# Source code for rdflib.graph

    from rdflib.term import Literal  # required for doctests
    assert Literal # avoid warning
    from rdflib.namespace import Namespace  # required for doctests
    assert Namespace # avoid warning
    from rdflib.py3compat import format_doctest_out

    __doc__ = format_doctest_out("""\

    RDFLib defines the following kinds of Graphs:

    * :class:`~rdflib.graph.Graph`
    * :class:`~rdflib.graph.QuotedGraph`
    * :class:`~rdflib.graph.ConjunctiveGraph`
    * :class:`~rdflib.graph.Dataset`

    Graph
    -----

    An RDF graph is a set of RDF triples. Graphs support the python ``in``
    operator, as well as iteration and some operations like union,
    difference and intersection.

    see :class:`~rdflib.graph.Graph`

    Conjunctive Graph
    -----------------

    A Conjunctive Graph is the most relevant collection of graphs that are
    considered to be the boundary for closed world assumptions.  This
    boundary is equivalent to that of the store instance (which is itself
    uniquely identified and distinct from other instances of
    :class:`Store` that signify other Conjunctive Graphs).  It is
    equivalent to all the named graphs within it and associated with a
    ``_default_`` graph which is automatically assigned a :class:`BNode`
    for an identifier - if one isn't given.

    see :class:`~rdflib.graph.ConjunctiveGraph`

    Quoted graph
    ------------

    The notion of an RDF graph [14] is extended to include the concept of
    a formula node. A formula node may occur wherever any other kind of
    node can appear. Associated with a formula node is an RDF graph that
    is completely disjoint from all other graphs; i.e. has no nodes in
    common with any other graph. (It may contain the same labels as other
    RDF graphs; because this is, by definition, a separate graph,
    considerations of tidiness do not apply between the graph at a formula
    node and any other graph.)

    This is intended to map the idea of "{ N3-expression }" that is used
    by N3 into an RDF graph upon which RDF semantics is defined.

    see :class:`~rdflib.graph.QuotedGraph`

    Dataset
    -------

    The RDF 1.1 Dataset, a small extension to the Conjunctive Graph. The
    primary term is "graphs in the datasets" and not "contexts with quads"
    so there is a separate method to set/retrieve a graph in a dataset and
    to operate with dataset graphs. As a consequence of this approach,
    dataset graphs cannot be identified with blank nodes, a name is always
    required (RDFLib will automatically add a name if one is not provided
    at creation time). This implementation includes a convenience method
    to directly add a single quad to a dataset graph.

    see :class:`~rdflib.graph.Dataset`

    Working with graphs
    ===================

    Instantiating Graphs with default store (IOMemory) and default identifier
    (a BNode):

        >>> g = Graph()
        >>> g.store.__class__
        <class 'rdflib.plugins.memory.IOMemory'>
        >>> g.identifier.__class__
        <class 'rdflib.term.BNode'>

    Instantiating Graphs with a IOMemory store and an identifier -
    <http://rdflib.net>:

        >>> g = Graph('IOMemory', URIRef("http://rdflib.net"))
        >>> g.identifier
        rdflib.term.URIRef(%(u)s'http://rdflib.net')
        >>> str(g) # doctest: +NORMALIZE_WHITESPACE
        "<http://rdflib.net> a rdfg:Graph;rdflib:storage
         [a rdflib:Store;rdfs:label 'IOMemory']."

    Creating a ConjunctiveGraph - The top level container for all named Graphs
    in a 'database':

        >>> g = ConjunctiveGraph()
        >>> str(g.default_context)
        "[a rdfg:Graph;rdflib:storage [a rdflib:Store;rdfs:label 'IOMemory']]."

    Adding / removing reified triples to Graph and iterating over it directly or
    via triple pattern:

        >>> g = Graph()
        >>> statementId = BNode()
        >>> print(len(g))
        0
        >>> g.add((statementId, RDF.type, RDF.Statement))
        >>> g.add((statementId, RDF.subject,
        ...     URIRef(%(u)s'http://rdflib.net/store/ConjunctiveGraph')))
        >>> g.add((statementId, RDF.predicate, RDFS.label))
        >>> g.add((statementId, RDF.object, Literal("Conjunctive Graph")))
        >>> print(len(g))
        4
        >>> for s, p, o in g:
        ...     print(type(s))
        ...
        <class 'rdflib.term.BNode'>
        <class 'rdflib.term.BNode'>
        <class 'rdflib.term.BNode'>
        <class 'rdflib.term.BNode'>

        >>> for s, p, o in g.triples((None, RDF.object, None)):
        ...     print(o)
        ...
        Conjunctive Graph
        >>> g.remove((statementId, RDF.type, RDF.Statement))
        >>> print(len(g))
        3

    ``None`` terms in calls to :meth:`~rdflib.graph.Graph.triples` can be
    thought of as "open variables".

    Graph support set-theoretic operators, you can add/subtract graphs, as
    well as intersection (with multiplication operator g1*g2) and xor (g1
    ^ g2).

    Note that BNode IDs are kept when doing set-theoretic operations, this
    may or may not be what you want. Two named graphs within the same
    application probably want share BNode IDs, two graphs with data from
    different sources probably not.  If your BNode IDs are all generated
    by RDFLib they are UUIDs and unique.

        >>> g1 = Graph()
        >>> g2 = Graph()
        >>> u = URIRef(%(u)s'http://example.com/foo')
        >>> g1.add([u, RDFS.label, Literal('foo')])
        >>> g1.add([u, RDFS.label, Literal('bar')])
        >>> g2.add([u, RDFS.label, Literal('foo')])
        >>> g2.add([u, RDFS.label, Literal('bing')])
        >>> len(g1 + g2)  # adds bing as label
        3
        >>> len(g1 - g2)  # removes foo
        1
        >>> len(g1 * g2)  # only foo
        1
        >>> g1 += g2  # now g1 contains everything

    Graph Aggregation - ConjunctiveGraphs and ReadOnlyGraphAggregate within
    the same store:

        >>> store = plugin.get('IOMemory', Store)()
        >>> g1 = Graph(store)
        >>> g2 = Graph(store)
        >>> g3 = Graph(store)
        >>> stmt1 = BNode()
        >>> stmt2 = BNode()
        >>> stmt3 = BNode()
        >>> g1.add((stmt1, RDF.type, RDF.Statement))
        >>> g1.add((stmt1, RDF.subject,
        ...     URIRef(%(u)s'http://rdflib.net/store/ConjunctiveGraph')))
        >>> g1.add((stmt1, RDF.predicate, RDFS.label))
        >>> g1.add((stmt1, RDF.object, Literal("Conjunctive Graph")))
        >>> g2.add((stmt2, RDF.type, RDF.Statement))
        >>> g2.add((stmt2, RDF.subject,
        ...     URIRef(%(u)s'http://rdflib.net/store/ConjunctiveGraph')))
        >>> g2.add((stmt2, RDF.predicate, RDF.type))
        >>> g2.add((stmt2, RDF.object, RDFS.Class))
        >>> g3.add((stmt3, RDF.type, RDF.Statement))
        >>> g3.add((stmt3, RDF.subject,
        ...     URIRef(%(u)s'http://rdflib.net/store/ConjunctiveGraph')))
        >>> g3.add((stmt3, RDF.predicate, RDFS.comment))
        >>> g3.add((stmt3, RDF.object, Literal(
        ...     "The top-level aggregate graph - The sum " +
        ...     "of all named graphs within a Store")))
        >>> len(list(ConjunctiveGraph(store).subjects(RDF.type, RDF.Statement)))
        3
        >>> len(list(ReadOnlyGraphAggregate([g1,g2]).subjects(
        ...     RDF.type, RDF.Statement)))
        2

