don't allow leading '_' on custom properties, whenever allow_custom is true

stix2/base.py

@@ -169,10 +169,16 @@ class _STIXBase(Mapping):
             extra_kwargs = list(set(kwargs) - set(self._properties))
             if extra_kwargs:
                 raise ExtraPropertiesError(cls, extra_kwargs)
-        if custom_props:
+        else:
+            # because allow_custom is true, any extra kwargs are custom
+            extra_kwargs = list(set(kwargs) - set(self._properties))
+
+        if custom_props or extra_kwargs:
             self._allow_custom = True
             if self.get_class_version() == "v21":
-                for prop_name, prop_value in custom_props.items():
+                all_custom_prop_names = extra_kwargs
+                all_custom_prop_names.extend(list(custom_props.keys()))
+                for prop_name in all_custom_prop_names:
                     if not re.match(PREFIX_21_REGEX, prop_name):
                         raise InvalidValueError(
                             self.__class__, prop_name,

stix2/test/v21/test_custom.py

@@ -20,6 +20,18 @@ IDENTITY_CUSTOM_PROP = stix2.v21.Identity(
 
 
 def test_identity_custom_property():
+    identity = stix2.v21.Identity(
+        id=IDENTITY_ID,
+        created="2015-12-21T19:59:11Z",
+        modified="2015-12-21T19:59:11Z",
+        name="John Smith",
+        identity_class="individual",
+        custom_properties={
+            "foo": "bar",
+        },
+    )
+    assert identity.foo == "bar"
+
     with pytest.raises(ValueError) as excinfo:
         stix2.v21.Identity(
             id=IDENTITY_ID,
@@ -45,6 +57,8 @@ def test_identity_custom_property():
         )
     assert "Unexpected properties for Identity" in str(excinfo.value)
 
+    # leading numeric character is illegal in 2.1
+
     with pytest.raises(stix2.exceptions.InvalidValueError) as excinfo:
         stix2.v21.Identity(
             id=IDENTITY_ID,
@@ -58,6 +72,8 @@ def test_identity_custom_property():
         )
     assert "must begin with an alpha character." in str(excinfo.value)
 
+    # leading "_" is illegal in 2.1
+
     with pytest.raises(stix2.exceptions.InvalidValueError) as excinfo:
         stix2.v21.Identity(
             id=IDENTITY_ID,
@@ -71,17 +87,17 @@ def test_identity_custom_property():
         )
     assert "must begin with an alpha character." in str(excinfo.value)
 
-    identity = stix2.v21.Identity(
-        id=IDENTITY_ID,
-        created="2015-12-21T19:59:11Z",
-        modified="2015-12-21T19:59:11Z",
-        name="John Smith",
-        identity_class="individual",
-        custom_properties={
-            "foo": "bar",
-        },
-    )
-    assert identity.foo == "bar"
+    with pytest.raises(stix2.exceptions.InvalidValueError) as excinfo:
+        identity = stix2.v21.Identity(
+            id=IDENTITY_ID,
+            created="2015-12-21T19:59:11Z",
+            modified="2015-12-21T19:59:11Z",
+            name="John Smith",
+            identity_class="individual",
+            _x_foo="bar",
+            allow_custom=True,
+        )
+    assert "must begin with an alpha character." in str(excinfo.value)
 
 
 def test_identity_custom_property_invalid():