Create custom exception class for modifying an immutable object.

stix2/base.py

@@ -4,7 +4,8 @@ import collections
 import datetime as dt
 import json
 
-from .exceptions import ExtraFieldsError, InvalidValueError, MissingFieldsError
+from .exceptions import ExtraFieldsError, ImmutableError, InvalidValueError, \
+                        MissingFieldsError
 from .utils import format_datetime, get_timestamp, NOW
 
 __all__ = ['STIXJSONEncoder', '_STIXBase']
@@ -85,8 +86,7 @@ class _STIXBase(collections.Mapping):
 
     def __setattr__(self, name, value):
         if name != '_inner' and not name.startswith("_STIXBase__"):
-            print(name)
-            raise ValueError("Cannot modify properties after creation.")
+            raise ImmutableError
         super(_STIXBase, self).__setattr__(name, value)
 
     def __str__(self):

stix2/exceptions.py

@@ -42,3 +42,10 @@ class ExtraFieldsError(STIXError, TypeError):
         msg = "Unexpected field(s) for {0}: ({1})."
         return msg.format(self.cls.__name__,
                           ", ".join(x for x in self.fields))
+
+
+class ImmutableError(STIXError, ValueError):
+    """Attempted to modify an object after creation"""
+
+    def __init__(self):
+        super(ImmutableError, self).__init__("Cannot modify properties after creation.")

stix2/test/test_indicator.py

@@ -125,7 +125,7 @@ def test_indicator_revoked_invalid():
 
 
 def test_cannot_assign_to_indicator_attributes(indicator):
-    with pytest.raises(ValueError) as excinfo:
+    with pytest.raises(stix2.exceptions.ImmutableError) as excinfo:
         indicator.valid_from = dt.datetime.now()
 
     assert str(excinfo.value) == "Cannot modify properties after creation."

stix2/test/test_malware.py

@@ -89,7 +89,7 @@ def test_malware_required_field_name():
 
 
 def test_cannot_assign_to_malware_attributes(malware):
-    with pytest.raises(ValueError) as excinfo:
+    with pytest.raises(stix2.exceptions.ImmutableError) as excinfo:
         malware.name = "Cryptolocker II"
 
     assert str(excinfo.value) == "Cannot modify properties after creation."

stix2/test/test_relationship.py

@@ -98,7 +98,7 @@ def test_relationship_required_field_target_ref():
 
 
 def test_cannot_assign_to_relationship_attributes(relationship):
-    with pytest.raises(ValueError) as excinfo:
+    with pytest.raises(stix2.exceptions.ImmutableError) as excinfo:
         relationship.relationship_type = "derived-from"
 
     assert str(excinfo.value) == "Cannot modify properties after creation."