Autodetect parse() format

rdflib/graph.py

@@ -2,12 +2,15 @@
 from __future__ import division
 from __future__ import print_function
 
+from xml.sax import SAXParseException
+
 from rdflib.term import Literal  # required for doctests
 
 assert Literal  # avoid warning
 from rdflib.namespace import Namespace  # required for doctests
 
 assert Namespace  # avoid warning
+import rdflib.util  # avoid circular dependency
 
 
 __doc__ = """\
@@ -1070,13 +1073,24 @@ def parse(
         )
         if format is None:
             format = source.content_type
+        assumed_xml = False
         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"
+            try:
+                format = rdflib.util.guess_format(source.file.name)
+            except (AttributeError, TypeError):
+                pass
+            if format is None:
+                format = "application/rdf+xml"
+                assumed_xml = True
         parser = plugin.get(format, Parser)()
         try:
             parser.parse(source, self, **args)
+        except SAXParseException as saxpe:
+            if assumed_xml:
+                logger.warning(
+                    "Could not guess format for %r, so assumed xml."
+                    " You can explicitly specify format using the format argument." % source)
+            raise saxpe
         finally:
             if source.auto_close:
                 source.close()

rdflib/util.py

@@ -46,8 +46,7 @@
 from rdflib.exceptions import ObjectTypeError
 from rdflib.exceptions import PredicateTypeError
 from rdflib.exceptions import SubjectTypeError
-from rdflib.graph import Graph
-from rdflib.graph import QuotedGraph
+import rdflib.graph  # avoid circular dependency
 from rdflib.namespace import Namespace
 from rdflib.namespace import NamespaceManager
 from rdflib.term import BNode
@@ -146,7 +145,7 @@ def from_n3(s, default=None, backend=None, nsm=None):
         >>> from rdflib import RDFS
         >>> from_n3('rdfs:label') == RDFS['label']
         True
-        >>> nsm = NamespaceManager(Graph())
+        >>> nsm = NamespaceManager(rdflib.graph.Graph())
         >>> nsm.bind('dbpedia', 'http://dbpedia.org/resource/')
         >>> berlin = URIRef('http://dbpedia.org/resource/Berlin')
         >>> from_n3('dbpedia:Berlin', nsm=nsm) == berlin
@@ -192,16 +191,16 @@ def from_n3(s, default=None, backend=None, nsm=None):
         return Literal(int(s))
     elif s.startswith('{'):
         identifier = from_n3(s[1:-1])
-        return QuotedGraph(backend, identifier)
+        return rdflib.graph.QuotedGraph(backend, identifier)
     elif s.startswith('['):
         identifier = from_n3(s[1:-1])
-        return Graph(backend, identifier)
+        return rdflib.graph.Graph(backend, identifier)
     elif s.startswith("_:"):
         return BNode(s[2:])
     elif ':' in s:
         if nsm is None:
             # instantiate default NamespaceManager and rely on its defaults
-            nsm = NamespaceManager(Graph())
+            nsm = NamespaceManager(rdflib.graph.Graph())
         prefix, last_part = s.split(':', 1)
         ns = dict(nsm.namespaces())[prefix]
         return Namespace(ns)[last_part]

test/test_parse_file_guess_format.py

@@ -0,0 +1,32 @@
+import unittest
+from pathlib import Path
+from shutil import copyfile
+from tempfile import TemporaryDirectory
+
+from xml.sax import SAXParseException
+
+from rdflib import Graph, logger as graph_logger
+
+
+class FileParserGuessFormatTest(unittest.TestCase):
+    def test_ttl(self):
+        g = Graph()
+        self.assertIsInstance(g.parse("test/w3c/turtle/IRI_subject.ttl"), Graph)
+
+    def test_n3(self):
+        g = Graph()
+        self.assertIsInstance(g.parse("test/n3/example-lots_of_graphs.n3"), Graph)
+
+    def test_warning(self):
+        g = Graph()
+        with TemporaryDirectory() as tmpdirname:
+            newpath = Path(tmpdirname).joinpath("no_file_ext")
+            copyfile("test/w3c/turtle/IRI_subject.ttl", str(newpath))
+            with self.assertLogs(graph_logger, "WARNING") as log_cm:
+                with self.assertRaises(SAXParseException):
+                    g.parse(str(newpath))
+            self.assertTrue(any("Could not guess format" in msg for msg in log_cm.output))
+
+
+if __name__ == '__main__':
+    unittest.main()