ajor=2))
        "Version('2')"
        """
        result = "Version('{}".format(self)
        for name in ('micro', 'releaselevel', 'serial'):
            value = getattr(self, name)
            if not value:
                break
            result += '.{}'.format(value)
        return result + "')"

    def __add__(self, other):
        """Return next version.

        >>> Version('3.1') + 1
        Version('3.2')
        >>> Version('2') + '1'
        Version('3')
        """
        result = Version(self)
        if self.minor is None:
            result.major += int(other)
        else:
            result.minor += int(other)
        return result

    def __sub__(self, other):
        """Return previous version.

        >>> Version('3.1') - 1
        Version('3.0')
        >>> Version('3') - '1'
        Version('2')
        """
        result = Version(self)
        if self.minor is None:
            result.major -= int(other)
            new = result.major
        else:
            result.minor -= int(other)
            new = result.minor
        if new < 0:
            raise ValueError('cannot decrease version further')
        return result

    def __eq__(self, other):
        return self.__cmp(other) == 0

    def __lt__(self, other):
        return self.__cmp(other) < 0

    def __le__(self, other):
        return self.__cmp(other) <= 0

    def __gt__(self, other):
        return self.__cmp(other) > 0

    def __ge__(self, other):
        return self.__cmp(other) >= 0

    def __lshift__(self, other):
        """Compare major.minor or major only (if minor is not set).

        >>> Version('2.6') << Version('2.7')
        True
        >>> Version('2.6') << Version('2.6.6')
        False
        >>> Version('3') << Version('2')
        False
        >>> Version('3.1') << Version('2')
        False
        >>> Version('2') << Version('3.2.1.alpha.3')
        True
        """
        if not isinstance(other, Version):
            other = Version(other)
        if self.minor is None or other.minor is None:
            return self.__cmp(other, ignore='minor') < 0
        else:
            return self.__cmp(other, ignore='micro') < 0

    def __rshift__(self, other):
        """Compare major.minor or major only (if minor is not set).

        >>> Version('2.6') >> Version('2.7')
        False
        >>> Version('2.6.7') >> Version('2.6.6')
        False
        >>> Version('3') >> Version('2')
        True
        >>> Version('3.1') >> Version('2')
        True
        >>> Version('2.1') >> Version('3.2.1.alpha.3')
        False
        """
        if not isinstance(other, Version):
            other = Version(other)
        if self.minor is None or other.minor is None:
            return self.__cmp(other, ignore='minor') > 0
        else:
            return self.__cmp(other, ignore='micro') > 0

    def __cmp(self, other, ignore=None):
        if not isinstance(other, Version):
            other = Version(other)
        for name in ('major', 'minor', 'micro', 'releaselevel', 'serial'):
            if name == ignore:
                break
            value1 = getattr(self, name) or 0
            value2 = getattr(other, name) or 0
            if name == 'releaselevel':
                rmap = {'alpha': -3, 'beta': -2, 'candidate': -1, 'final': 0}
                value1 = rmap.get(value1, 0)
                value2 = rmap.get(value2, 0)
            if value1 == value2:
                continue
            return (value1 > value2) - (value1 < value2)
        return 0


def get_requested_versions(vrange=None, available=None):
    """Return a set of requested and supported Python versions.

    :param available: if set to `True`, return installed versions only,
        if set to `False`, return requested versions that are not installed.
        By default returns all requested versions.
    :type available: bool

    >>> sorted(get_requested_versions([(3, 0), None]))
    [(3, 2), (3, 3)]
    >>> sorted(get_requested_versions('')) == sorted(SUPPORTED)
    True
    >>> sorted(get_requested_versions([None, None])) == sorted(SUPPORTED)
    True
    >>> get_requested_versions([(5, 0), None]) == set()
    True
    """
    if isinstance(vrange, str):
        vrange = parse_vrange(vrange)

    if not vrange or list(vrange) == [None, None]:
        versions = set(SUPPORTED)
    else:
        minv = (0, 0) if vrange[0] is None else vrange[0]
        maxv = (99, 99) if vrange[1] is None else vrange[1]
        if minv == maxv:
            versions = set((minv,) if minv in SUPPORTED else tuple())
        else:
            versions = set(v for v in SUPPORTED if minv <= v < maxv)

    if available:
        versions = set(v for v in versions
                       if exists("/usr/bin/python%d.%d" % v))
    elif available is False:
        versions = set(v for v in versions
                       if not exists("/usr/bin/python%d.%d" % v))

    return versions


def parse_vrange(value):
    """Return minimum and maximum Python version from given range.

