# Source code for rdflib.term

    """
    This module defines the different types of terms. Terms are the kinds of
    objects that can appear in a quoted/asserted triple. This includes those
    that are core to RDF:

    * :class:`Blank Nodes <rdflib.term.BNode>`
    * :class:`URI References <rdflib.term.URIRef>`
    * :class:`Literals <rdflib.term.Literal>` (which consist of a literal value,datatype and language tag)

    Those that extend the RDF model into N3:

    * :class:`Formulae <rdflib.graph.QuotedGraph>`
    * :class:`Universal Quantifications (Variables) <rdflib.term.Variable>`

    And those that are primarily for matching against 'Nodes' in the
    underlying Graph:

    * REGEX Expressions
    * Date Ranges
    * Numerical Ranges

    """

    __all__ = [
        'bind',

        'Node',
        'Identifier',

        'URIRef',
        'BNode',
        'Literal',

        'Variable',
        'Statement',
    ]

    import logging
    logger = logging.getLogger(__name__)
    import warnings
    import math

    import base64
    import xml.dom.minidom

    from urllib.parse import urlparse, urljoin, urldefrag
    from datetime import date, time, datetime
    from re import sub, compile
    from collections import defaultdict

    from isodate import parse_time, parse_date, parse_datetime

    try:
        from hashlib import md5
        assert md5
    except ImportError:
        from md5 import md5

    import rdflib
    from . import py3compat
    from rdflib.compat import numeric_greater

    b = py3compat.b

    skolem_genid = "/.well-known/genid/"
    rdflib_skolem_genid = "/.well-known/genid/rdflib/"
    skolems = {}

    _invalid_uri_chars = '<>" {}|\\^`'

    def _is_valid_uri(uri):
        for c in _invalid_uri_chars:
            if c in uri: return False
        return True

    _lang_tag_regex = compile('^[a-zA-Z]+(?:-[a-zA-Z0-9]+)*$')

    def _is_valid_langtag(tag):
        return bool(_lang_tag_regex.match(tag))

    def _is_valid_unicode(value):
        """
        Verify that the provided value can be converted into a Python
        unicode object.
        """
        if isinstance(value, bytes):
            coding_func, param = getattr(value, 'decode'), 'utf-8'
        elif py3compat.PY3:
            coding_func, param = str, value
        else:
            coding_func, param = str, value

        # try to convert value into unicode
        try:
            coding_func(param)
        except UnicodeError:
            return False
        return True

    class Node(object):
        """
        A Node in the Graph.
        """

        __slots__ = ()

    class Identifier(Node, str):  # allow Identifiers to be Nodes in the Graph
        """
        See http://www.w3.org/2002/07/rdf-identifer-terminology/
        regarding choice of terminology.
        """

        __slots__ = ()

        def __new__(cls, value):
            return str.__new__(cls, value)

        def eq(self, other):
            """A "semantic"/interpreted equality function,
            by default, same as __eq__"""
            return self.__eq__(other)

        def neq(self, other):
            """A "semantic"/interpreted not equal function,
            by default, same as __ne__"""
            return self.__ne__(other)

        def __ne__(self, other):
            return not self.__eq__(other)

        def __eq__(self, other):
            """
            Equality for Nodes.

            >>> BNode("foo")==None
            False
            >>> BNode("foo")==URIRef("foo")
            False
            >>> URIRef("foo")==BNode("foo")
            False
            >>> BNode("foo")!=URIRef("foo")
            True
            >>> URIRef("foo")!=BNode("foo")
            True
            >>> Variable('a')!=URIRef('a')
            True
            >>> Variable('a')!=Variable('a')
            False
            """

            if type(self) == type(other):
                return str(self) == str(other)
            else:
                return False

        def __gt__(self, other):
            """
            This implements ordering for Nodes,

            This tries to implement this:
            http://www.w3.org/TR/sparql11-query/#modOrderBy

            Variables are not included in the SPARQL list, but
            they are greater than BNodes and smaller than everything else

            """
            if other is None:
                return True  # everything bigger than None
            elif type(self) == type(other):
                return str(self) > str(other)
            elif isinstance(other, Node):
                return _ORDERING[type(self)] > _ORDERING[type(other)]

            return NotImplemented

        def __lt__(self, other):
            if other is None:
                return False  # Nothing is less than None
            elif type(self) == type(other):
                return str(self) < str(other)
            elif isinstance(other, Node):
                return _ORDERING[type(self)] < _ORDERING[type(other)]

            return NotImplemented

        def __le__(self, other):
            r = self.__lt__(other)
            if r:
                return True
            return self == other

        def __ge__(self, other):
            r = self.__gt__(other)
            if r:
                return True
            return self == other

        def __hash__(self):
            t = type(self)
            fqn = t.__module__ + '.' + t.__name__
            return hash(fqn) ^ hash(str(self))

    class URIRef(Identifier):
        """
        RDF URI Reference: http://www.w3.org/TR/rdf-concepts/#section-Graph-URIref
        """

        __slots__ = ()

        def __new__(cls, value, base=None):
            if base is not None:
                ends_in_hash = value.endswith("#")
                value = urljoin(base, value, allow_fragments=1)
                if ends_in_hash:
                    if not value.endswith("#"):
                        value += "#"

            if not _is_valid_uri(value):
                logger.warning('%s does not look like a valid URI, trying to serialize this will break.'%value)

            try:
                rt = str.__new__(cls, value)
            except UnicodeDecodeError:
                rt = str.__new__(cls, value, 'utf-8')
            return rt

        def toPython(self):
            return str(self)

        def n3(self, namespace_manager = None):
            """
            This will do a limited check for valid URIs,
            essentially just making sure that the string includes no illegal
            characters (``<, >, ", {, }, |, \\, `, ^``)

            :param namespace_manager: if not None, will be used to make up
                 a prefixed name
            """

            if not _is_valid_uri(self):
                raise Exception('"%s" does not look like a valid URI, I cannot serialize this as N3/Turtle. Perhaps you wanted to urlencode it?'%self)

            if namespace_manager:
                return namespace_manager.normalizeUri(self)
            else:
                return "<%s>" % self

        def defrag(self):
            if "#" in self:
                url, frag = urldefrag(self)
                return URIRef(url)
            else:
                return self

        def __reduce__(self):
            return (URIRef, (str(self),))

        def __getnewargs__(self):
            return (str(self), )

        if not py3compat.PY3:
            def __str__(self):
                return self.encode()

        def __repr__(self):
            if self.__class__ is URIRef:
                clsName = "rdflib.term.URIRef"
            else:
                clsName = self.__class__.__name__

            return """%s(%s)""" % (clsName, super(URIRef, self).__repr__())

        def __add__(self, other):
            return self.__class__(str(self) + other)

        def __radd__(self, other):
            return self.__class__(other + str(self))

        def __mod__(self, other):
            return self.__class__(str(self) % other)

        def md5_term_hash(self):
            """a string of hex that will be the same for two URIRefs that
            are the same. It is not a suitable unique id.

