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

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