CODEWITHSUNDEEP
pythonpython-3.xset
Thalecress
Thalecress

Reputation: 3451

How do I create a Python set with only one element?

If I have a string, and want to create a set that initially contains only that string, is there a more Pythonic approach than the following?

mySet = set()
mySet.add(myString)

The following gives me a set of the letters in myString:

mySet = set(myString)

Upvotes: 75

Views: 66848

Answers (6)

matt2000
matt2000

Reputation: 1073

use mySet = {mystring}

 Python 3.6.9 (default, Sep 24 2019, 14:35:19)                                                                                             
 Type 'copyright', 'credits' or 'license' for more information                                                                             
 IPython 7.8.0 -- An enhanced Interactive Python. Type '?' for help.                                                                       

 In [1]: def s(i):                                                                                                                         
     ...:     r = set()                                                                                                                     
     ...:     r.add(i)                                                                                                                      
     ...:     return r                                                                                                                      
     ...:                                                                                                                                   

 In [2]: %timeit s(1234)                                                                                                                   
  218 ns ± 5.99 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)                                                                

 In [3]: %timeit set([1234])                                                                                                               
  201 ns ± 3 ns per loop (mean ± std. dev. of 7 runs, 10000000 loops each)                                                                  

 In [4]: %timeit {1234}                                                                                                                    
  51.7 ns ± 1.7 ns per loop (mean ± std. dev. of 7 runs, 10000000 loops each)

Upvotes: 1

Anatoly Scherbakov
Anatoly Scherbakov

Reputation: 1762

For example, this easy way:

mySet = set([myString])

Upvotes: 25

dstromberg
dstromberg

Reputation: 7177

In 2.7 as well as 3.x, you can use:

mySet = {'abc'}

Upvotes: 85

SZIEBERTH Ádám
SZIEBERTH Ádám

Reputation: 4184

set(obj) is going to iterate trough obj and add all unique elements to the set. Since strings are also iterable, if you pass a string to set() then you get the unique letters in your set. You can put your obj to a list first:

set(["mystring"])

However that is not elegant IMO. You know, even for the creation of an empty dictionary we prefer {} over dict(). Same here. I wolud use the following syntax:

myset = {"mystring"}

Note that for tuples, you need a comma after it:

mytuple = ("mystring",)

Upvotes: 3

Ashwini Chaudhary
Ashwini Chaudhary

Reputation: 251001

For Python2.7+:

set_display ::=  "{" (expression_list | comprehension) "}"

Example:

>>> myString = 'foobar'
>>> s = {myString}
>>> s
set(['foobar'])

>>> s = {'spam'}
>>> s
set(['spam'])

Note that an empty {} is not a set, its a dict.

Help on set:

class set(object)
 |  set() -> new empty set object
 |  set(iterable) -> new set object

As you can see set() expects an iterable and strings are iterable too, so it converts the string characters to a set.

Put the string in some iterable and pass it to set():

>>> set(('foo',))  #tuple
set(['foo'])
>>> set(['foo'])   #list
set(['foo'])

Upvotes: 7

Simeon Visser
Simeon Visser

Reputation: 122376

If the set also isn't likely to change, consider using a frozenset:

mySet = frozenset([myString])

Upvotes: 0

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