            Supported for backwards compatibility; new code should
            probably just use __hash__
            """
            warnings.warn(
                "method md5_term_hash is deprecated, and will be removed " +
                "in the future. If you use this please let rdflib-dev know!",
                category=DeprecationWarning, stacklevel=2)
            d = md5(self.encode())
            d.update(b("U"))
            return d.hexdigest()

        def de_skolemize(self):
            """ Create a Blank Node from a skolem URI, in accordance
            with http://www.w3.org/TR/rdf11-concepts/#section-skolemization.
            This function accepts only rdflib type skolemization, to provide
            a round-tripping within the system.

            .. versionadded:: 4.0
            """
            if isinstance(self, RDFLibGenid):
                parsed_uri = urlparse("%s" % self)
                return BNode(
                    value=parsed_uri.path[len(rdflib_skolem_genid):])
            elif isinstance(self, Genid):
                bnode_id = "%s" % self
                if bnode_id in skolems:
                    return skolems[bnode_id]
                else:
                    retval = BNode()
                    skolems[bnode_id] = retval
                    return retval
            else:
                raise Exception("<%s> is not a skolem URI" % self)

    class Genid(URIRef):
        __slots__ = ()

        @staticmethod
        def _is_external_skolem(uri):
            if not isinstance(uri, str):
                uri = str(uri)
            parsed_uri = urlparse(uri)
            gen_id = parsed_uri.path.rfind(skolem_genid)
            if gen_id != 0:
                return False
            return True

    class RDFLibGenid(Genid):
        __slots__ = ()

        @staticmethod
        def _is_rdflib_skolem(uri):
            if not isinstance(uri, str):
                uri = str(uri)
            parsed_uri = urlparse(uri)
            if parsed_uri.params != "" \
                    or parsed_uri.query != "" \
                    or parsed_uri.fragment != "":
                return False
            gen_id = parsed_uri.path.rfind(rdflib_skolem_genid)
            if gen_id != 0:
                return False
            return True

    def _unique_id():
        # Used to read: """Create a (hopefully) unique prefix"""
        # now retained merely to leave interal API unchanged.
        # From BNode.__new__() below ...
        #
        # acceptable bnode value range for RDF/XML needs to be
        # something that can be serialzed as a nodeID for N3
        #
        # BNode identifiers must be valid NCNames" _:[A-Za-z][A-Za-z0-9]*
        # http://www.w3.org/TR/2004/REC-rdf-testcases-20040210/#nodeID
        return "N"  # ensure that id starts with a letter

    def _serial_number_generator():
        """
        Generates UUID4-based but ncname-compliant identifiers.
        """
        from uuid import uuid4

        def _generator():
            return uuid4().hex

        return _generator

    class BNode(Identifier):
        """
        Blank Node: http://www.w3.org/TR/rdf-concepts/#section-blank-nodes

        """
        __slots__ = ()

        def __new__(cls, value=None,
                    _sn_gen=_serial_number_generator(), _prefix=_unique_id()):
            """
            # only store implementations should pass in a value
            """
            if value is None:
                # so that BNode values do not collide with ones created with
                # a different instance of this module at some other time.
                node_id = _sn_gen()
                value = "%s%s" % (_prefix, node_id)
            else:
                # TODO: check that value falls within acceptable bnode value range
                # for RDF/XML needs to be something that can be serialzed
                # as a nodeID for N3 ??  Unless we require these
                # constraints be enforced elsewhere?
                pass  # assert is_ncname(unicode(value)), "BNode identifiers
                     # must be valid NCNames" _:[A-Za-z][A-Za-z0-9]*
                     # http://www.w3.org/TR/2004/REC-rdf-testcases-20040210/#nodeID
            return Identifier.__new__(cls, value)

        def toPython(self):
            return str(self)

        def n3(self, namespace_manager=None):
            return "_:%s" % self

        def __getnewargs__(self):
            return (str(self), )

        def __reduce__(self):
            return (BNode, (str(self),))

        if not py3compat.PY3:
            def __str__(self):
                return self.encode()

        def __repr__(self):
            if self.__class__ is BNode:
                clsName = "rdflib.term.BNode"
            else:
                clsName = self.__class__.__name__
            return """%s('%s')""" % (clsName, str(self))

        def md5_term_hash(self):
            """a string of hex that will be the same for two BNodes that
            are the same. It is not a suitable unique id.

            Supported for backwards compatibility; new code should
            probably just use __hash__
            """
            warnings.warn(
                "method md5_term_hash is deprecated, and will be removed " +
                "in the future. If you use this please let rdflib-dev know!",
                category=DeprecationWarning, stacklevel=2)
            d = md5(self.encode())
            d.update(b("B"))
            return d.hexdigest()

        def skolemize(self, authority="http://rdlib.net/"):
            """ Create a URIRef "skolem" representation of the BNode, in accordance
            with http://www.w3.org/TR/rdf11-concepts/#section-skolemization

            .. versionadded:: 4.0
            """
            skolem = "%s%s" % (rdflib_skolem_genid, str(self))
            return URIRef(urljoin(authority, skolem))

    class Literal(Identifier):
        __doc__ = py3compat.format_doctest_out("""
        RDF Literal: http://www.w3.org/TR/rdf-concepts/#section-Graph-Literal

        The lexical value of the literal is the unicode object
        The interpreted, datatyped value is available from .value

        Language tags must be valid according to :rfc:5646

        For valid XSD datatypes, the lexical form is optionally normalized
        at construction time. Default behaviour is set by rdflib.NORMALIZE_LITERALS
        and can be overridden by the normalize parameter to __new__

        Equality and hashing of Literals are done based on the lexical form, i.e.:

        >>> from rdflib.namespace import XSD

        >>> Literal('01')!=Literal('1') # clear - strings differ
        True

        but with data-type they get normalized:

        >>> Literal('01', datatype=XSD.integer)!=Literal('1', datatype=XSD.integer)
        False

        unless disabled:

        >>> Literal('01', datatype=XSD.integer, normalize=False)!=Literal('1', datatype=XSD.integer)
        True

        Value based comparison is possible:

        >>> Literal('01', datatype=XSD.integer).eq(Literal('1', datatype=XSD.float))
        True

        The eq method also provides limited support for basic python types:

        >>> Literal(1).eq(1) # fine - int compatible with xsd:integer
        True
        >>> Literal('a').eq('b') # fine - str compatible with plain-lit
        False
        >>> Literal('a', datatype=XSD.string).eq('a') # fine - str compatible with xsd:string
        True
        >>> Literal('a').eq(1) # not fine, int incompatible with plain-lit
        NotImplemented

        Greater-than/less-than ordering comparisons are also done in value
        space, when compatible datatypes are used.  Incompatible datatypes
        are ordered by DT, or by lang-tag.  For other nodes the ordering
        is None < BNode < URIRef < Literal

        Any comparison with non-rdflib Node are "NotImplemented"
        In PY2.X some stable order will be made up by python

        In PY3 this is an error.

