I don't like consuming the iterator in the exception case. You might expect just one, but have a fallback approach for more. You could build the safer behavior using itertools.tee() or itertools.chain().

On Mon, Jul 27, 2020, 3:10 PM Noam Yorav-Raphael <noamraph@gmail.com> wrote:

Hi,

There's a simple function that I use many times, and I think may be a good fit to be added to itertools. A function that gets an iterator, and if it has exactly one element returns it, and otherwise raises an exception. This is very useful for cases where I do some sort of query that I expect to get exactly one result, and I want an exception to be raised if I'm wrong. For example:

jack = one(p for p in people if p.id == '1234')

sqlalchemy already has such a function for queries: https://docs.sqlalchemy.org/en/13/orm/query.html#sqlalchemy.orm.query.Query.one

This is my implementation:

def one(iterable):
    it = iter(iterable)
    try:
        r = next(it)
    except StopIteration:
        raise ValueError("Iterator is empty")
    try:
        next(it)
    except StopIteration:
        return r
    else:
        raise ValueError("Iterator has more than one item")

What do you think?

Thanks,

Noam

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