Code review feedback.

common/djangoapps/util/string_utils.py

@@ -2,14 +2,11 @@
 Utilities for string manipulation.
 """
 
-import ast
-
 def str_to_bool(str):
     """
     Converts "true" (case-insensitive) to the boolean True.
-    Everything else will return False.
+    Everything else will return False (including None).
+
+    An error will be thrown for non-string input (besides None).
     """
-    try:
-        return ast.literal_eval(str.title())
-    except:
-        return False
+    return False if str is None else str.lower() == "true"