Allow shorter syntax for creating relationships.

stix2/__init__.py

@@ -194,7 +194,8 @@ class Relationship(_STIXBase):
         'target_ref',
     ]
 
-    def __init__(self, **kwargs):
+    def __init__(self, source_ref=None, relationship_type=None, target_ref=None,
+                 **kwargs):
         # TODO:
         # - created_by_ref
         # - revoked
@@ -204,6 +205,13 @@ class Relationship(_STIXBase):
 
         # - description
 
+        if source_ref and not kwargs.get('source_ref'):
+            kwargs['source_ref'] = source_ref
+        if relationship_type and not kwargs.get('relationship_type'):
+            kwargs['relationship_type'] = relationship_type
+        if target_ref and not kwargs.get('target_ref'):
+            kwargs['target_ref'] = target_ref
+
         # TODO: do we care about the performance penalty of creating this
         # if we won't need it?
         now = datetime.now(tz=pytz.UTC)

stix2/test/test_stix2.py

@@ -336,3 +336,11 @@ def test_create_relationship_from_objects_rather_than_ids(indicator, malware):
     assert relationship.relationship_type == 'indicates'
     assert relationship.source_ref == INDICATOR_ID
     assert relationship.target_ref == MALWARE_ID
+
+
+def test_create_relationship_with_positional_args(indicator, malware):
+    relationship = stix2.Relationship(indicator, 'indicates', malware)
+
+    assert relationship.relationship_type == 'indicates'
+    assert relationship.source_ref == INDICATOR_ID
+    assert relationship.target_ref == MALWARE_ID