What is the simplest way to create an empty iterable using yield in Python?

Question

I was playing around with iterables and more specifically the yield operator in Python. While using test driven development to start writing a new iterable, I wondered what is the shortest code that could make this simple test for an iterable to pass:

def test():
    for x in my_iterable():
        pass

The shortest version I could think of was:

def my_iterable():
    for i in []:
        yield i

Is it possible to write a simpler, shorter or more beautiful (pythonic) version?

Note that if your test iterable is empty and doesn't produce anything -- as shown and in most or all of the answers -- your test won't exercise any of the code in the `for` loop. — martineau, May 16 '12 at 17:08

score 43 · Answer 1 · answered May 16 '12 at 15:22

43

Yes, there is:

return iter([])

answered May 16 '12 at 15:22

James Youngman

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that's nice but I got: `F999` warning – lang2 Sep 05 '17 at 06:05
4

You may want to consider the pythonic [`yield from ()`](https://stackoverflow.com/a/36658865/3077939) instead. – aluriak Feb 05 '20 at 13:38
`[]` creates a new object every time the expression is evaluated. It is better to use empty tuple instead `()`. Empty tuple can even be a singleton (not guaranteed) - every empty tuple refers to the same object. – pabouk - Ukraine stay strong Jun 04 '22 at 21:18

score 29 · Accepted Answer · answered Apr 16 '16 at 00:32

29

Another solution, in Python 3, is to use the new yield from syntax:

def empty_gen():
    yield from ()

Which is readable, and keep empty_gen as a generator.

answered Apr 16 '16 at 00:32

aluriak

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**This is it.** While answers like `lambda: iter(())` satisfy man's instinctual drive for cleverness, they're also unreadable and fail to produce an actual *generator.* In particular, `isinstance(iter(()), types.GeneratorType) == False`. As the relative success of [`mypy`](http://mypy-lang.org/) shows, types sometimes *do* matter. Always `yield from`, Python 3 people. – Cecil Curry Aug 08 '17 at 06:20

score 14 · Answer 3 · edited May 17 '12 at 13:06

14

You can use the lambda and iter functions to create an empty iterable in Python.

my_iterable = lambda: iter(())

edited May 17 '12 at 13:06

octopusgrabbus

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answered May 16 '12 at 15:22

Ignacio Vazquez-Abrams

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georg · Answer 4 · 2012-05-16T16:07:24.803

11

How about

my_iterable = str

this passes your test.

To speak seriously, Iterable in the collections module provides:

def __iter__(self):
    while False:
        yield None

This can be considered "most pythonic" because this is what python itself uses.

Note that technically all answers so far provide iterators (__iter__ + next), not iterables (just __iter__).

edited May 16 '12 at 16:07

answered May 16 '12 at 15:58

georg

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Accepted because of reference to Python standard library. – Adam Lindberg May 21 '12 at 07:10
1

That makes me curious. Why is `str` iterable? – Daniel Walker May 14 '20 at 17:45
@DanielWalker You can iterate over the characters in the string. – MEMark Oct 30 '22 at 20:11
@MEMark, yes, but you can't iterate over `str` itself. – Daniel Walker Nov 02 '22 at 14:44

score 2 · Answer 5 · answered May 16 '12 at 15:26

2

def do_yield():
    return
    yield None

if usage of yield is important for you, one of the other answers otherwise.

answered May 16 '12 at 15:26

glglgl

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score 0 · Answer 6 · answered May 18 '12 at 07:10

Another answer, as I provide a completely new solution with a different approach.

In one of by libraries, I have an EmptyIterator such as

class EmptyIter(object):
    __name__ = 'EmptyIter'
    """Iterable which is False and empty"""
    def __len__(self): return 0
    def next(self): raise StopIteration # even that is redundant
    def __getitem__(self, index): raise IndexError

It is an alternative approach which uses the following properties:

alternative iteration via the sequence protocol (see here)
alternative "falseness" protocol as described here.

What is the simplest way to create an empty iterable using yield in Python?

6 Answers6