        >>> from rdflib import Literal, XSD
        >>> lit2006 = Literal('2006-01-01',datatype=XSD.date)
        >>> lit2006.toPython()
        datetime.date(2006, 1, 1)
        >>> lit2006 < Literal('2007-01-01',datatype=XSD.date)
        True
        >>> Literal(datetime.utcnow()).datatype
        rdflib.term.URIRef(%(u)s'http://www.w3.org/2001/XMLSchema#dateTime')
        >>> Literal(1) > Literal(2) # by value
        False
        >>> Literal(1) > Literal(2.0) # by value
        False
        >>> Literal('1') > Literal(1) # by DT
        True
        >>> Literal('1') < Literal('1') # by lexical form
        False
        >>> Literal('a', lang='en') > Literal('a', lang='fr') # by lang-tag
        False
        >>> Literal(1) > URIRef('foo') # by node-type
        True

        The > < operators will eat this NotImplemented and either make up
        an ordering (py2.x) or throw a TypeError (py3k):

        >>> Literal(1).__gt__(2.0)
        NotImplemented

        """)

        if not py3compat.PY3:
            __slots__ = ("language", "datatype", "value", "_language",
                         "_datatype", "_value")
        else:
            __slots__ = ("_language", "_datatype", "_value")

        def __new__(cls, lexical_or_value, lang=None, datatype=None, normalize=None):

            if lang == '':
                lang = None  # no empty lang-tags in RDF

            normalize = normalize if normalize != None else rdflib.NORMALIZE_LITERALS

            if lang is not None and datatype is not None:
                raise TypeError(
                    "A Literal can only have one of lang or datatype, "
                    "per http://www.w3.org/TR/rdf-concepts/#section-Graph-Literal")

            if lang and not _is_valid_langtag(lang):
                raise Exception("'%s' is not a valid language tag!"%lang)

            if datatype:
                datatype = URIRef(datatype)

            value = None
            if isinstance(lexical_or_value, Literal):
                # create from another Literal instance

                lang = lang or lexical_or_value.language
                if datatype:
                    # override datatype
                    value = _castLexicalToPython(lexical_or_value, datatype)
                else:
                    datatype = lexical_or_value.datatype
                    value = lexical_or_value.value

            elif isinstance(lexical_or_value, str):
                    # passed a string
                    # try parsing lexical form of datatyped literal
                    value = _castLexicalToPython(lexical_or_value, datatype)

                    if value is not None and normalize:
                        _value, _datatype = _castPythonToLiteral(value)
                        if _value is not None and _is_valid_unicode(_value):
                            lexical_or_value = _value

            else:
                # passed some python object
                value = lexical_or_value
                _value, _datatype = _castPythonToLiteral(lexical_or_value)

                datatype = datatype or _datatype
                if _value is not None:
                    lexical_or_value = _value
                if datatype:
                    lang = None

            if py3compat.PY3 and isinstance(lexical_or_value, bytes):
                lexical_or_value = lexical_or_value.decode('utf-8')

            try:
                inst = str.__new__(cls, lexical_or_value)
            except UnicodeDecodeError:
                inst = str.__new__(cls, lexical_or_value, 'utf-8')

            inst._language = lang
            inst._datatype = datatype
            inst._value = value
            return inst

        @py3compat.format_doctest_out
        def normalize(self):
            """
            Returns a new literal with a normalised lexical representation
            of this literal
            >>> from rdflib import XSD
            >>> Literal("01", datatype=XSD.integer, normalize=False).normalize()
            rdflib.term.Literal(%(u)s'1', datatype=rdflib.term.URIRef(%(u)s'http://www.w3.org/2001/XMLSchema#integer'))

            Illegal lexical forms for the datatype given are simply passed on
            >>> Literal("a", datatype=XSD.integer, normalize=False)
            rdflib.term.Literal(%(u)s'a', datatype=rdflib.term.URIRef(%(u)s'http://www.w3.org/2001/XMLSchema#integer'))

            """

            if self.value != None:
                return Literal(self.value, datatype=self.datatype, lang=self.language)
            else:
                return self

        @property
        def value(self):
            return self._value

        @property
        def language(self):
            return self._language

        @property
        def datatype(self):
            return self._datatype

        def __reduce__(self):
            return (Literal, (str(self), self.language, self.datatype),)

        def __getstate__(self):
            return (None, dict(language=self.language, datatype=self.datatype))

        def __setstate__(self, arg):
            _, d = arg
            self._language = d["language"]
            self._datatype = d["datatype"]

        @py3compat.format_doctest_out
        def __add__(self, val):
            """
            >>> Literal(1) + 1
            rdflib.term.Literal(%(u)s'2', datatype=rdflib.term.URIRef(%(u)s'http://www.w3.org/2001/XMLSchema#integer'))
            >>> Literal("1") + "1"
            rdflib.term.Literal(%(u)s'11')
            """

            py = self.toPython()
            if not isinstance(py, Literal):
                try:
                    return Literal(py + val)
                except TypeError:
                    pass  # fall-through

            s = str.__add__(self, val)
            return Literal(s, self.language, self.datatype)

        def __bool__(self):
            """
            Is the Literal "True"
            This is used for if statements, bool(literal), etc.
            """
            if self.value != None:
                return bool(self.value)
            return len(self) != 0

        @py3compat.format_doctest_out
        def __neg__(self):
            """
            >>> (- Literal(1))
            rdflib.term.Literal(%(u)s'-1', datatype=rdflib.term.URIRef(%(u)s'http://www.w3.org/2001/XMLSchema#integer'))
            >>> (- Literal(10.5))
            rdflib.term.Literal(%(u)s'-10.5', datatype=rdflib.term.URIRef(%(u)s'http://www.w3.org/2001/XMLSchema#double'))
            >>> from rdflib.namespace import XSD
            >>> (- Literal("1", datatype=XSD.integer))
            rdflib.term.Literal(%(u)s'-1', datatype=rdflib.term.URIRef(%(u)s'http://www.w3.org/2001/XMLSchema#integer'))

            >>> (- Literal("1"))
            Traceback (most recent call last):
              File "<stdin>", line 1, in <module>
            TypeError: Not a number; rdflib.term.Literal(%(u)s'1')
            >>>
            """

            if isinstance(self.value, (int, float)):
                return Literal(self.value.__neg__())
            else:
                raise TypeError("Not a number; %s" % repr(self))

