Pythonic style

[Chris Angelico]

On Mon, Sep 21, 2020 at 11:37 PM Tim Chase
<python.list at tim.thechases.com> wrote:
>
> On 2020-09-20 18:34, Stavros Macrakis wrote:
> > Consider a simple function which returns the first element of an
> > iterable if it has exactly one element, and throws an exception
> > otherwise. It should work even if the iterable doesn't terminate.
> > I've written this function in multiple ways, all of which feel a
> > bit clumsy.
> >
> > I'd be interested to hear thoughts on which of these solutions is
> > most Pythonic in style. And of course if there is a more elegant
> > way to solve this, I'm all ears! I'm probably missing something
> > obvious!
>
> You can use tuple unpacking assignment and Python will take care of
> the rest for you:
>
>   >>> x, = tuple() # no elements
>   Traceback (most recent call last):
>     File "<stdin>", line 1, in <module>
>   ValueError: not enough values to unpack (expected 1, got 0)
>   >>> x, = (1, )  # one element
>   >>> x, = itertools.repeat("hello") # 2 to infinite elements
>   Traceback (most recent call last):
>     File "<stdin>", line 1, in <module>
>   ValueError: too many values to unpack (expected 1)
>
> so you can do
>
>   def fn(iterable):
>     x, = iterable
>     return x
>
> The trailing comma can be hard to spot, so I usually draw a little
> extra attention to it with either
>
>   (x, ) = iterable
>
> or
>
>   x, = iterable # unpack one value
>
> I'm not sure it qualifies as Pythonic, but it uses Pythonic features
> like tuple unpacking and the code is a lot more concise.

Or:

[x] = iterable

I'd definitely recommend using unpacking as the most obvious way to do
this. Among other advantages, it gives different messages for the "too
many" and "too few" cases.

ChrisA