Loop over empty iterator does not raise exception

Question

I am on Python 3.6.7.

I just noticed that a for loop over an empty list does not loop even once. After some thought, that made some sense to me. I.e.

Answer 1

There is no builtin way to directly check for empty iterators. Empty iterators are generally not considered exceptional. However, you can define compact helpers to explicitly check for empty iterators.

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Detecting empty iterators up-front is generally not possible. In some cases, one can guess from the source of an iterator whether it is empty -- for example, iterators of sequences and mappings are empty if their parent is boolean false. However, iterators themselves do not have any indication for content and may in fact be "empty" unless iterated over.

The only reliable means is to check whether an iterator provides items while iterating. One can define a wrapper iterator that raises an error if an iterator provides no items at all.

def non_empty(iterable):
    """Helper to ensure that ``iterable`` is not empty during iteration"""
    iterator = iter(iterable)
    try:
        yield next(iterator)  # explicitly check first item
    except StopIteration:
        raise LookupError(f'{iterable} is empty') from None
    yield from iterator       # forward iteration of later items

Such a helper can be wrapped around both iterables and iterators, and works in for loops, explicit iter iterators and any other iteration scenario.

>>> iterable = []
>>> for element in non_empty(iterable):
...    assert isinstance(element, int)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 6, in non_empty
LookupError: [] is empty

Answer 2

Why do you need to do something if the loop wont run because the list is empty? It's perfectly normal behavior. If you expect for some reason to have at least one item (maybe having no items in array is a measure of error in your program) then check if len(iterator) == 0 as you suggested earlier. Its a perfectly valid way to do it.

By the way exeptions are the way to go in those cases when you are checking the values of variables etc. Asserts are more of the hard-coded info for critical failures and SHOULD NOT be used for reporting usual errors as they cant be handled.

Answer 3

You can use the Statements, and else Clauses on Loops.

Loop statements may have an else clause; it is executed when the loop terminates through exhaustion of the iterable.

For example:

iterator = []
for element in iterator:
    print('This wont print..')
else:
    assert iterator

This will results with:

Traceback (most recent call last):
  File "<pyshell#2>", line 4, in <module>
    assert iterator
AssertionError