    ConjunctiveGraphs have a :meth:`~rdflib.graph.ConjunctiveGraph.quads` method
    which returns quads instead of triples, where the fourth item is the Graph
    (or subclass thereof) instance in which the triple was asserted:

        >>> uniqueGraphNames = set(
        ...     [graph.identifier for s, p, o, graph in ConjunctiveGraph(store
        ...     ).quads((None, RDF.predicate, None))])
        >>> len(uniqueGraphNames)
        3
        >>> unionGraph = ReadOnlyGraphAggregate([g1, g2])
        >>> uniqueGraphNames = set(
        ...     [graph.identifier for s, p, o, graph in unionGraph.quads(
        ...     (None, RDF.predicate, None))])
        >>> len(uniqueGraphNames)
        2

    Parsing N3 from a string

        >>> g2 = Graph()
        >>> src = '''
        ... @prefix rdf:  <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
        ... @prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> .
        ... [ a rdf:Statement ;
        ...   rdf:subject <http://rdflib.net/store#ConjunctiveGraph>;
        ...   rdf:predicate rdfs:label;
        ...   rdf:object "Conjunctive Graph" ] .
        ... '''
        >>> g2 = g2.parse(data=src, format='n3')
        >>> print(len(g2))
        4

    Using Namespace class:

        >>> RDFLib = Namespace('http://rdflib.net/')
        >>> RDFLib.ConjunctiveGraph
        rdflib.term.URIRef(%(u)s'http://rdflib.net/ConjunctiveGraph')
        >>> RDFLib['Graph']
        rdflib.term.URIRef(%(u)s'http://rdflib.net/Graph')

    """)

    import logging
    logger = logging.getLogger(__name__)

    # import md5
    import random
    import warnings

    from hashlib import md5

    try:
        from io import BytesIO
        assert BytesIO
    except ImportError:
        try:
            from cStringIO import StringIO as BytesIO
            assert BytesIO
        except ImportError:
            from StringIO import StringIO as BytesIO
            assert BytesIO

    from rdflib.namespace import RDF, RDFS, SKOS

    from rdflib import plugin, exceptions, query

    from rdflib.term import Node, URIRef, Genid
    from rdflib.term import BNode

    import rdflib.term

    from rdflib.paths import Path

    from rdflib.store import Store
    from rdflib.serializer import Serializer
    from rdflib.parser import Parser
    from rdflib.parser import create_input_source
    from rdflib.namespace import NamespaceManager
    from rdflib.resource import Resource
    from rdflib import py3compat
    b = py3compat.b

    import os
    import shutil
    import tempfile
    from urlparse import urlparse

    __all__ = [
        'Graph', 'ConjunctiveGraph', 'QuotedGraph', 'Seq',
        'ModificationException', 'Dataset',
        'UnSupportedAggregateOperation', 'ReadOnlyGraphAggregate']

    class Graph(Node):
        """An RDF Graph

        The constructor accepts one argument, the 'store'
        that will be used to store the graph data (see the 'store'
        package for stores currently shipped with rdflib).

        Stores can be context-aware or unaware.  Unaware stores take up
        (some) less space but cannot support features that require
        context, such as true merging/demerging of sub-graphs and
        provenance.

        The Graph constructor can take an identifier which identifies the Graph
        by name.  If none is given, the graph is assigned a BNode for its
        identifier.
        For more on named graphs, see: http://www.w3.org/2004/03/trix/

        """

        def __init__(self, store='default', identifier=None,
                     namespace_manager=None):
            super(Graph, self).__init__()
            self.__identifier = identifier or BNode()

            if not isinstance(self.__identifier, Node):
                self.__identifier = URIRef(self.__identifier)

            if not isinstance(store, Store):
                # TODO: error handling
                self.__store = store = plugin.get(store, Store)()
            else:
                self.__store = store
            self.__namespace_manager = namespace_manager
            self.context_aware = False
            self.formula_aware = False
            self.default_union = False

        def __get_store(self):
            return self.__store
        store = property(__get_store)  # read-only attr

        def __get_identifier(self):
            return self.__identifier
        identifier = property(__get_identifier)  # read-only attr

        def _get_namespace_manager(self):
            if self.__namespace_manager is None:
                self.__namespace_manager = NamespaceManager(self)
            return self.__namespace_manager

        def _set_namespace_manager(self, nm):
            self.__namespace_manager = nm

        namespace_manager = property(_get_namespace_manager,
                                     _set_namespace_manager,
                                     doc="this graph's namespace-manager")

        def __repr__(self):
            return "<Graph identifier=%s (%s)>" % (self.identifier, type(self))

        def __str__(self):
            if isinstance(self.identifier, URIRef):
                return ("%s a rdfg:Graph;rdflib:storage " +
                        "[a rdflib:Store;rdfs:label '%s'].") % (
                            self.identifier.n3(),
                            self.store.__class__.__name__)
            else:
                return ("[a rdfg:Graph;rdflib:storage " +
                        "[a rdflib:Store;rdfs:label '%s']].") % (
                            self.store.__class__.__name__)

        def toPython(self):
            return self

        def destroy(self, configuration):
            """Destroy the store identified by `configuration` if supported"""
            self.__store.destroy(configuration)

        # Transactional interfaces (optional)
        def commit(self):
            """Commits active transactions"""
            self.__store.commit()

        def rollback(self):
            """Rollback active transactions"""
            self.__store.rollback()

        def open(self, configuration, create=False):
            """Open the graph store

            Might be necessary for stores that require opening a connection to a
            database or acquiring some resource.
            """
            return self.__store.open(configuration, create)

        def close(self, commit_pending_transaction=False):
            """Close the graph store

            Might be necessary for stores that require closing a connection to a
            database or releasing some resource.
            """
            self.__store.close(
                commit_pending_transaction=commit_pending_transaction)

        def add(self, (s, p, o)):
            """Add a triple with self as context"""
            assert isinstance(s, Node), \
                "Subject %s must be an rdflib term" % (s,)
            assert isinstance(p, Node), \
                "Predicate %s must be an rdflib term" % (p,)
            assert isinstance(o, Node), \
                "Object %s must be an rdflib term" % (o,)
            self.__store.add((s, p, o), self, quoted=False)

        def addN(self, quads):
            """Add a sequence of triple with context"""

            self.__store.addN((s, p, o, c) for s, p, o, c in quads
                              if isinstance(c, Graph)
                              and c.identifier is self.identifier
                              and _assertnode(s,p,o)
                              )

        def remove(self, (s, p, o)):
            """Remove a triple from the graph

            If the triple does not provide a context attribute, removes the triple
            from all contexts.
            """
            self.__store.remove((s, p, o), context=self)

        def triples(self, (s, p, o)):
            """Generator over the triple store

            Returns triples that match the given triple pattern. If triple pattern
            does not provide a context, all contexts will be searched.
            """
            if isinstance(p, Path):
                for _s, _o in p.eval(self, s, o):
                    yield (_s, p, _o)
            else:
                for (s, p, o), cg in self.__store.triples((s, p, o), context=self):
                    yield (s, p, o)

        @py3compat.format_doctest_out
        def __getitem__(self, item):
            """
            A graph can be "sliced" as a shortcut for the triples method
            The python slice syntax is (ab)used for specifying triples.
            A generator over matches is returned,
            the returned tuples include only the parts not given

            >>> import rdflib
            >>> g = rdflib.Graph()
            >>> g.add((rdflib.URIRef('urn:bob'), rdflib.RDFS.label, rdflib.Literal('Bob')))

            >>> list(g[rdflib.URIRef('urn:bob')]) # all triples about bob
            [(rdflib.term.URIRef(%(u)s'http://www.w3.org/2000/01/rdf-schema#label'), rdflib.term.Literal(%(u)s'Bob'))]

            >>> list(g[:rdflib.RDFS.label]) # all label triples
            [(rdflib.term.URIRef(%(u)s'urn:bob'), rdflib.term.Literal(%(u)s'Bob'))]

            >>> list(g[::rdflib.Literal('Bob')]) # all triples with bob as object
            [(rdflib.term.URIRef(%(u)s'urn:bob'), rdflib.term.URIRef(%(u)s'http://www.w3.org/2000/01/rdf-schema#label'))]

            Combined with SPARQL paths, more complex queries can be
            written concisely:

            Name of all Bobs friends:

            g[bob : FOAF.knows/FOAF.name ]

            Some label for Bob:

            g[bob : DC.title|FOAF.name|RDFS.label]

            All friends and friends of friends of Bob

            g[bob : FOAF.knows * '+']

            etc.