        @py3compat.format_doctest_out
        def __pos__(self):
            """
            >>> (+ Literal(1))
            rdflib.term.Literal(%(u)s'1', datatype=rdflib.term.URIRef(%(u)s'http://www.w3.org/2001/XMLSchema#integer'))
            >>> (+ Literal(-1))
            rdflib.term.Literal(%(u)s'-1', datatype=rdflib.term.URIRef(%(u)s'http://www.w3.org/2001/XMLSchema#integer'))
            >>> from rdflib.namespace import XSD
            >>> (+ Literal("-1", datatype=XSD.integer))
            rdflib.term.Literal(%(u)s'-1', datatype=rdflib.term.URIRef(%(u)s'http://www.w3.org/2001/XMLSchema#integer'))

            >>> (+ Literal("1"))
            Traceback (most recent call last):
              File "<stdin>", line 1, in <module>
            TypeError: Not a number; rdflib.term.Literal(%(u)s'1')
            """
            if isinstance(self.value, (int, float)):
                return Literal(self.value.__pos__())
            else:
                raise TypeError("Not a number; %s" % repr(self))

        @py3compat.format_doctest_out
        def __abs__(self):
            """
            >>> abs(Literal(-1))
            rdflib.term.Literal(%(u)s'1', datatype=rdflib.term.URIRef(%(u)s'http://www.w3.org/2001/XMLSchema#integer'))

            >>> from rdflib.namespace import XSD
            >>> abs( Literal("-1", datatype=XSD.integer))
            rdflib.term.Literal(%(u)s'1', datatype=rdflib.term.URIRef(%(u)s'http://www.w3.org/2001/XMLSchema#integer'))

            >>> abs(Literal("1"))
            Traceback (most recent call last):
              File "<stdin>", line 1, in <module>
            TypeError: Not a number; rdflib.term.Literal(%(u)s'1')
            """
            if isinstance(self.value, (int, float)):
                return Literal(self.value.__abs__())
            else:
                raise TypeError("Not a number; %s" % repr(self))

        @py3compat.format_doctest_out
        def __invert__(self):
            """
            >>> ~(Literal(-1))
            rdflib.term.Literal(%(u)s'0', datatype=rdflib.term.URIRef(%(u)s'http://www.w3.org/2001/XMLSchema#integer'))

            >>> from rdflib.namespace import XSD
            >>> ~( Literal("-1", datatype=XSD.integer))
            rdflib.term.Literal(%(u)s'0', datatype=rdflib.term.URIRef(%(u)s'http://www.w3.org/2001/XMLSchema#integer'))

            Not working:

            >>> ~(Literal("1"))
            Traceback (most recent call last):
              File "<stdin>", line 1, in <module>
            TypeError: Not a number; rdflib.term.Literal(%(u)s'1')
            """
            if isinstance(self.value, (int, float)):
                return Literal(self.value.__invert__())
            else:
                raise TypeError("Not a number; %s" % repr(self))

        def __gt__(self, other):
            """

            This implements ordering for Literals,
            the other comparison methods delegate here

            This tries to implement this:
            http://www.w3.org/TR/sparql11-query/#modOrderBy

            In short, Literals with compatible data-types are orderd in value space,
            i.e.
            >>> from rdflib import XSD

            >>> Literal(1)>Literal(2) # int/int
            False
            >>> Literal(2.0)>Literal(1) # double/int
            True
            >>> from decimal import Decimal
            >>> Literal(Decimal("3.3")) > Literal(2.0) # decimal/double
            True
            >>> Literal(Decimal("3.3")) < Literal(4.0) # decimal/double
            True
            >>> Literal('b')>Literal('a') # plain lit/plain lit
            True
            >>> Literal('b')>Literal('a', datatype=XSD.string) # plain lit/xsd:string
            True

            Incompatible datatype mismatches ordered by DT

            >>> Literal(1)>Literal("2") # int>string
            False

            Langtagged literals by lang tag
            >>> Literal("a", lang="en")>Literal("a", lang="fr")
            False
            """
            if other is None:
                return True  # Everything is greater than None
            if isinstance(other, Literal):

                if self.datatype in _NUMERIC_LITERAL_TYPES and \
                        other.datatype in _NUMERIC_LITERAL_TYPES:
                    return numeric_greater(self.value, other.value)

                # plain-literals and xsd:string literals
                # are "the same"
                dtself = self.datatype or _XSD_STRING
                dtother = other.datatype or _XSD_STRING

                if dtself != dtother:
                    if rdflib.DAWG_LITERAL_COLLATION:
                        return NotImplemented
                    else:
                        return dtself > dtother

                if self.language != other.language:
                    if not self.language:
                        return False
                    elif not other.language:
                        return True
                    else:
                        return self.language > other.language

                if self.value != None and other.value != None:
                    return self.value > other.value

                if str(self) != str(other):
                    return str(self) > str(other)

                # same language, same lexical form, check real dt
                # plain-literals come before xsd:string!
                if self.datatype != other.datatype:
                    if not self.datatype:
                        return False
                    elif not other.datatype:
                        return True
                    else:
                        return self.datatype > other.datatype

                return False  # they are the same

            elif isinstance(other, Node):
                return True  # Literal are the greatest!
            else:
                return NotImplemented  # we can only compare to nodes

        def __lt__(self, other):
            if other is None:
                return False  # Nothing is less than None
            if isinstance(other, Literal):
                try:
                    return not self.__gt__(other) and not self.eq(other)
                except TypeError:
                    return NotImplemented
            if isinstance(other, Node):
                return False  # all nodes are less-than Literals

            return NotImplemented

        def __le__(self, other):
            """
            >>> from rdflib.namespace import XSD
            >>> Literal('2007-01-01T10:00:00', datatype=XSD.dateTime
            ...     ) <= Literal('2007-01-01T10:00:00', datatype=XSD.dateTime)
            True
            """
            r = self.__lt__(other)
            if r:
                return True
            try:
                return self.eq(other)
            except TypeError:
                return NotImplemented

        def __ge__(self, other):
            r = self.__gt__(other)
            if r:
                return True
            try:
                return self.eq(other)
            except TypeError:
                return NotImplemented

        def _comparable_to(self, other):
            """
            Helper method to decide which things are meaningful to
            rich-compare with this literal
            """
            if isinstance(other, Literal):
                if (self.datatype and other.datatype):
                    # two datatyped literals
                    if not self.datatype in XSDToPython or not other.datatype in XSDToPython:
                        # non XSD DTs must match
                        if self.datatype != other.datatype:
                            return False

                else:
                    # xsd:string may be compared with plain literals
                    if not (self.datatype == _XSD_STRING and not other.datatype) or \
                            (other.datatype == _XSD_STRING and not self.datatype):
                        return False

                    # if given lang-tag has to be case insensitive equal
                    if (self.language or "").lower() != (other.language or "").lower():
                        return False

            return True

        def __hash__(self):
            """
            >>> from rdflib.namespace import XSD
            >>> a = {Literal('1', datatype=XSD.integer):'one'}
            >>> Literal('1', datatype=XSD.double) in a
            False

            "Called for the key object for dictionary operations,
            and by the built-in function hash(). Should return
            a 32-bit integer usable as a hash value for
            dictionary operations. The only required property
            is that objects which compare equal have the same
            hash value; it is advised to somehow mix together
            (e.g., using exclusive or) the hash values for the
            components of the object that also play a part in
            comparison of objects." -- 3.4.1 Basic customization (Python)

            "Two literals are equal if and only if all of the following hold:
            * The strings of the two lexical forms compare equal, character by
            character.
            * Either both or neither have language tags.
            * The language tags, if any, compare equal.
            * Either both or neither have datatype URIs.
            * The two datatype URIs, if any, compare equal, character by
            character."
            -- 6.5.1 Literal Equality (RDF: Concepts and Abstract Syntax)

            """
            res = super(Literal, self).__hash__()
            if self.language:
                res ^= hash(self.language.lower())
            if self.datatype:
                res ^= hash(self.datatype)
            return res

        @py3compat.format_doctest_out
        def __eq__(self, other):
            """
            Literals are only equal to other literals.

