Evaluate error string lazily for performance.

factory/builder.py

@@ -365,9 +365,9 @@ def __getattr__(self, name):
             self.__values[name] = value
             return value
         else:
-            raise AttributeError(
+            raise errors.LazyAttributeError(
                 "The parameter %r is unknown. Evaluated attributes are %r, "
-                "definitions are %r." % (name, self.__values, self.__declarations))
+                "definitions are %r.", name, self.__values, self.__declarations)
 
     def __setattr__(self, name, value):
         """Prevent setting attributes once __init__ is done."""

factory/errors.py

@@ -27,3 +27,17 @@ class InvalidDeclarationError(FactoryError):
     This means that the user declared 'foo__bar' without adding a declaration
     at 'foo'.
     """
+
+
+class LazyAttributeError(AttributeError):
+    """ A lazily evaluated version of AttributeError.
+
+    To be used when generating the error message is expensive.
+    """
+    def __init__(self, message, *args):
+        super().__init__()
+        self._message = message
+        self._args = args
+
+    def __str__(self):
+        return self._message % self._args