            .. versionadded:: 4.0

            """

            if isinstance(item, slice):

                s,p,o=item.start,item.stop,item.step
                if s is None and p is None and o is None:
                    return self.triples((s,p,o))
                elif s is None and p is None:
                    return self.subject_predicates(o)
                elif s is None and o is None:
                    return self.subject_objects(p)
                elif p is None and o is None:
                    return self.predicate_objects(s)
                elif s is None:
                    return self.subjects(p,o)
                elif p is None:
                    return self.predicates(s,o)
                elif o is None:
                    return self.objects(s,p)
                else:
                    # all given
                    return (s,p,o) in self

            elif isinstance(item, (Path,Node)):

                return self.predicate_objects(item)

            else:
                raise TypeError("You can only index a graph by a single rdflib term or path, or a slice of rdflib terms.")

        def __len__(self):
            """Returns the number of triples in the graph

            If context is specified then the number of triples in the context is
            returned instead.
            """
            return self.__store.__len__(context=self)

        def __iter__(self):
            """Iterates over all triples in the store"""
            return self.triples((None, None, None))

        def __contains__(self, triple):
            """Support for 'triple in graph' syntax"""
            for triple in self.triples(triple):
                return True
            return False

        def __hash__(self):
            return hash(self.identifier)

        def md5_term_hash(self):
            d = md5(str(self.identifier))
            d.update("G")
            return d.hexdigest()

        def __cmp__(self, other):
            if other is None:
                return -1
            elif isinstance(other, Graph):
                return cmp(self.identifier, other.identifier)
            else:
                # Note if None is considered equivalent to owl:Nothing
                # Then perhaps a graph with length 0 should be considered
                # equivalent to None (if compared to it)?
                return 1

        def __eq__(self, other):
            return isinstance(other, Graph) \
                and self.identifier == other.identifier

        def __lt__(self, other):
            return (other is None) \
                or (isinstance(other, Graph)
                    and self.identifier < other.identifier)

        def __le__(self, other):
            return self < other or self == other

        def __gt__(self, other):
            return (isinstance(other, Graph)
                    and self.identifier > other.identifier) \
                or (other is not None)

        def __ge__(self, other):
            return self > other or self == other

        def __iadd__(self, other):
            """Add all triples in Graph other to Graph.
               BNode IDs are not changed."""
            self.addN((s, p, o, self) for s, p, o in other)
            return self

        def __isub__(self, other):
            """Subtract all triples in Graph other from Graph.
               BNode IDs are not changed."""
            for triple in other:
                self.remove(triple)
            return self

        def __add__(self, other):
            """Set-theoretic union
               BNode IDs are not changed."""
            retval = Graph()
            for (prefix, uri) in set(
                    list(self.namespaces()) + list(other.namespaces())):
                retval.bind(prefix, uri)
            for x in self:
                retval.add(x)
            for y in other:
                retval.add(y)
            return retval

        def __mul__(self, other):
            """Set-theoretic intersection.
               BNode IDs are not changed."""
            retval = Graph()
            for x in other:
                if x in self:
                    retval.add(x)
            return retval

        def __sub__(self, other):
            """Set-theoretic difference.
               BNode IDs are not changed."""
            retval = Graph()
            for x in self:
                if not x in other:
                    retval.add(x)
            return retval

        def __xor__(self, other):
            """Set-theoretic XOR.
               BNode IDs are not changed."""
            return (self - other) + (other - self)

        __or__ = __add__
        __and__ = __mul__

        # Conv. methods

        def set(self, triple):
            """Convenience method to update the value of object

            Remove any existing triples for subject and predicate before adding
            (subject, predicate, object).
            """
            (subject, predicate, object_) = triple
            assert subject is not None, \
                "s can't be None in .set([s,p,o]), as it would remove (*, p, *)"
            assert predicate is not None, \
                "p can't be None in .set([s,p,o]), as it would remove (s, *, *)"
            self.remove((subject, predicate, None))
            self.add((subject, predicate, object_))

        def subjects(self, predicate=None, object=None):
            """A generator of subjects with the given predicate and object"""
            for s, p, o in self.triples((None, predicate, object)):
                yield s

        def predicates(self, subject=None, object=None):
            """A generator of predicates with the given subject and object"""
            for s, p, o in self.triples((subject, None, object)):
                yield p

        def objects(self, subject=None, predicate=None):
            """A generator of objects with the given subject and predicate"""
            for s, p, o in self.triples((subject, predicate, None)):
                yield o

        def subject_predicates(self, object=None):
            """A generator of (subject, predicate) tuples for the given object"""
            for s, p, o in self.triples((None, None, object)):
                yield s, p

        def subject_objects(self, predicate=None):
            """A generator of (subject, object) tuples for the given predicate"""
            for s, p, o in self.triples((None, predicate, None)):
                yield s, o

        def predicate_objects(self, subject=None):
            """A generator of (predicate, object) tuples for the given subject"""
            for s, p, o in self.triples((subject, None, None)):
                yield p, o

        def triples_choices(self, (subject, predicate, object_), context=None):
            for (s, p, o), cg in self.store.triples_choices(
                    (subject, predicate, object_), context=self):
                yield (s, p, o)

        def value(self, subject=None, predicate=RDF.value, object=None,
                  default=None, any=True):
            """Get a value for a pair of two criteria

            Exactly one of subject, predicate, object must be None. Useful if one
            knows that there may only be one value.

            It is one of those situations that occur a lot, hence this
            'macro' like utility

            Parameters:
            subject, predicate, object  -- exactly one must be None
            default -- value to be returned if no values found
            any -- if True, return any value in the case there is more than one,
            else, raise UniquenessError
            """
            retval = default

            if (subject is None and predicate is None) or \
                    (subject is None and object is None) or \
                    (predicate is None and object is None):
                return None

            if object is None:
                values = self.objects(subject, predicate)
            if subject is None:
                values = self.subjects(predicate, object)
            if predicate is None:
                values = self.predicates(subject, object)

            try:
                retval = values.next()
            except StopIteration:
                retval = default
            else:
                if any is False:
                    try:
                        values.next()
                        msg = ("While trying to find a value for (%s, %s, %s) the"
                               " following multiple values where found:\n" %
                               (subject, predicate, object))
                        triples = self.store.triples(
                            (subject, predicate, object), None)
                        for (s, p, o), contexts in triples:
                            msg += "(%s, %s, %s)\n (contexts: %s)\n" % (
                                s, p, o, list(contexts))
                        raise exceptions.UniquenessError(msg)
                    except StopIteration:
                        pass
            return retval

        def label(self, subject, default=''):
            """Query for the RDFS.label of the subject

            Return default if no label exists or any label if multiple exist.
            """
            if subject is None:
                return default
            return self.value(subject, RDFS.label, default=default, any=True)

        @py3compat.format_doctest_out
        def preferredLabel(self, subject, lang=None, default=None,
                           labelProperties=(SKOS.prefLabel, RDFS.label)):
            """
            Find the preferred label for subject.