            "Two literals are equal if and only if all of the following hold:
            * The strings of the two lexical forms compare equal, character by character.
            * Either both or neither have language tags.
            * The language tags, if any, compare equal.
            * Either both or neither have datatype URIs.
            * The two datatype URIs, if any, compare equal, character by character."
            -- 6.5.1 Literal Equality (RDF: Concepts and Abstract Syntax)

            >>> Literal("1", datatype=URIRef("foo")) == Literal("1", datatype=URIRef("foo"))
            True
            >>> Literal("1", datatype=URIRef("foo")) == Literal("1", datatype=URIRef("foo2"))
            False

            >>> Literal("1", datatype=URIRef("foo")) == Literal("2", datatype=URIRef("foo"))
            False
            >>> Literal("1", datatype=URIRef("foo")) == "asdf"
            False
            >>> from rdflib import XSD
            >>> Literal('2007-01-01', datatype=XSD.date) == Literal('2007-01-01', datatype=XSD.date)
            True
            >>> Literal('2007-01-01', datatype=XSD.date) == date(2007, 1, 1)
            False
            >>> Literal("one", lang="en") == Literal("one", lang="en")
            True
            >>> Literal("hast", lang='en') == Literal("hast", lang='de')
            False
            >>> Literal("1", datatype=XSD.integer) == Literal(1)
            True
            >>> Literal("1", datatype=XSD.integer) == Literal("01", datatype=XSD.integer)
            True

            """
            if self is other:
                return True
            if other is None:
                return False
            if isinstance(other, Literal):
                return self.datatype == other.datatype \
                    and (self.language.lower() if self.language else None) == (other.language.lower() if other.language else None) \
                    and str.__eq__(self, other)

            return False

        def eq(self, other):
            """
            Compare the value of this literal with something else

            Either, with the value of another literal
            comparisons are then done in literal "value space",
            and according to the rules of XSD subtype-substitution/type-promotion

            OR, with a python object:

            basestring objects can be compared with plain-literals,
            or those with datatype xsd:string

            bool objects with xsd:boolean

            a int, long or float with numeric xsd types

            isodate date,time,datetime objects with xsd:date,xsd:time or xsd:datetime

            Any other operations returns NotImplemented

            """
            if isinstance(other, Literal):

                if self.datatype in _NUMERIC_LITERAL_TYPES  \
                        and other.datatype in _NUMERIC_LITERAL_TYPES:
                    if self.value != None and other.value != None:
                        return self.value == other.value
                    else:
                        if str.__eq__(self, other):
                            return True
                        raise TypeError(
                            'I cannot know that these two lexical forms do not map to the same value: %s and %s' % (self, other))
                if (self.language or "").lower() != (other.language or "").lower():
                    return False

                dtself = self.datatype or _XSD_STRING
                dtother = other.datatype or _XSD_STRING

                if (dtself == _XSD_STRING and dtother == _XSD_STRING):
                    # string/plain literals, compare on lexical form
                    return str.__eq__(self, other)

                if dtself != dtother:
                    if rdflib.DAWG_LITERAL_COLLATION:
                        raise TypeError("I don't know how to compare literals with datatypes %s and %s" % (
                            self.datatype, other.datatype))
                    else:
                        return False

                # matching non-string DTs now - do we compare values or
                # lexical form first?  comparing two ints is far quicker -
                # maybe there are counter examples

                if self.value != None and other.value != None:

                    if self.datatype in (_RDF_XMLLITERAL, _RDF_HTMLLITERAL):
                        return _isEqualXMLNode(self.value, other.value)

                    return self.value == other.value
                else:

                    if str.__eq__(self, other):
                        return True

                    if self.datatype == _XSD_STRING:
                        return False  # string value space=lexical space

                    # matching DTs, but not matching, we cannot compare!
                    raise TypeError(
                        'I cannot know that these two lexical forms do not map to the same value: %s and %s' % (self, other))

            elif isinstance(other, Node):
                return False  # no non-Literal nodes are equal to a literal

            elif isinstance(other, str):
                # only plain-literals can be directly compared to strings

                # TODO: Is "blah"@en eq "blah" ?
                if self.language is not None:
                    return False

                if (self.datatype == _XSD_STRING or self.datatype is None):
                    return str(self) == other

            elif isinstance(other, (int, float)):
                if self.datatype in _NUMERIC_LITERAL_TYPES:
                    return self.value == other
            elif isinstance(other, (date, datetime, time)):
                if self.datatype in (_XSD_DATETIME, _XSD_DATE, _XSD_TIME):
                    return self.value == other
            elif isinstance(other, bool):
                if self.datatype == _XSD_BOOLEAN:
                    return self.value == other

            return NotImplemented

        def neq(self, other):
            return not self.eq(other)

        @py3compat.format_doctest_out
        def n3(self, namespace_manager = None):
            r'''
            Returns a representation in the N3 format.

            Examples::

                >>> Literal("foo").n3()
                %(u)s'"foo"'

            Strings with newlines or triple-quotes::

                >>> Literal("foo\nbar").n3()
                %(u)s'"""foo\nbar"""'

                >>> Literal("''\'").n3()
                %(u)s'"\'\'\'"'

                >>> Literal('"""').n3()
                %(u)s'"\\"\\"\\""'

            Language::

                >>> Literal("hello", lang="en").n3()
                %(u)s'"hello"@en'

            Datatypes::

                >>> Literal(1).n3()
                %(u)s'"1"^^<http://www.w3.org/2001/XMLSchema#integer>'

                >>> Literal(1.0).n3()
                %(u)s'"1.0"^^<http://www.w3.org/2001/XMLSchema#double>'

                >>> Literal(True).n3()
                %(u)s'"true"^^<http://www.w3.org/2001/XMLSchema#boolean>'

            Datatype and language isn't allowed (datatype takes precedence)::

                >>> Literal(1, lang="en").n3()
                %(u)s'"1"^^<http://www.w3.org/2001/XMLSchema#integer>'

            Custom datatype::