    >>> parse_vrange('3.0-')
    ((3, 0), None)
    >>> parse_vrange('3.1-3.3')
    ((3, 1), (3, 3))
    >>> parse_vrange('3.2-4.0')
    ((3, 2), (4, 0))
    >>> parse_vrange('-3.7')
    (None, (3, 7))
    >>> parse_vrange('3.2')
    ((3, 2), (3, 2))
    >>> parse_vrange('') == parse_vrange('-') == (None, None)
    True
    """
    if value in ('', '-'):
        return None, None

    match = RANGE_RE.match(value)
    if not match:
        raise ValueError("version range is invalid: %s" % value)
    groups = match.groups()

    if list(groups).count(None) == 3:  # only one version is allowed
        minv = tuple(int(i) for i in groups[1].split('.'))
        return minv, minv

    minv = maxv = None
    if groups[0]:  # maximum version only
        maxv = groups[1]
    else:
        minv = groups[1]
        maxv = groups[3]

    minv = tuple(int(i) for i in minv.split('.')) if minv else None
    maxv = tuple(int(i) for i in maxv.split('.')) if maxv else None

    if maxv and minv and minv > maxv:
        raise ValueError("version range is invalid: %s" % value)

    return minv, maxv


def parse_pycentral_vrange(value):
    """Parse X-Python3-Version.

    >>> parse_pycentral_vrange('>= 3.1')
    ((3, 1), None)
    >>> parse_pycentral_vrange('<< 4.0')
    (None, (4, 0))
    >>> parse_pycentral_vrange('3.1')
    ((3, 1), (3, 1))
    >>> parse_pycentral_vrange('3.1, 3.2')
    ((3, 1), None)
    """
    #get = lambda x: get_requested_versions(parse_vrange(x))

    minv = maxv = None
    hardcoded = set()

    for item in value.split(','):
        item = item.strip()

        match = re.match(r'>=\s*([\d\.]+)', item)
        if match:
            minv = "%.3s" % match.group(1)
            continue
        match = re.match(r'<<\s*([\d\.]+)', item)
        if match:
            maxv = "%.3s" % match.group(1)
            continue
        match = re.match(r'^[\d\.]+$', item)
        if match:
            hardcoded.add("%.3s" % match.group(0))

    if len(hardcoded) == 1:
        ver = hardcoded.pop()
        return getver(ver), getver(ver)

    if not minv and hardcoded:
        # yeah, no maxv!
        minv = sorted(hardcoded)[0]

    return getver(minv) if minv else None, getver(maxv) if maxv else None


def vrange_str(vrange):
    """Return version range string from given range.

    >>> vrange_str(((3, 4), None))
    '3.4-'
    >>> vrange_str(((3, 4), (3, 6)))
    '3.4-3.6'
    >>> vrange_str(((3, 4), (4, 0)))
    '3.4-4.0'
    >>> vrange_str((None, (3, 7)))
    '-3.7'
    >>> vrange_str(((3, 5), (3, 5)))
    '3.5'
    >>> vrange_str((None, None))
    '-'
    """
    if vrange[0] is vrange[1] is None:
        return '-'
    if vrange[0] == vrange[1]:
        return '.'.join(str(i) for i in vrange[0])
    elif vrange[0] is None:
        return '-' + '.'.join(str(i) for i in vrange[1])
    elif vrange[1] is None:
        return '.'.join(str(i) for i in vrange[0]) + '-'
    else:
        return "%s-%s" % ('.'.join(str(i) for i in vrange[0]),
                          '.'.join(str(i) for i in vrange[1]))


def vrepr(value):
    """
    >>> vrepr(([3, 4], [3, 2]))
    ['3.4', '3.2']
    >>> vrepr(('2.6', '3.1'))
    ['2.6', '3.1']
    >>> vrepr('3.2')
    '3.2'
    >>> vrepr((3, 2))
    '3.2'
    """
    if isinstance(value, str):
        return value
    elif not isinstance(value, (GeneratorType, set)) and isinstance(value[0], int):
        return '.'.join(str(i) for i in value)

    result = []
    for version in value:
        if isinstance(version, str):
            result.append(version)
        else:
            result.append('.'.join(str(i) for i in version))
    return result


def getver(value):
    """Return pair of integers that represent version.

    >>> getver('3.2')
    (3, 2)
    >>> getver('3.1.4')
    (3, 1)
    >>> getver((3, 3, 1))
    (3, 3)
    >>> getver(None)
    ''
    """
    if not value:
        return ''
    if isinstance(value, tuple):
        return value[:2]
    return tuple(int(i) for i in value.split('.', 2))[:2]


def debsorted(versions, return_str=None):
    """Return sorted list of versions starting with default Python
    version (if available) then list of suppored versions greater than default
    one followed by reversed list of older versions.

    List of versions sorted this way can be used in Depends field.

    :param vrepr: return string represenatations of versions, by default the
        same format is used as in :param:`versions`

    >>> debsorted([(2, 6), (3, 1), (3, 2), (3, 3), (2, 7)])[0] == DEFAULT
    True
    >>> debsorted(('2.4', '3.2', '2.6', '2.7'))[-1]
    (2, 4)
    >>> debsorted(set([(2, 1), (2, 2)]))
    [(2, 2), (2, 1)]
    >>> debsorted([(2, 1), (2, 2)], return_str=True)
    ['2.2', '2.1']
    """
    result = []
    old_versions = []
    for version in sorted(versions):
        if isinstance(version, str):
            version = getver(version)
        if version < DEFAULT:
            old_versions.append(version)
        else:
            result.append(version)
    result.extend(reversed(old_versions))
    if return_str and result:
        return vrepr(result)
    return result