            By default prefers skos:prefLabels over rdfs:labels. In case at least
            one prefLabel is found returns those, else returns labels. In case a
            language string (e.g., 'en', 'de' or even '' for no lang-tagged
            literals) is given, only such labels will be considered.

            Return a list of (labelProp, label) pairs, where labelProp is either
            skos:prefLabel or rdfs:label.

            >>> from rdflib import ConjunctiveGraph, URIRef, RDFS, Literal
            >>> from rdflib.namespace import SKOS
            >>> from pprint import pprint
            >>> g = ConjunctiveGraph()
            >>> u = URIRef(%(u)s'http://example.com/foo')
            >>> g.add([u, RDFS.label, Literal('foo')])
            >>> g.add([u, RDFS.label, Literal('bar')])
            >>> pprint(sorted(g.preferredLabel(u)))
            [(rdflib.term.URIRef(%(u)s'http://www.w3.org/2000/01/rdf-schema#label'),
              rdflib.term.Literal(%(u)s'bar')),
             (rdflib.term.URIRef(%(u)s'http://www.w3.org/2000/01/rdf-schema#label'),
              rdflib.term.Literal(%(u)s'foo'))]
            >>> g.add([u, SKOS.prefLabel, Literal('bla')])
            >>> pprint(g.preferredLabel(u))
            [(rdflib.term.URIRef(%(u)s'http://www.w3.org/2004/02/skos/core#prefLabel'),
              rdflib.term.Literal(%(u)s'bla'))]
            >>> g.add([u, SKOS.prefLabel, Literal('blubb', lang='en')])
            >>> sorted(g.preferredLabel(u)) #doctest: +NORMALIZE_WHITESPACE
            [(rdflib.term.URIRef(%(u)s'http://www.w3.org/2004/02/skos/core#prefLabel'),
              rdflib.term.Literal(%(u)s'bla')),
              (rdflib.term.URIRef(%(u)s'http://www.w3.org/2004/02/skos/core#prefLabel'),
              rdflib.term.Literal(%(u)s'blubb', lang='en'))]
            >>> g.preferredLabel(u, lang='') #doctest: +NORMALIZE_WHITESPACE
            [(rdflib.term.URIRef(%(u)s'http://www.w3.org/2004/02/skos/core#prefLabel'),
              rdflib.term.Literal(%(u)s'bla'))]
            >>> pprint(g.preferredLabel(u, lang='en'))
            [(rdflib.term.URIRef(%(u)s'http://www.w3.org/2004/02/skos/core#prefLabel'),
              rdflib.term.Literal(%(u)s'blubb', lang='en'))]
            """

            if default is None:
                default = []

            # setup the language filtering
            if lang is not None:
                if lang == '':  # we only want not language-tagged literals
                    langfilter = lambda l: l.language is None
                else:
                    langfilter = lambda l: l.language == lang
            else:  # we don't care about language tags
                langfilter = lambda l: True

            for labelProp in labelProperties:
                labels = filter(langfilter, self.objects(subject, labelProp))
                if len(labels) == 0:
                    continue
                else:
                    return [(labelProp, l) for l in labels]
            return default

        def comment(self, subject, default=''):
            """Query for the RDFS.comment of the subject

            Return default if no comment exists
            """
            if subject is None:
                return default
            return self.value(subject, RDFS.comment, default=default, any=True)

        def items(self, list):
            """Generator over all items in the resource specified by list

            list is an RDF collection.
            """
            chain = set([list])
            while list:
                item = self.value(list, RDF.first)
                if item is not None:
                    yield item
                list = self.value(list, RDF.rest)
                if list in chain:
                    raise ValueError("List contains a recursive rdf:rest reference")
                chain.add(list)

        def transitiveClosure(self, func, arg, seen=None):
            """
            Generates transitive closure of a user-defined
            function against the graph

            >>> from rdflib.collection import Collection
            >>> g=Graph()
            >>> a=BNode('foo')
            >>> b=BNode('bar')
            >>> c=BNode('baz')
            >>> g.add((a,RDF.first,RDF.type))
            >>> g.add((a,RDF.rest,b))
            >>> g.add((b,RDF.first,RDFS.label))
            >>> g.add((b,RDF.rest,c))
            >>> g.add((c,RDF.first,RDFS.comment))
            >>> g.add((c,RDF.rest,RDF.nil))
            >>> def topList(node,g):
            ...    for s in g.subjects(RDF.rest,node):
            ...       yield s
            >>> def reverseList(node,g):
            ...    for f in g.objects(node,RDF.first):
            ...       print(f)
            ...    for s in g.subjects(RDF.rest,node):
            ...       yield s

            >>> [rt for rt in g.transitiveClosure(
            ...     topList,RDF.nil)] # doctest: +NORMALIZE_WHITESPACE
            [rdflib.term.BNode('baz'),
             rdflib.term.BNode('bar'),
             rdflib.term.BNode('foo')]

            >>> [rt for rt in g.transitiveClosure(
            ...     reverseList,RDF.nil)] # doctest: +NORMALIZE_WHITESPACE
            http://www.w3.org/2000/01/rdf-schema#comment
            http://www.w3.org/2000/01/rdf-schema#label
            http://www.w3.org/1999/02/22-rdf-syntax-ns#type
            [rdflib.term.BNode('baz'),
             rdflib.term.BNode('bar'),
             rdflib.term.BNode('foo')]

            """
            if seen is None:
                seen = {}
            elif arg in seen:
                return
            seen[arg] = 1
            for rt in func(arg, self):
                yield rt
                for rt_2 in self.transitiveClosure(func, rt, seen):
                    yield rt_2

        def transitive_objects(self, subject, property, remember=None):
            """Transitively generate objects for the ``property`` relationship

            Generated objects belong to the depth first transitive closure of the
            ``property`` relationship starting at ``subject``.
            """
            if remember is None:
                remember = {}
            if subject in remember:
                return
            remember[subject] = 1
            yield subject
            for object in self.objects(subject, property):
                for o in self.transitive_objects(object, property, remember):
                    yield o

        def transitive_subjects(self, predicate, object, remember=None):
            """Transitively generate objects for the ``property`` relationship

            Generated objects belong to the depth first transitive closure of the
            ``property`` relationship starting at ``subject``.
            """
            if remember is None:
                remember = {}
            if object in remember:
                return
            remember[object] = 1
            yield object
            for subject in self.subjects(predicate, object):
                for s in self.transitive_subjects(predicate, subject, remember):
                    yield s

        def seq(self, subject):
            """Check if subject is an rdf:Seq

            If yes, it returns a Seq class instance, None otherwise.
            """
            if (subject, RDF.type, RDF.Seq) in self:
                return Seq(self, subject)
            else:
                return None

        def qname(self, uri):
            return self.namespace_manager.qname(uri)

        def compute_qname(self, uri, generate=True):
            return self.namespace_manager.compute_qname(uri, generate)

        def bind(self, prefix, namespace, override=True):
            """Bind prefix to namespace

            If override is True will bind namespace to given prefix even
            if namespace was already bound to a different prefix.

            for example:  graph.bind('foaf', 'http://xmlns.com/foaf/0.1/')

            """
            return self.namespace_manager.bind(
                prefix, namespace, override=override)

        def namespaces(self):
            """Generator over all the prefix, namespace tuples"""
            for prefix, namespace in self.namespace_manager.namespaces():
                yield prefix, namespace

        def absolutize(self, uri, defrag=1):
            """Turn uri into an absolute URI if it's not one already"""
            return self.namespace_manager.absolutize(uri, defrag)

        def serialize(self, destination=None, format="xml",
                      base=None, encoding=None, **args):
            """Serialize the Graph to destination

            If destination is None serialize method returns the serialization as a
            string. Format defaults to xml (AKA rdf/xml).