                >>> footype = URIRef("http://example.org/ns#foo")
                >>> Literal("1", datatype=footype).n3()
                %(u)s'"1"^^<http://example.org/ns#foo>'

            Passing a namespace-manager will use it to abbreviate datatype URIs:

                >>> from rdflib import Graph
                >>> Literal(1).n3(Graph().namespace_manager)
                %(u)s'"1"^^xsd:integer'
            '''
            if namespace_manager:
                return self._literal_n3(qname_callback =
                                        namespace_manager.normalizeUri)
            else:
                return self._literal_n3()

        @py3compat.format_doctest_out
        def _literal_n3(self, use_plain=False, qname_callback=None):
            '''
            Using plain literal (shorthand) output::
                >>> from rdflib.namespace import XSD

                >>> Literal(1)._literal_n3(use_plain=True)
                %(u)s'1'

                >>> Literal(1.0)._literal_n3(use_plain=True)
                %(u)s'1e+00'

                >>> Literal(1.0, datatype=XSD.decimal)._literal_n3(use_plain=True)
                %(u)s'1.0'

                >>> Literal(1.0, datatype=XSD.float)._literal_n3(use_plain=True)
                %(u)s'"1.0"^^<http://www.w3.org/2001/XMLSchema#float>'

                >>> Literal("foo", datatype=XSD.string)._literal_n3(
                ...         use_plain=True)
                %(u)s'"foo"^^<http://www.w3.org/2001/XMLSchema#string>'

                >>> Literal(True)._literal_n3(use_plain=True)
                %(u)s'true'

                >>> Literal(False)._literal_n3(use_plain=True)
                %(u)s'false'

                >>> Literal(1.91)._literal_n3(use_plain=True)
                %(u)s'1.91e+00'

                Only limited precision available for floats:
                >>> Literal(0.123456789)._literal_n3(use_plain=True)
                %(u)s'1.234568e-01'

                >>> Literal('0.123456789',
                ...     datatype=XSD.decimal)._literal_n3(use_plain=True)
                %(u)s'0.123456789'

            Using callback for datatype QNames::

                >>> Literal(1)._literal_n3(
                ...         qname_callback=lambda uri: "xsd:integer")
                %(u)s'"1"^^xsd:integer'

            '''
            if use_plain and self.datatype in _PLAIN_LITERAL_TYPES:
                if self.value is not None:
                    # If self is inf or NaN, we need a datatype
                    # (there is no plain representation)
                    if self.datatype in _NUMERIC_INF_NAN_LITERAL_TYPES:
                        try:
                            v = float(self)
                            if math.isinf(v) or math.isnan(v):
                                return self._literal_n3(False, qname_callback)
                        except ValueError:
                            return self._literal_n3(False, qname_callback)

                    # this is a bit of a mess -
                    # in py >=2.6 the string.format function makes this easier
                    # we try to produce "pretty" output
                    if self.datatype == _XSD_DOUBLE:
                        return sub("\\.?0*e", "e", '%e' % float(self))
                    elif self.datatype == _XSD_DECIMAL:
                        s = '%s' % self
                        if '.' not in s:
                            s += '.0'
                        return s

                    elif self.datatype == _XSD_BOOLEAN:
                        return ('%s' % self).lower()
                    else:
                        return '%s' % self

            encoded = self._quote_encode()

            datatype = self.datatype
            quoted_dt = None
            if datatype:
                if qname_callback:
                    quoted_dt = qname_callback(datatype)
                if not quoted_dt:
                    quoted_dt = "<%s>" % datatype
                if datatype in _NUMERIC_INF_NAN_LITERAL_TYPES:
                    try:
                        v = float(self)
                        if math.isinf(v):
                            # py string reps: float: 'inf', Decimal: 'Infinity"
                            # both need to become "INF" in xsd datatypes
                            encoded = encoded.replace('inf', 'INF').replace(
                                'Infinity', 'INF')
                        if math.isnan(v):
                            encoded = encoded.replace('nan', 'NaN')
                    except ValueError:
                        # if we can't cast to float something is wrong, but we can
                        # still serialize. Warn user about it
                        warnings.warn("Serializing weird numerical %r" % self)

            language = self.language
            if language:
                return '%s@%s' % (encoded, language)
            elif datatype:
                return '%s^^%s' % (encoded, quoted_dt)
            else:
                return '%s' % encoded

        def _quote_encode(self):
            # This simpler encoding doesn't work; a newline gets encoded as "\\n",
            # which is ok in sourcecode, but we want "\n".
            # encoded = self.encode('unicode-escape').replace(
            #        '\\', '\\\\').replace('"','\\"')
            # encoded = self.replace.replace('\\', '\\\\').replace('"','\\"')

            # NOTE: Could in theory chose quotes based on quotes appearing in the
            # string, i.e. '"' and "'", but N3/turtle doesn't allow "'"(?).

            if "\n" in self:
                # Triple quote this string.
                encoded = self.replace('\\', '\\\\')
                if '"""' in self:
                    # is this ok?
                    encoded = encoded.replace('"""', '\\"\\"\\"')
                if encoded[-1] == '"' and encoded[-2] != '\\':
                    encoded = encoded[:-1] + '\\' + '"'

                return '"""%s"""' % encoded.replace('\r', '\\r')
            else:
                return '"%s"' % self.replace(
                    '\n', '\\n').replace(
                        '\\', '\\\\').replace(
                            '"', '\\"').replace(
                                '\r', '\\r')

        if not py3compat.PY3:
            def __str__(self):
                return self.encode()

        def __repr__(self):
            args = [super(Literal, self).__repr__()]
            if self.language is not None:
                args.append("lang=%s" % repr(self.language))
            if self.datatype is not None:
                args.append("datatype=%s" % repr(self.datatype))
            if self.__class__ == Literal:
                clsName = "rdflib.term.Literal"
            else:
                clsName = self.__class__.__name__
            return """%s(%s)""" % (clsName, ", ".join(args))

        def toPython(self):
            """
            Returns an appropriate python datatype derived from this RDF Literal
            """

            if self.value is not None:
                return self.value
            return self

        def md5_term_hash(self):
            """a string of hex that will be the same for two Literals that
            are the same. It is not a suitable unique id.

