In my personal toolbox of utility functions, this is by far the function I use most often, although it's implemented slightly differently and I call it `only`. I think it's very useful and it would be great to have in the standard library to encourage people to write safer code.
Often this is part of a larger expression, especially if I'm drilling into some nested data structure. This can lead to having a prefix operation (the function call) breaking a chain of postfix operations (attributes, method calls, subscripting...) which is ugly and less readable. It would be nice if this could also be available as a method on lists, tuples, and sets to keep the data flowing left to right. Plus it would save an import. On Tue, Jul 28, 2020 at 9:29 PM Noam Yorav-Raphael <noamr...@gmail.com> wrote: > Hello, > > 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 > > more-itertools has this exact function: > https://more-itertools.readthedocs.io/en/latest/api.html#more_itertools.one > > Here is a simple implementation: > > def one(iterable): > it = iter(iterable) > try: > first = next(it) > except StopIteration: > raise ValueError("Iterator is empty") > try: > second = next(it) > except StopIteration: > return first > else: > raise ValueError("Iterator has more than one item") > > I brought this up on python-dev, but it should be discussed here. > Here is the discussion: > > https://mail.python.org/archives/list/python-...@python.org/thread/D52MPKLIN4VEXBOCKVMTWAK66MAOEINY/ > > Brett Cannon said that this idea has been brought up at least twice > before. I found: > > https://mail.python.org/archives/list/python-ideas@python.org/thread/FTJ6JRDTZ57HUVZ3PVIZV2NHU2NLAC4X/#RMWV3SNZ2N4KZLPKPIDE42H46QDEIVHE > > > https://mail.python.org/archives/list/python-ideas@python.org/thread/REYDJFCXQNQG4SAWKELQMCGM77IZG47Q/#ITR2ILPVCKYR52U2D7RHGENASZTNVDHN > > The first thread hasn't reached any operative conclusion. The second > thread was very long, and seemed to focus mostly on another function, > first(), that doesn't check if there is more than one item. Joao S. O Bueno > said that it passed "general approval". I think that perhaps a new > discussion, focused just on one (no pun intended) function in the itertools > module may reach a conclusion. > > It was suggested that instead of an additional function, one can use > iterator unpacking: > > jack, = (p for p in people if p.id == '1234') > or > [jack] = (p for p in people if p.id == '1234') > > I still think that having a one() function would be useful, since: > 1. I think it spells the intention more clearly. It is not symbols that > you need to understand their meaning in order to understand that I expect > the iterable to have exactly one item, it's spelled in code. > 2. The exception would be easier to understand, since errors in tuple > unpacking usually mean something else. > 3. The one() function allows you to use the result inside an expression > without assigning it to a variable. Therefore, I think it encourages > writing better code. It's very easy to write: > print([p for p in people if p.id == '1234][0]) > (which has the problem of not verifying the assumption that there's no > more than one result), and I find it easier to replace _[0] with one(_) > than to be required to name a new variable, and instead of having an > operation on the iterable, change the way I'm assigning to it. > > WDYT? > > Cheers, > Noam > > > _______________________________________________ > Python-ideas mailing list -- python-ideas@python.org > To unsubscribe send an email to python-ideas-le...@python.org > https://mail.python.org/mailman3/lists/python-ideas.python.org/ > Message archived at > https://mail.python.org/archives/list/python-ideas@python.org/message/6OLEL4XTUWXRI7ENODKEDOYFBRVDYKI7/ > Code of Conduct: http://python.org/psf/codeofconduct/ >
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