Don't misuse KeyError for the custom names function.

README.rst

@@ -277,9 +277,34 @@ You can also hand the handling of names over to a function, if you prefer:
     3
 
 That was a bit of a silly example, but you could use this for pulling values
-from a database or file, say, or doing some kind of caching system.
+from a database or file, looking up spreadsheet cells, say, or doing some kind of caching system.
 
-The two default names that are provided are ``True`` and ``False``.  So if you want to provide your own names, but want ``True`` and ``False`` to keep working, either provide them yourself, or ``.copy()`` and ``.update`` the ``DEFAULT_NAMES``. (See functions example above).
+In general, when it attempts to find a variable by name, if it cannot find one,
+then it will look in the ``functions`` for a function of that name.  If you want your name handler
+function to return a "I can't find that name!", then it should raise a ``simpleeval.NameNotDefined`` 
+exception. Eg:
+
+.. code-block:: python
+
+   >>> def name_handler(node):
+   ...     if node.id[0] == 'a':
+   ...         return 21
+   ...     raise NameNotDefined(node.id[0], "Not found")
+   ...
+   ... simple_eval('a + a', names=name_handler, functions={"b": 100})
+   
+   42
+
+   >>> simple_eval('a + b', names=name_handler, functions={'b': 100})
+   121
+
+(Note: in that example, putting a number directly into the functions dict was done just to
+show the fall-back to functions.  Normally only put actual callables in there.)
+
+
+The two default names that are provided are ``True`` and ``False``.  So if you want to provide
+your own names, but want ``True`` and ``False`` to keep working, either provide them yourself,
+or ``.copy()`` and ``.update`` the ``DEFAULT_NAMES``. (See functions example above).
 
 Creating an Evaluator Class
 ---------------------------

simpleeval.py

@@ -526,24 +526,30 @@ def _eval_name(self, node):
         try:
             # This happens at least for slicing
             # This is a safe thing to do because it is impossible
-            # that there is a true exression assigning to none
+            # that there is a true expression assigning to none
             # (the compiler rejects it, so you can't even
             # pass that to ast.parse)
-            if hasattr(self.names, "__getitem__"):
-                return self.names[node.id]
-            if callable(self.names):
+            return self.names[node.id]
+
+        except (TypeError, KeyError):
+            pass
+
+        if callable(self.names):
+            try:
                 return self.names(node)
+            except NameNotDefined:
+                pass
+        elif not hasattr(self.names, "__getitem__"):
             raise InvalidExpression(
                 'Trying to use name (variable) "{0}"'
                 ' when no "names" defined for'
                 " evaluator".format(node.id)
             )
 
-        except KeyError:
-            if node.id in self.functions:
-                return self.functions[node.id]
+        if node.id in self.functions:
+            return self.functions[node.id]
 
-            raise NameNotDefined(node.id, self.expr)
+        raise NameNotDefined(node.id, self.expr)
 
     def _eval_subscript(self, node):
         container = self._eval(node.value)

test_simpleeval.py

@@ -933,6 +933,36 @@ def name_handler(node):
         self.t("a", 1)
         self.t("a + b", 3)
 
+    def test_name_handler_name_not_found(self):
+        def name_handler(node):
+            if node.id[0] == "a":
+                return 21
+            raise NameNotDefined(node.id[0], "not found")
+
+        self.s.names = name_handler
+        self.s.functions = {"b": lambda: 100}
+        self.t("a + a", 42)
+
+        self.t("b()", 100)
+
+        with self.assertRaises(NameNotDefined):
+            self.t("c", None)
+
+    def test_name_handler_raises_error(self):
+        # What happens if our name-handler raises a different kind of error?
+        # we want it to ripple up all the way...
+
+        def name_handler(_node):
+            return {}["test"]
+
+        self.s.names = name_handler
+
+        # This should never be accessed:
+        self.s.functions = {"c": 42}
+
+        with self.assertRaises(KeyError):
+            self.t("c", None)
+
 
 class TestWhitespace(DRYTest):
     """test that incorrect whitespace (preceding/trailing) doesn't matter."""