            Supported for backwards compatibility; new code should
            probably just use __hash__
            """
            warnings.warn(
                "method md5_term_hash is deprecated, and will be removed " +
                "removed in the future. If you use this please let rdflib-dev know!",
                category=DeprecationWarning, stacklevel=2)
            d = md5(self.encode())
            d.update(b("L"))
            return d.hexdigest()

    def _parseXML(xmlstring):
        if not py3compat.PY3:
            xmlstring = xmlstring.encode('utf-8')
        retval = xml.dom.minidom.parseString(
            "<rdflibtoplevelelement>%s</rdflibtoplevelelement>" % xmlstring)
        retval.normalize()
        return retval

    def _parseHTML(htmltext):
        try:
            import html5lib
            parser = html5lib.HTMLParser(
                tree=html5lib.treebuilders.getTreeBuilder("dom"))
            retval = parser.parseFragment(htmltext)
            retval.normalize()
            return retval
        except ImportError:
            raise ImportError(
                "HTML5 parser not available. Try installing" +
                " html5lib <http://code.google.com/p/html5lib>")

    def _writeXML(xmlnode):
        if isinstance(xmlnode, xml.dom.minidom.DocumentFragment):
            d = xml.dom.minidom.Document()
            d.childNodes += xmlnode.childNodes
            xmlnode = d
        s = xmlnode.toxml('utf-8')
        # for clean round-tripping, remove headers -- I have great and
        # specific worries that this will blow up later, but this margin
        # is too narrow to contain them
        if s.startswith(b('<?xml version="1.0" encoding="utf-8"?>')):
            s = s[38:]
        if s.startswith(b('<rdflibtoplevelelement>')):
            s = s[23:-24]
        if s == b('<rdflibtoplevelelement/>'):
            s = b('')
        return s

    # Cannot import Namespace/XSD because of circular dependencies
    _XSD_PFX = 'http://www.w3.org/2001/XMLSchema#'
    _RDF_PFX = 'http://www.w3.org/1999/02/22-rdf-syntax-ns#'

    _RDF_XMLLITERAL = URIRef(_RDF_PFX + 'XMLLiteral')
    _RDF_HTMLLITERAL = URIRef(_RDF_PFX + 'HTML')

    _XSD_STRING = URIRef(_XSD_PFX + 'string')

    _XSD_FLOAT = URIRef(_XSD_PFX + 'float')
    _XSD_DOUBLE = URIRef(_XSD_PFX + 'double')
    _XSD_DECIMAL = URIRef(_XSD_PFX + 'decimal')
    _XSD_INTEGER = URIRef(_XSD_PFX + 'integer')
    _XSD_BOOLEAN = URIRef(_XSD_PFX + 'boolean')

    _XSD_DATETIME = URIRef(_XSD_PFX + 'dateTime')
    _XSD_DATE = URIRef(_XSD_PFX + 'date')
    _XSD_TIME = URIRef(_XSD_PFX + 'time')

    # TODO: duration, gYearMonth, gYear, gMonthDay, gDay, gMonth

    _NUMERIC_LITERAL_TYPES = (
        _XSD_INTEGER,
        _XSD_DECIMAL,
        _XSD_DOUBLE,
        URIRef(_XSD_PFX + 'float'),

        URIRef(_XSD_PFX + 'byte'),
        URIRef(_XSD_PFX + 'int'),
        URIRef(_XSD_PFX + 'long'),
        URIRef(_XSD_PFX + 'negativeInteger'),
        URIRef(_XSD_PFX + 'nonNegativeInteger'),
        URIRef(_XSD_PFX + 'nonPositiveInteger'),
        URIRef(_XSD_PFX + 'positiveInteger'),
        URIRef(_XSD_PFX + 'short'),
        URIRef(_XSD_PFX + 'unsignedByte'),
        URIRef(_XSD_PFX + 'unsignedInt'),
        URIRef(_XSD_PFX + 'unsignedLong'),
        URIRef(_XSD_PFX + 'unsignedShort'),

    )

    # these have "native" syntax in N3/SPARQL
    _PLAIN_LITERAL_TYPES = (
        _XSD_INTEGER,
        _XSD_BOOLEAN,
        _XSD_DOUBLE,
        _XSD_DECIMAL,
    )

    # these have special INF and NaN XSD representations
    _NUMERIC_INF_NAN_LITERAL_TYPES = (
        URIRef(_XSD_PFX + 'float'),
        _XSD_DOUBLE,
        _XSD_DECIMAL,
    )

    def _castPythonToLiteral(obj):
        """
        Casts a python datatype to a tuple of the lexical value and a
        datatype URI (or None)
        """
        for pType, (castFunc, dType) in _PythonToXSD:
            if isinstance(obj, pType):
                if castFunc:
                    return castFunc(obj), dType
                elif dType:
                    return obj, dType
                else:
                    return obj, None
        return obj, None  # TODO: is this right for the fall through case?

    from decimal import Decimal

    # Mappings from Python types to XSD datatypes and back (borrowed from sparta)
    # datetime instances are also instances of date... so we need to order these.

    # SPARQL/Turtle/N3 has shortcuts for integer, double, decimal
    # python has only float - to be in tune with sparql/n3/turtle
    # we default to XSD.double for float literals

    # python ints are promoted to longs when overflowing
    # python longs have no limit
    # both map to the abstract integer type,
    # rather than some concrete bit-limited datatype

    _PythonToXSD = [
        (str, (None, None)),
        (float, (None, _XSD_DOUBLE)),
        (bool, (lambda i:str(i).lower(), _XSD_BOOLEAN)),
        (int, (None, _XSD_INTEGER)),
        (int, (None, _XSD_INTEGER)),
        (Decimal, (None, _XSD_DECIMAL)),
        (datetime, (lambda i:i.isoformat(), _XSD_DATETIME)),
        (date, (lambda i:i.isoformat(), _XSD_DATE)),
        (time, (lambda i:i.isoformat(), _XSD_TIME)),
        (xml.dom.minidom.Document, (_writeXML, _RDF_XMLLITERAL)),
        # this is a bit dirty - by accident the html5lib parser produces
        # DocumentFragments, and the xml parser Documents, letting this
        # decide what datatype to use makes roundtripping easier, but it a
        # bit random
        (xml.dom.minidom.DocumentFragment, (_writeXML, _RDF_HTMLLITERAL))
    ]

    XSDToPython = {
        None : None, # plain literals map directly to value space
        URIRef(_XSD_PFX + 'time'): parse_time,
        URIRef(_XSD_PFX + 'date'): parse_date,
        URIRef(_XSD_PFX + 'gYear'): parse_date,
        URIRef(_XSD_PFX + 'gYearMonth'): parse_date,
        URIRef(_XSD_PFX + 'dateTime'): parse_datetime,
        URIRef(_XSD_PFX + 'string'): None,
        URIRef(_XSD_PFX + 'normalizedString'): None,
        URIRef(_XSD_PFX + 'token'): None,
        URIRef(_XSD_PFX + 'language'): None,
        URIRef(_XSD_PFX + 'boolean'): lambda i: i.lower() in ['1', 'true'],
        URIRef(_XSD_PFX + 'decimal'): Decimal,
        URIRef(_XSD_PFX + 'integer'): int,
        URIRef(_XSD_PFX + 'nonPositiveInteger'): int,
        URIRef(_XSD_PFX + 'long'): int,
        URIRef(_XSD_PFX + 'nonNegativeInteger'): int,
        URIRef(_XSD_PFX + 'negativeInteger'): int,
        URIRef(_XSD_PFX + 'int'): int,
        URIRef(_XSD_PFX + 'unsignedLong'): int,
        URIRef(_XSD_PFX + 'positiveInteger'): int,
        URIRef(_XSD_PFX + 'short'): int,
        URIRef(_XSD_PFX + 'unsignedInt'): int,
        URIRef(_XSD_PFX + 'byte'): int,
        URIRef(_XSD_PFX + 'unsignedShort'): int,
        URIRef(_XSD_PFX + 'unsignedByte'): int,
        URIRef(_XSD_PFX + 'float'): float,
        URIRef(_XSD_PFX + 'double'): float,
        URIRef(_XSD_PFX + 'base64Binary'): lambda s: base64.b64decode(s),
        URIRef(_XSD_PFX + 'anyURI'): None,
        _RDF_XMLLITERAL: _parseXML,
        _RDF_HTMLLITERAL: _parseHTML
    }

