Pythonic style

[Tim Chase]

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.

-tim