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Empty set literal?

James Craig |

[] = empty list

() = empty tuple

{} = empty dict

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

12

7 Answers

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

5

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.

5

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

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

More here:

5

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

2

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
4

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.

5

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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