    _toPythonMapping = {}

    _toPythonMapping.update(XSDToPython)

    def _castLexicalToPython(lexical, datatype):
        """
        Map a lexical form to the value-space for the given datatype
        :returns: a python object for the value or ``None``
        """
        convFunc = _toPythonMapping.get(datatype, False)
        if convFunc:
            try:
                return convFunc(lexical)
            except:
                # not a valid lexical representation for this dt
                return None
        elif convFunc is None:
            # no conv func means 1-1 lexical<->value-space mapping
            try:
                return str(lexical)
            except UnicodeDecodeError:
                return str(lexical, 'utf-8')
        else:
            # no convFunc - unknown data-type
            return None

    def bind(datatype, pythontype, constructor=None, lexicalizer=None):
        """
        register a new datatype<->pythontype binding

        :param constructor: an optional function for converting lexical forms
                            into a Python instances, if not given the pythontype
                            is used directly

        :param lexicalizer: an optinoal function for converting python objects to
                            lexical form, if not given object.__str__ is used

        """
        if datatype in _toPythonMapping:
            logger.warning("datatype '%s' was already bound. Rebinding." %
                            datatype)

        if constructor == None:
            constructor = pythontype
        _toPythonMapping[datatype] = constructor
        _PythonToXSD.append((pythontype, (lexicalizer, datatype)))

    class Variable(Identifier):
        """
        A Variable - this is used for querying, or in Formula aware
        graphs, where Variables can stored in the graph
        """
        __slots__ = ()

        def __new__(cls, value):
            if len(value) == 0:
                raise Exception(
                    "Attempted to create variable with empty string as name!")
            if value[0] == '?':
                value = value[1:]
            return str.__new__(cls, value)

        def __repr__(self):
            if self.__class__ is Variable:
                clsName = "rdflib.term.Variable"
            else:
                clsName = self.__class__.__name__

            return """%s(%s)""" % (clsName, super(Variable, self).__repr__())

        def toPython(self):
            return "?%s" % self

        def n3(self, namespace_manager = None):
            return "?%s" % self

        def __reduce__(self):
            return (Variable, (str(self),))

        def md5_term_hash(self):
            """a string of hex that will be the same for two Variables that
            are the same. It is not a suitable unique id.

            Supported for backwards compatibility; new code should
            probably just use __hash__
            """
            warnings.warn(
                "method md5_term_hash is deprecated, and will be removed " +
                "removed in the future. If you use this please let rdflib-dev know!",
                category=DeprecationWarning, stacklevel=2)
            d = md5(self.encode())
            d.update(b("V"))
            return d.hexdigest()

    class Statement(Node, tuple):

        def __new__(cls, xxx_todo_changeme, context):
            (subject, predicate, object) = xxx_todo_changeme
            warnings.warn(
                "Class Statement is deprecated, and will be removed in " +
                "the future. If you use this please let rdflib-dev know!",
                category=DeprecationWarning, stacklevel=2)
            return tuple.__new__(cls, ((subject, predicate, object), context))

        def __reduce__(self):
            return (Statement, (self[0], self[1]))

        def toPython(self):
            return (self[0], self[1])

    # Nodes are ordered like this
    # See http://www.w3.org/TR/sparql11-query/#modOrderBy
    # we leave "space" for more subclasses of Node elsewhere
    # default-dict to grazefully fail for new subclasses
    _ORDERING = defaultdict(int)
    _ORDERING.update({
        BNode: 10,
        Variable: 20,
        URIRef: 30,
        Literal: 40
        })

    def _isEqualXMLNode(node, other):
        from xml.dom.minidom import Node

        def recurse():
            # Recursion through the children
            # In Python2, the semantics of 'map' is such that the check on
            # length would be unnecessary. In Python 3,
            # the semantics of map has changed (why, oh why???) and the check
            # for the length becomes necessary...
            if len(node.childNodes) != len(other.childNodes):
                return False
            for (nc, oc) in map(
                    lambda x, y: (x, y), node.childNodes, other.childNodes):
                if not _isEqualXMLNode(nc, oc):
                    return False
            # if we got here then everything is fine:
            return True

        if node is None or other is None:
            return False

        if node.nodeType != other.nodeType:
            return False

        if node.nodeType in [Node.DOCUMENT_NODE, Node.DOCUMENT_FRAGMENT_NODE]:
            return recurse()

        elif node.nodeType == Node.ELEMENT_NODE:
            # Get the basics right
            if not (node.tagName == other.tagName
                    and node.namespaceURI == other.namespaceURI):
                return False

            # Handle the (namespaced) attributes; the namespace setting key
            # should be ignored, though
            # Note that the minidom orders the keys already, so we do not have
            # to worry about that, which is a bonus...
            n_keys = [
                k for k in node.attributes.keysNS()
                if k[0] != 'http://www.w3.org/2000/xmlns/']
            o_keys = [
                k for k in other.attributes.keysNS()
                if k[0] != 'http://www.w3.org/2000/xmlns/']
            if len(n_keys) != len(o_keys):
                return False
            for k in n_keys:
                if not (k in o_keys
                        and node.getAttributeNS(k[0], k[1]) ==
                        other.getAttributeNS(k[0], k[1])):
                    return False

            # if we got here, the attributes are all right, we can go down
            # the tree recursively
            return recurse()

        elif node.nodeType in [
                Node.TEXT_NODE, Node.COMMENT_NODE, Node.CDATA_SECTION_NODE,
                Node.NOTATION_NODE]:
            return node.data == other.data

        elif node.nodeType == Node.PROCESSING_INSTRUCTION_NODE:
            return node.data == other.data and node.target == other.target

        elif node.nodeType == Node.ENTITY_NODE:
            return node.nodeValue == other.nodeValue

        elif node.nodeType == Node.DOCUMENT_TYPE_NODE:
            return node.publicId == other.publicId \
                and node.systemId == other.system.Id

        else:
            # should not happen, in fact
            raise Exception(
                'I dont know how to compare XML Node type: %s' % node.nodeType)

    if __name__ == '__main__':
        import doctest
        doctest.testmod()