ConjunctiveStore.add() accepts values that make serialize() fail badly

[dmm]

For example:

from rdflib.graph import Graph, ConjunctiveGraph
from rdflib import URIRef, Namespace

ns = Namespace('http://example.org/')
store = ConjunctiveGraph(identifier='temp')
store.add((ns['abc'], ns['attribute'], 'qwer'))
store.serialize(destination='test.n3', format='n3')

Fails with:

Traceback (most recent call last):
  File "test.py", line 7, in <module>
    store.serialize(destination='test.n3', format='n3')
  File "/usr/local/lib/python2.7/dist-packages/rdflib/graph.py", line 684, in serialize
    serializer.serialize(stream, base=base, encoding=encoding, **args)
  File "/usr/local/lib/python2.7/dist-packages/rdflib/plugins/serializers/turtle.py", line 180, in serialize
    if self.statement(subject) and not firstTime:
  File "/usr/local/lib/python2.7/dist-packages/rdflib/plugins/serializers/n3.py", line 91, in statement
    or super(N3Serializer, self).statement(subject))
  File "/usr/local/lib/python2.7/dist-packages/rdflib/plugins/serializers/turtle.py", line 239, in statement
    return self.s_squared(subject) or self.s_default(subject)
  File "/usr/local/lib/python2.7/dist-packages/rdflib/plugins/serializers/turtle.py", line 244, in s_default
    self.predicateList(subject)
  File "/usr/local/lib/python2.7/dist-packages/rdflib/plugins/serializers/turtle.py", line 342, in predicateList
    self.objectList(properties[propList[0]])
  File "/usr/local/lib/python2.7/dist-packages/rdflib/plugins/serializers/turtle.py", line 357, in objectList
    self.path(objects[0], OBJECT)
  File "/usr/local/lib/python2.7/dist-packages/rdflib/plugins/serializers/n3.py", line 95, in path
    super(N3Serializer, self).path(node, position, newline)
  File "/usr/local/lib/python2.7/dist-packages/rdflib/plugins/serializers/turtle.py", line 260, in path
    or self.p_default(node, position, newline)):
  File "/usr/local/lib/python2.7/dist-packages/rdflib/plugins/serializers/turtle.py", line 266, in p_default
    self.write(self.label(node, position))
  File "/usr/local/lib/python2.7/dist-packages/rdflib/plugins/serializers/turtle.py", line 279, in label
    return self.getQName(node, position==VERB) or node.n3()
AttributeError: 'str' object has no attribute 'n3'

The problem is that the object of the added triple should be a rdflib.term.Literal instead of a str. But the error doesn't occur until serialize() is called. As a result it isn't really clear what or where the problem is. ConjunctiveStore.add() should check that tuple it is given for valid values.

[gromgull]

The question is only really where to raise the exception. I guess an isinstance(Node) check in graph.add is the best place.

We have a related issue with graph identifiers for the conjuctive graph for some stores, URIs given as strings are generally turned into URIRefs, but not always.

Edit: which is #167

[dmm]

That's a good point. You don't want to have lots of paranoid code constantly checking types. My first intuition would be to deem some subset as the external API and check types there or at least clearly document what is and is not acceptable for parameters.