docs: def one(src, default=None, key=None):
      """Along the same lines as builtins, :func:`all` and :func:`any`, and
      similar to :func:`first`, ``one()`` returns the single object in
      the given iterable *src* that evaluates to ``True``, as determined
      by callable *key*. If unset, *key* defaults to :class:`bool`. If
      no such objects are found, *default* is returned. If *default* is
      not passed, ``None`` is returned.
  
      If *src* has more than one object that evaluates to ``True``, or
      if there is no object that fulfills such condition, return
      *default*. It's like an `XOR`_ over an iterable.
  
      >>> one((True, False, False))
      True
      >>> one((True, False, True))
      >>> one((0, 0, 'a'))
      'a'
      >>> one((0, False, None))
      >>> one((True, True), default=False)
      False
      >>> bool(one(('', 1)))
      True
      >>> one((10, 20, 30, 42), key=lambda i: i > 40)
      42
  
      See `Martín Gaitán's original repo`_ for further use cases.
  
      .. _Martín Gaitán's original repo: https://github.com/mgaitan/one
      .. _XOR: https://en.wikipedia.org/wiki/Exclusive_or
  
      """
      ones = list(itertools.islice(filter(key, src), 2))
      return ones[0] if len(ones) == 1 else default