Creating an empty set

Question

I have some code which tots up a set of selected values. I would like to define an empty set and add to it, but {} keeps turning into a dictionary. I have found if I populate the set with a dummy value I can use it, but it's not very elegant. Can someone tell me the proper way to do this? Thanks.

inversIndex = {'five': {1}, 'ten': {2}, 'twenty': {3},
               'two': {0, 1, 2}, 'eight': {2}, 'four': {1},
               'six': {1}, 'seven': {1}, 'three': {0, 2},
               'nine': {2}, 'twelve': {2}, 'zero': {0, 1, 3},
               'eleven': {2}, 'one': {0}}

query = ['four', 'two', 'three']

def orSearch(inverseIndex, query):
    b = [ inverseIndex[c] for c in query ]
    x = {'dummy'}
    for y in b:
        { x.add(z) for z in y }
    x.remove('dummy')
    return x

orSearch(inverseIndex, query)

{0, 1, 2}

A `set`? Just use `set()`. – Brian Cain Jul 15 '13 at 20:29 — Brian Cain, Jul 15 '13 at 20:29
Using a set comprehension only for its side effects? – Josh Lee Jul 15 '13 at 20:32 — Josh Lee, Jul 15 '13 at 20:32
Yeah, a very hacky newbie ;^) – Chris Degnen Jul 15 '13 at 20:36 — Chris Degnen, Jul 15 '13 at 20:36

score 62 · Accepted Answer · edited Jul 15 '13 at 20:32

62

You can just construct a set:

>>> s = set()

will do the job.

edited Jul 15 '13 at 20:32

Rohit Jain

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answered Jul 15 '13 at 20:29

Jon Kiparsky

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inspectorG4dget · Answer 2 · 2013-07-15T20:41:54.460

8

The "proper" way to do it:

myset = set()

The {...} notation cannot be used to initialize an empty set

edited Jul 15 '13 at 20:41

answered Jul 15 '13 at 20:29

inspectorG4dget

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2

Not true; curly braces can be used for nonempty set literals. – Taymon Jul 15 '13 at 20:39

score 3 · Answer 3 · answered Jul 15 '13 at 20:38

As has been pointed out - the way to get an empy set literal is via set(), however, if you re-wrote your code, you don't need to worry about this, eg (and using set()):

from operator import itemgetter
query = ['four', 'two', 'three']
result = set().union(*itemgetter(*query)(inversIndex))
# set([0, 1, 2])

pycoder · Answer 4 · 2019-11-12T05:33:25.393

3

A set literal is just a tuple of values in curly braces:

x = {2, 3, 5, 7}

So, you can create an empty set with empty tuple of values in curly braces:

x = {*()}

Still, just because you can, doesn't mean you should.

Unless it's an obfuscated programming, or a codegolf where every character matters, I'd suggest an explicit x = set() instead.

"Explicit is better than implicit."

edited Nov 12 '19 at 05:33

answered Oct 17 '19 at 22:35

pycoder

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`{*()}` could very easily be constant-folded (and therefore execute slightly faster), unlike `set()`; although as of Python 3.10, it isn’t yet. – user3840170 Feb 20 '22 at 17:27
– user3840170 Apr 15 '22 at 10:10

Creating an empty set

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