            Format support can be extended with plugins,
            but 'xml', 'n3', 'turtle', 'nt', 'pretty-xml', trix' are built in.
            """
            serializer = plugin.get(format, Serializer)(self)
            if destination is None:
                stream = BytesIO()
                serializer.serialize(stream, base=base, encoding=encoding, **args)
                return stream.getvalue()
            if hasattr(destination, "write"):
                stream = destination
                serializer.serialize(stream, base=base, encoding=encoding, **args)
            else:
                location = destination
                scheme, netloc, path, params, _query, fragment = urlparse(location)
                if netloc != "":
                    print("WARNING: not saving as location" +
                          "is not a local file reference")
                    return
                fd, name = tempfile.mkstemp()
                stream = os.fdopen(fd, "wb")
                serializer.serialize(stream, base=base, encoding=encoding, **args)
                stream.close()
                if hasattr(shutil, "move"):
                    shutil.move(name, path)
                else:
                    shutil.copy(name, path)
                    os.remove(name)

        def parse(self, source=None, publicID=None, format=None,
                  location=None, file=None, data=None, **args):
            """
            Parse source adding the resulting triples to the Graph.

            The source is specified using one of source, location, file or
            data.

            :Parameters:

              - `source`: An InputSource, file-like object, or string. In the case
                of a string the string is the location of the source.
              - `location`: A string indicating the relative or absolute URL of the
                source. Graph's absolutize method is used if a relative location
                is specified.
              - `file`: A file-like object.
              - `data`: A string containing the data to be parsed.
              - `format`: Used if format can not be determined from source.
                Defaults to rdf/xml. Format support can be extended with plugins,
                but 'xml', 'n3', 'nt', 'trix', 'rdfa' are built in.
              - `publicID`: the logical URI to use as the document base. If None
                specified the document location is used (at least in the case where
                there is a document location).

            :Returns:

              - self, the graph instance.

            Examples:

            >>> my_data = '''
            ... <rdf:RDF
            ...   xmlns:rdf='http://www.w3.org/1999/02/22-rdf-syntax-ns#'
            ...   xmlns:rdfs='http://www.w3.org/2000/01/rdf-schema#'
            ... >
            ...   <rdf:Description>
            ...     <rdfs:label>Example</rdfs:label>
            ...     <rdfs:comment>This is really just an example.</rdfs:comment>
            ...   </rdf:Description>
            ... </rdf:RDF>
            ... '''
            >>> import tempfile
            >>> fd, file_name = tempfile.mkstemp()
            >>> f = os.fdopen(fd, 'w')
            >>> dummy = f.write(my_data)  # Returns num bytes written on py3
            >>> f.close()

            >>> g = Graph()
            >>> result = g.parse(data=my_data, format="application/rdf+xml")
            >>> len(g)
            2

            >>> g = Graph()
            >>> result = g.parse(location=file_name, format="application/rdf+xml")
            >>> len(g)
            2

            >>> g = Graph()
            >>> result = g.parse(file=open(file_name, "r"),
            ...     format="application/rdf+xml")
            >>> len(g)
            2

            >>> os.remove(file_name)

            """

            source = create_input_source(source=source, publicID=publicID,
                                         location=location, file=file,
                                         data=data, format=format)
            if format is None:
                format = source.content_type
            if format is None:
                # raise Exception("Could not determine format for %r. You can" + \
                # "expicitly specify one with the format argument." % source)
                format = "application/rdf+xml"
            parser = plugin.get(format, Parser)()
            parser.parse(source, self, **args)
            return self

        def load(self, source, publicID=None, format="xml"):
            self.parse(source, publicID, format)

        def query(self, query_object, processor='sparql',
                  result='sparql', initNs=None, initBindings=None,
                  use_store_provided=True, **kwargs):
            """
            Query this graph.

            A type of 'prepared queries' can be realised by providing
            initial variable bindings with initBindings

            Initial namespaces are used to resolve prefixes used in the query,
            if none are given, the namespaces from the graph's namespace manager
            are used.

            :returntype: rdflib.query.QueryResult

            """

            initBindings = initBindings or {}
            initNs = initNs or dict(self.namespaces())

            if hasattr(self.store, "query") and use_store_provided:
                try:
                    return self.store.query(
                        query_object, initNs, initBindings,
                        self.default_union
                        and '__UNION__'
                        or self.identifier,
                        **kwargs)
                except NotImplementedError:
                    pass  # store has no own implementation

            if not isinstance(result, query.Result):
                result = plugin.get(result, query.Result)
            if not isinstance(processor, query.Processor):
                processor = plugin.get(processor, query.Processor)(self)

            return result(processor.query(
                query_object, initBindings, initNs, **kwargs))

        def update(self, update_object, processor='sparql',
                  initNs={}, initBindings={},
                  use_store_provided=True, **kwargs):
            """
            """
            if hasattr(self.store, "update") and use_store_provided:
                try:
                    return self.store.update(
                        update_object, initNs, initBindings,
                        self.default_union
                        and '__UNION__'
                        or self.identifier,
                        **kwargs)
                except NotImplementedError:
                    pass  # store has no own implementation

            if not isinstance(processor, query.UpdateProcessor):
                processor = plugin.get(processor, query.UpdateProcessor)(self)

            return processor.update(update_object, initBindings, initNs, **kwargs)

        def n3(self):
            """return an n3 identifier for the Graph"""
            return "[%s]" % self.identifier.n3()

        def __reduce__(self):
            return (Graph, (self.store, self.identifier,))

        def isomorphic(self, other):
            """
            does a very basic check if these graphs are the same
            If no BNodes are involved, this is accurate.

            See rdflib.compare for a correct implementation of isomorphism checks
            """
            # TODO: this is only an approximation.
            if len(self) != len(other):
                return False
            for s, p, o in self:
                if not isinstance(s, BNode) and not isinstance(o, BNode):
                    if not (s, p, o) in other:
                        return False
            for s, p, o in other:
                if not isinstance(s, BNode) and not isinstance(o, BNode):
                    if not (s, p, o) in self:
                        return False
            # TODO: very well could be a false positive at this point yet.
            return True

        def connected(self):
            """Check if the Graph is connected

            The Graph is considered undirectional.

            Performs a search on the Graph, starting from a random node. Then
            iteratively goes depth-first through the triplets where the node is
            subject and object. Return True if all nodes have been visited and
            False if it cannot continue and there are still unvisited nodes left.
            """
            all_nodes = list(self.all_nodes())
            discovered = []

            # take a random one, could also always take the first one, doesn't
            # really matter.
            if not all_nodes:
                return False

            visiting = [all_nodes[random.randrange(len(all_nodes))]]
            while visiting:
                x = visiting.pop()
                if x not in discovered:
                    discovered.append(x)
                for new_x in self.objects(subject=x):
                    if new_x not in discovered and new_x not in visiting:
                        visiting.append(new_x)
                for new_x in self.subjects(object=x):
                    if new_x not in discovered and new_x not in visiting:
                        visiting.append(new_x)

            # optimisation by only considering length, since no new objects can
            # be introduced anywhere.
            if len(all_nodes) == len(discovered):
                return True
            else:
                return False

        def all_nodes(self):
            res = set(self.objects())
            res.update(self.subjects())
            return res

        def resource(self, identifier):
            """Create a new ``Resource`` instance.

