s frozenset proposal on the above expression is some 60% faster!

>>> ϕ = frozenset()

>>> %timeit (ϕ &amp; ϕ) | ϕ
100 ns ± 0.51 ns per loop (mean ± std. dev. of 7 runs, 10000000 loops each)

NB: As ctrueden noticed in comments, {()} is not an empty set. It’s a set with 1 element: empty tuple.

The Answer 6

3 people think this answer is useful

Adding to the crazy ideas: with Python 3 accepting unicode identifiers, you could declare a variable ϕ = frozenset() (ϕ is U+03D5) and use it instead.

The Answer 7

-2 people think this answer is useful

There are few ways to create empty Set in Python :

  1. Using set() method
    This is the built-in method in python that creates Empty set in that variable.
  2. Using clear() method (creative Engineer Technique LOL)
    See this Example:

    sets={“Hi”,”How”,”are”,”You”,”All”}
    type(sets)  (This Line Output : set)
    sets.clear()
    print(sets)  (This Line Output : {})
    type(sets)  (This Line Output : set)

So, This are 2 ways to create empty Set.

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