Don’t use mutable object as a default parameter

`def foo(bar=[]):` is dangerous because the list will be
initialized once and the state will be shared across future
calls.

Author: acdha

bagit.py

@@ -553,7 +553,10 @@ class BagError(Exception):
 
 
 class BagValidationError(BagError):
-    def __init__(self, message, details=[]):
+    def __init__(self, message, details=None):
+        if details is None:
+            details = []
+
         self.message = message
         self.details = details