            Parameters:

            - ``identifier``: a URIRef or BNode instance.

            Example::

                >>> graph = Graph()
                >>> uri = URIRef("http://example.org/resource")
                >>> resource = graph.resource(uri)
                >>> assert isinstance(resource, Resource)
                >>> assert resource.identifier is uri
                >>> assert resource.graph is graph

            """
            if not isinstance(identifier, Node):
                identifier = URIRef(identifier)
            return Resource(self, identifier)

        def _process_skolem_tuples(self, target, func):
            for t in self.triples((None, None, None)):
                target.add(func(t))

        def skolemize(self, new_graph=None, bnode=None):
            def do_skolemize(bnode, t):
                (s, p, o) = t
                if s == bnode:
                    s = s.skolemize()
                if o == bnode:
                    o = o.skolemize()
                return (s, p, o)

            def do_skolemize2(t):
                (s, p, o) = t
                if isinstance(s, BNode):
                    s = s.skolemize()
                if isinstance(o, BNode):
                    o = o.skolemize()
                return (s, p, o)

            retval = Graph() if new_graph is None else new_graph

            if bnode is None:
                self._process_skolem_tuples(retval, do_skolemize2)
            elif isinstance(bnode, BNode):
                self._process_skolem_tuples(
                    retval, lambda t: do_skolemize(bnode, t))

            return retval

        def de_skolemize(self, new_graph=None, uriref=None):
            def do_de_skolemize(uriref, t):
                (s, p, o) = t
                if s == uriref:
                    s = s.de_skolemize()
                if o == uriref:
                    o = o.de_skolemize()
                return (s, p, o)

            def do_de_skolemize2(t):
                (s, p, o) = t
                if isinstance(s, Genid):
                    s = s.de_skolemize()
                if isinstance(o, Genid):
                    o = o.de_skolemize()
                return (s, p, o)

            retval = Graph() if new_graph is None else new_graph

            if uriref is None:
                self._process_skolem_tuples(retval, do_de_skolemize2)
            elif isinstance(uriref, Genid):
                self._process_skolem_tuples(
                    retval, lambda t: do_de_skolemize(uriref, t))

            return retval

    class ConjunctiveGraph(Graph):

        """
        A ConjunctiveGraph is an (unamed) aggregation of all the named
        graphs in a store.

        It has a ``default`` graph, whose name is associated with the
        graph throughout its life. :meth:`__init__` can take an identifier
        to use as the name of this default graph or it will assign a
        BNode.

        All methods that add triples work against this default graph.

        All queries are carried out against the union of all graphs.

        """

        def __init__(self, store='default', identifier=None):
            super(ConjunctiveGraph, self).__init__(store, identifier=identifier)
            assert self.store.context_aware, ("ConjunctiveGraph must be backed by"
                                              " a context aware store.")
            self.context_aware = True
            self.default_union = True # Conjunctive!
            self.default_context = Graph(store=self.store,
                                         identifier=identifier or BNode())

        def __str__(self):
            pattern = ("[a rdflib:ConjunctiveGraph;rdflib:storage "
                       "[a rdflib:Store;rdfs:label '%s']]")
            return pattern % self.store.__class__.__name__

        def _spoc(self, triple_or_quad, default=False):
            """
            helper method for having methods that support
            either triples or quads
            """
            if triple_or_quad is None:
                return (None, None, None, self.default_context if default else None)
            if len(triple_or_quad) == 3:
                c = self.default_context if default else None
                (s, p, o) = triple_or_quad
            elif len(triple_or_quad) == 4:
                (s, p, o, c) = triple_or_quad
                c = self._graph(c)
            return s,p,o,c

        def __contains__(self, triple_or_quad):
            """Support for 'triple/quad in graph' syntax"""
            s,p,o,c = self._spoc(triple_or_quad)
            for t in self.triples((s,p,o), context=c):
                return True
            return False

        def add(self, triple_or_quad):

            """
            Add a triple or quad to the store.

            if a triple is given it is added to the default context
            """

            s,p,o,c = self._spoc(triple_or_quad, default=True)

            _assertnode(s,p,o)

            self.store.add((s, p, o), context=c, quoted=False)

        def _graph(self, c):
            if c is None: return None
            if not isinstance(c, Graph):
                return self.get_context(c)
            else:
                return c

        def addN(self, quads):
            """Add a sequence of triples with context"""

            self.store.addN(
                (s, p, o, self._graph(c)) for s, p, o, c in quads if
                _assertnode(s, p, o)
                )

        def remove(self, triple_or_quad):
            """
            Removes a triple or quads

            if a triple is given it is removed from all contexts

            a quad is removed from the given context only

            """
            s,p,o,c = self._spoc(triple_or_quad)

            self.store.remove((s, p, o), context=c)

        def triples(self, triple_or_quad, context=None):
            """
            Iterate over all the triples in the entire conjunctive graph

            For legacy reasons, this can take the context to query either
            as a fourth element of the quad, or as the explicit context
            keyword paramater. The kw param takes precedence.
            """

            s,p,o,c = self._spoc(triple_or_quad)
            context = self._graph(context or c)

            if self.default_union:
                if context==self.default_context:
                    context = None
            else:
                if context is None:
                    context = self.default_context

            if isinstance(p, Path):
                if context is None:
                    context = self

                for s, o in p.eval(context, s, o):
                    yield (s, p, o)
            else:
                for (s, p, o), cg in self.store.triples((s, p, o), context=context):
                    yield s, p, o

        def quads(self, triple_or_quad=None):
            """Iterate over all the quads in the entire conjunctive graph"""

            s,p,o,c = self._spoc(triple_or_quad)

            for (s, p, o), cg in self.store.triples((s, p, o), context=c):
                for ctx in cg:
                    yield s, p, o, ctx

        def triples_choices(self, (s, p, o), context=None):
            """Iterate over all the triples in the entire conjunctive graph"""

            if context is None:
                if not self.default_union:
                    context=self.default_context
            else:
                context = self._graph(context)

            for (s1, p1, o1), cg in self.store.triples_choices((s, p, o),
                                                               context=context):
                yield (s1, p1, o1)

        def __len__(self):
            """Number of triples in the entire conjunctive graph"""
            return self.store.__len__()

        def contexts(self, triple=None):
            """Iterate over all contexts in the graph

            If triple is specified, iterate over all contexts the triple is in.
            """
            for context in self.store.contexts(triple):
                if isinstance(context, Graph):
                    # TODO: One of these should never happen and probably
                    # should raise an exception rather than smoothing over
                    # the weirdness - see #225
                    yield context
                else:
                    yield self.get_context(context)

        def get_context(self, identifier, quoted=False):
            """Return a context graph for the given identifier

            identifier must be a URIRef or BNode.
            """
            return Graph(store=self.store, identifier=identifier,
                         namespace_manager=self)

        def remove_context(self, context):
            """Removes the given context from the graph"""
            self.store.remove((None, None, None), context)

        def context_id(self, uri, context_id=None):
            """URI#context"""
            uri = uri.split("#", 1)[0]
            if context_id is None:
                context_id = "#context"
            return URIRef(context_id, base=uri)

        def parse(self, source=None, publicID=None, format="xml",
                  location=None, file=None, data=None, **args):
            """
            Parse source adding the resulting triples to its own context
            (sub graph of this graph).

            See :meth:`rdflib.graph.Graph.parse` for documentation on arguments.

