I would think that you can use tuple unpacking for this?

    single_element, = some_iterator

Will attempt to unpack the some_iterator into single_element and fail if single_element doesn't have exactly 1 element. It also has the added benefit that it works for any number of elements:

one, two = some_iterator

Example:

>>> a = [1]
>>> one, = a
>>> one
1
>>> b = [1, 2]
>>> one, = b
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-5-9b70ee0300f5> in <module>
----> 1 one, = b

ValueError: too many values to unpack (expected 1)
>>> c = []
>>> one, = c
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-7-413c1b218942> in <module>
----> 1 one, = c

ValueError: not enough values to unpack (expected 1, got 0)

Best,
Paul

I second this as being useful.

However the “pythonic” way (whatever that means nowadays) is to do a for break else loop, which I think is kinda difficult to read as you need to make a few assumptions.

Rollo

On 27 Jul 2020, at 20:06, 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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