More repr tests.

stix2/base.py

@@ -104,6 +104,6 @@ class _STIXBase(collections.Mapping):
                           separators=(",", ": "))  # Don't include spaces after commas.
 
     def __repr__(self):
-        props = [(k, self[k]) for k in sorted(self._properties)]
+        props = [(k, self[k]) for k in sorted(self._properties) if self.get(k)]
         return "{0}({1})".format(self.__class__.__name__,
                                  ", ".join(["{0!s}={1!r}".format(k, v) for k, v in props]))

stix2/test/test_external_reference.py

@@ -34,6 +34,7 @@ def test_external_reference_capec():
     )
 
     assert str(ref) == CAPEC
+    assert repr(ref) == "ExternalReference(external_id='CAPEC-550', source_name='capec')"
 
 
 CAPEC_URL = """{
@@ -100,6 +101,9 @@ def test_external_reference_offline():
     )
 
     assert str(ref) == OFFLINE
+    assert repr(ref) == "ExternalReference(description='Threat report', source_name='ACME Threat Intel')"
+    # Yikes! This works
+    assert eval("stix2." + repr(ref)) == ref
 
 
 def test_external_reference_source_required():