            :Returns:

            The graph into which the source was parsed. In the case of n3
            it returns the root context.
            """

            source = create_input_source(
                source=source, publicID=publicID, location=location,
                file=file, data=data, format=format)

            g_id = publicID and publicID or source.getPublicId()
            if not isinstance(g_id, Node):
                g_id = URIRef(g_id)

            context = Graph(store=self.store, identifier=g_id)
            context.remove((None, None, None)) # hmm ?
            context.parse(source, publicID=publicID, format=format,
                          location=location, file=file, data=data, **args)
            return context

        def __reduce__(self):
            return (ConjunctiveGraph, (self.store, self.identifier))

    DATASET_DEFAULT_GRAPH_ID = URIRef('urn:x-rdflib:default')

    class Dataset(ConjunctiveGraph):
        __doc__ = format_doctest_out("""
        RDF 1.1 Dataset. Small extension to the Conjunctive Graph:
        - the primary term is graphs in the datasets and not contexts with quads,
        so there is a separate method to set/retrieve a graph in a dataset and
        operate with graphs
        - graphs cannot be identified with blank nodes
        - added a method to directly add a single quad

        Examples of usage:

        >>> # Create a new Dataset
        >>> ds = Dataset()
        >>> # simple triples goes to default graph
        >>> ds.add((URIRef('http://example.org/a'),
        ...    URIRef('http://www.example.org/b'),
        ...    Literal('foo')))
        >>>
        >>> # Create a graph in the dataset, if the graph name has already been
        >>> # used, the corresponding graph will be returned
        >>> # (ie, the Dataset keeps track of the constituent graphs)
        >>> g = ds.graph(URIRef('http://www.example.com/gr'))
        >>>
        >>> # add triples to the new graph as usual
        >>> g.add(
        ...     (URIRef('http://example.org/x'),
        ...     URIRef('http://example.org/y'),
        ...     Literal('bar')) )
        >>> # alternatively: add a quad to the dataset -> goes to the graph
        >>> ds.add(
        ...     (URIRef('http://example.org/x'),
        ...     URIRef('http://example.org/z'),
        ...     Literal('foo-bar'),g) )
        >>>
        >>> # querying triples return them all regardless of the graph
        >>> for t in ds.triples((None,None,None)):  # doctest: +SKIP
        ...     print(t)  # doctest: +NORMALIZE_WHITESPACE
        (rdflib.term.URIRef(%(u)s'http://example.org/a'),
         rdflib.term.URIRef(%(u)s'http://www.example.org/b'),
         rdflib.term.Literal(%(u)s'foo'))
        (rdflib.term.URIRef(%(u)s'http://example.org/x'),
         rdflib.term.URIRef(%(u)s'http://example.org/z'),
         rdflib.term.Literal(%(u)s'foo-bar'))
        (rdflib.term.URIRef(%(u)s'http://example.org/x'),
         rdflib.term.URIRef(%(u)s'http://example.org/y'),
         rdflib.term.Literal(%(u)s'bar'))
        >>>
        >>> # querying quads return quads; the fourth argument can be unrestricted
        >>> # or restricted to a graph
        >>> for q in ds.quads((None, None, None, None)):  # doctest: +SKIP
        ...     print(q)  # doctest: +NORMALIZE_WHITESPACE
        (rdflib.term.URIRef(%(u)s'http://example.org/a'),
         rdflib.term.URIRef(%(u)s'http://www.example.org/b'),
         rdflib.term.Literal(%(u)s'foo'),
         None)
        (rdflib.term.URIRef(%(u)s'http://example.org/x'),
         rdflib.term.URIRef(%(u)s'http://example.org/y'),
         rdflib.term.Literal(%(u)s'bar'),
         rdflib.term.URIRef(%(u)s'http://www.example.com/gr'))
        (rdflib.term.URIRef(%(u)s'http://example.org/x'),
         rdflib.term.URIRef(%(u)s'http://example.org/z'),
         rdflib.term.Literal(%(u)s'foo-bar'),
         rdflib.term.URIRef(%(u)s'http://www.example.com/gr'))
        >>>
        >>> for q in ds.quads((None,None,None,g)):  # doctest: +SKIP
        ...     print(q)  # doctest: +NORMALIZE_WHITESPACE
        (rdflib.term.URIRef(%(u)s'http://example.org/x'),
         rdflib.term.URIRef(%(u)s'http://example.org/y'),
         rdflib.term.Literal(%(u)s'bar'),
         rdflib.term.URIRef(%(u)s'http://www.example.com/gr'))
        (rdflib.term.URIRef(%(u)s'http://example.org/x'),
         rdflib.term.URIRef(%(u)s'http://example.org/z'),
         rdflib.term.Literal(%(u)s'foo-bar'),
         rdflib.term.URIRef(%(u)s'http://www.example.com/gr'))
        >>> # Note that in the call above -
        >>> # ds.quads((None,None,None,'http://www.example.com/gr'))
        >>> # would have been accepted, too
        >>>
        >>> # graph names in the dataset can be queried:
        >>> for c in ds.graphs():  # doctest: +SKIP
        ...     print(c)  # doctest:
        DEFAULT
        http://www.example.com/gr
        >>> # A graph can be created without specifying a name; a skolemized genid
        >>> # is created on the fly
        >>> h = ds.graph()
        >>> for c in ds.graphs():  # doctest: +SKIP
        ...     print(c)  # doctest: +NORMALIZE_WHITESPACE +ELLIPSIS
        DEFAULT
        http://rdlib.net/.well-known/genid/rdflib/N...
        http://www.example.com/gr
        >>> # Note that the Dataset.graphs() call returns names of empty graphs,
        >>> # too. This can be restricted:
        >>> for c in ds.graphs(empty=False):  # doctest: +SKIP
        ...     print(c)  # doctest: +NORMALIZE_WHITESPACE
        DEFAULT
        http://www.example.com/gr
        >>>
        >>> # a graph can also be removed from a dataset via ds.remove_graph(g)

        .. versionadded:: 4.0
        """)

        def __init__(self, store='default', default_union=False):
            super(Dataset, self).__init__(store=store, identifier=None)

            if not self.store.graph_aware:
                raise Exception("DataSet must be backed by a graph-aware store!")
            self.default_context = Graph(store=self.store, identifier=DATASET_DEFAULT_GRAPH_ID)

            self.default_union = default_union

        def __str__(self):
            pattern = ("[a rdflib:Dataset;rdflib:storage "
                       "[a rdflib:Store;rdfs:label '%s']]")
            return pattern % self.store.__class__.__name__

        def graph(self, identifier=None):
            if identifier is None:
                from rdflib.term import rdflib_skolem_genid
                self.bind(
                    "genid", "http://rdflib.net" + rdflib_skolem_genid,
                    override=False)
                identifier = BNode().skolemize()

            g = self._graph(identifier)

            self.store.add_graph(g)
            return g

        def parse(self, source=None, publicID=None, format="xml",
                  location=None, file=None, data=None, **args):
            c = ConjunctiveGraph.parse(self, source, publicID, format, location, file, data, **args)
            self.graph(c)
            return c

        def add_graph(self, g):
            """alias of graph for consistency"""
            return self.graph(g)

        def remove_graph(self, g):
            if not isinstance(g, Graph):
                g = self.get_context(g)

            self.store.remove_graph(g)
            if g is None or g == self.default_context:
                # default graph cannot be removed
                # only triples deleted, so add it back in
                self.store.add_graph(self.default_context)

        def contexts(self, triple=None):
            default = False
            for c in super(Dataset, self).contexts(triple):
                default |= c.identifier == DATASET_DEFAULT_GRAPH_ID
                yield c
            if not default:
                yield self.graph(DATASET_DEFAULT_GRAPH_ID)

        graphs = contexts

        def quads(self, quad):
            for s, p, o, c in super(Dataset, self).quads(quad):
                if c.identifier==self.default_context:
                    yield (s, p, o, None)
                else:
                    yield (s, p, o, c.identifier)

