1  # from more_itertools 9.0
       2  def only(iterable, default=None, too_long=None):
       3      """If *iterable* has only one item, return it.
       4      If it has zero items, return *default*.
       5      If it has more than one item, raise the exception given by *too_long*,
       6      which is ``ValueError`` by default.
       7      >>> only([], default='missing')
       8      'missing'
       9      >>> only([1])
      10      1
      11      >>> only([1, 2])  # doctest: +IGNORE_EXCEPTION_DETAIL
      12      Traceback (most recent call last):
      13      ...
      14      ValueError: Expected exactly one item in iterable, but got 1, 2,
      15       and perhaps more.'
      16      >>> only([1, 2], too_long=TypeError)  # doctest: +IGNORE_EXCEPTION_DETAIL
      17      Traceback (most recent call last):
      18      ...
      19      TypeError
      20      Note that :func:`only` attempts to advance *iterable* twice to ensure there
      21      is only one item.  See :func:`spy` or :func:`peekable` to check
      22      iterable contents less destructively.
      23      """
      24      it = iter(iterable)
      25      first_value = next(it, default)
      26  
      27      try:
      28          second_value = next(it)
      29      except StopIteration:
      30          pass
      31      else:
      32          msg = (
      33              'Expected exactly one item in iterable, but got {!r}, {!r}, '
      34              'and perhaps more.'.format(first_value, second_value)
      35          )
      36          raise too_long or ValueError(msg)
      37  
      38      return first_value