    class QuotedGraph(Graph):
        """
        Quoted Graphs are intended to implement Notation 3 formulae. They are
        associated with a required identifier that the N3 parser *must* provide
        in order to maintain consistent formulae identification for scenarios
        such as implication and other such processing.
        """
        def __init__(self, store, identifier):
            super(QuotedGraph, self).__init__(store, identifier)

        def add(self, (s, p, o)):
            """Add a triple with self as context"""
            assert isinstance(s, Node), \
                "Subject %s must be an rdflib term" % (s,)
            assert isinstance(p, Node), \
                "Predicate %s must be an rdflib term" % (p,)
            assert isinstance(o, Node), \
                "Object %s must be an rdflib term" % (o,)

            self.store.add((s, p, o), self, quoted=True)

        def addN(self, quads):
            """Add a sequence of triple with context"""

            self.store.addN(
                (s, p, o, c) for s, p, o, c in quads
                if isinstance(c, QuotedGraph)
                and c.identifier is self.identifier
                and _assertnode(s, p, o)
                )

        def n3(self):
            """Return an n3 identifier for the Graph"""
            return "{%s}" % self.identifier.n3()

        def __str__(self):
            identifier = self.identifier.n3()
            label = self.store.__class__.__name__
            pattern = ("{this rdflib.identifier %s;rdflib:storage "
                       "[a rdflib:Store;rdfs:label '%s']}")
            return pattern % (identifier, label)

        def __reduce__(self):
            return (QuotedGraph, (self.store, self.identifier))

    # Make sure QuotedGraph is ordered correctly
    # wrt to other Terms.
    # this must be done here, as the QuotedGraph cannot be
    # circularily imported in term.py
    rdflib.term._ORDERING[QuotedGraph]=11

    class Seq(object):
        """Wrapper around an RDF Seq resource

        It implements a container type in Python with the order of the items
        returned corresponding to the Seq content. It is based on the natural
        ordering of the predicate names _1, _2, _3, etc, which is the
        'implementation' of a sequence in RDF terms.
        """

        def __init__(self, graph, subject):
            """Parameters:

            - graph:
                the graph containing the Seq

            - subject:
                the subject of a Seq. Note that the init does not
                check whether this is a Seq, this is done in whoever
                creates this instance!
            """

            _list = self._list = list()
            LI_INDEX = URIRef(str(RDF) + "_")
            for (p, o) in graph.predicate_objects(subject):
                if p.startswith(LI_INDEX):  # != RDF.Seq: #
                    i = int(p.replace(LI_INDEX, ''))
                    _list.append((i, o))

            # here is the trick: the predicates are _1, _2, _3, etc. Ie,
            # by sorting the keys (by integer) we have what we want!
            _list.sort()

        def toPython(self):
            return self

        def __iter__(self):
            """Generator over the items in the Seq"""
            for _, item in self._list:
                yield item

        def __len__(self):
            """Length of the Seq"""
            return len(self._list)

        def __getitem__(self, index):
            """Item given by index from the Seq"""
            index, item = self._list.__getitem__(index)
            return item

    class ModificationException(Exception):

        def __init__(self):
            pass

        def __str__(self):
            return ("Modifications and transactional operations not allowed on "
                    "ReadOnlyGraphAggregate instances")

    class UnSupportedAggregateOperation(Exception):

        def __init__(self):
            pass

        def __str__(self):
            return ("This operation is not supported by ReadOnlyGraphAggregate "
                    "instances")

    class ReadOnlyGraphAggregate(ConjunctiveGraph):
        """Utility class for treating a set of graphs as a single graph

        Only read operations are supported (hence the name). Essentially a
        ConjunctiveGraph over an explicit subset of the entire store.
        """

        def __init__(self, graphs, store='default'):
            if store is not None:
                super(ReadOnlyGraphAggregate, self).__init__(store)
                Graph.__init__(self, store)
                self.__namespace_manager = None

            assert isinstance(graphs, list) \
                and graphs \
                and [g for g in graphs if isinstance(g, Graph)], \
                "graphs argument must be a list of Graphs!!"
            self.graphs = graphs

        def __repr__(self):
            return "<ReadOnlyGraphAggregate: %s graphs>" % len(self.graphs)

        def destroy(self, configuration):
            raise ModificationException()

        # Transactional interfaces (optional)
        def commit(self):
            raise ModificationException()

        def rollback(self):
            raise ModificationException()

        def open(self, configuration, create=False):
            # TODO: is there a use case for this method?
            for graph in self.graphs:
                graph.open(self, configuration, create)

        def close(self):
            for graph in self.graphs:
                graph.close()

        def add(self, (s, p, o)):
            raise ModificationException()

        def addN(self, quads):
            raise ModificationException()

        def remove(self, (s, p, o)):
            raise ModificationException()

        def triples(self, (s, p, o)):
            for graph in self.graphs:
                if isinstance(p, Path):
                    for s, o in p.eval(self, s, o):
                        yield s, p, o
                else:
                    for s1, p1, o1 in graph.triples((s, p, o)):
                        yield (s1, p1, o1)

        def __contains__(self, triple_or_quad):
            context = None
            if len(triple_or_quad) == 4:
                context = triple_or_quad[3]
            for graph in self.graphs:
                if context is None or graph.identifier == context.identifier:
                    if triple_or_quad[:3] in graph:
                        return True
            return False

        def quads(self, (s, p, o)):
            """Iterate over all the quads in the entire aggregate graph"""
            for graph in self.graphs:
                for s1, p1, o1 in graph.triples((s, p, o)):
                    yield (s1, p1, o1, graph)

        def __len__(self):
            return sum(len(g) for g in self.graphs)

        def __hash__(self):
            raise UnSupportedAggregateOperation()

        def __cmp__(self, other):
            if other is None:
                return -1
            elif isinstance(other, Graph):
                return -1
            elif isinstance(other, ReadOnlyGraphAggregate):
                return cmp(self.graphs, other.graphs)
            else:
                return -1

        def __iadd__(self, other):
            raise ModificationException()

        def __isub__(self, other):
            raise ModificationException()

        # Conv. methods

        def triples_choices(self, (subject, predicate, object_), context=None):
            for graph in self.graphs:
                choices = graph.triples_choices((subject, predicate, object_))
                for (s, p, o) in choices:
                    yield (s, p, o)

        def qname(self, uri):
            if hasattr(self, 'namespace_manager') and self.namespace_manager:
                return self.namespace_manager.qname(uri)
            raise UnSupportedAggregateOperation()

        def compute_qname(self, uri, generate=True):
            if hasattr(self, 'namespace_manager') and self.namespace_manager:
                return self.namespace_manager.compute_qname(uri, generate)
            raise UnSupportedAggregateOperation()

        def bind(self, prefix, namespace, override=True):
            raise UnSupportedAggregateOperation()

        def namespaces(self):
            if hasattr(self, 'namespace_manager'):
                for prefix, namespace in self.namespace_manager.namespaces():
                    yield prefix, namespace
            else:
                for graph in self.graphs:
                    for prefix, namespace in graph.namespaces():
                        yield prefix, namespace

        def absolutize(self, uri, defrag=1):
            raise UnSupportedAggregateOperation()

        def parse(self, source, publicID=None, format="xml", **args):
            raise ModificationException()

        def n3(self):
            raise UnSupportedAggregateOperation()

        def __reduce__(self):
            raise UnSupportedAggregateOperation()

    def _assertnode(*terms):
        for t in terms:
            assert isinstance(t, Node), \
                'Term %s must be an rdflib term' % (t,)
        return True

    def test():
        import doctest
        doctest.testmod()

    if __name__ == '__main__':
        test()