Don't misuse KeyError for the custom names function.

README.rst

@@ -157,7 +157,7 @@ The ``^`` operator is often mistaken for a exponent operator, not the bitwise
 operation that it is in python, so if you want ``3 ^ 2`` to equal ``9``, you can
 replace the operator like this:
 
-.. code-block:: python
+.. code-block:: pycon
 
     >>> import ast
     >>> from simpleeval import safe_power
@@ -200,15 +200,15 @@ If Expressions
 
 You can use python style ``if x then y else z`` type expressions:
 
-.. code-block:: python
+.. code-block:: pycon
 
     >>> simple_eval("'equal' if x == y else 'not equal'",
                     names={"x": 1, "y": 2})
     'not equal'
 
 which, of course, can be nested:
 
-.. code-block:: python
+.. code-block:: pycon
 
     >>> simple_eval("'a' if 1 == 2 else 'b' if 2 == 3 else 'c'")
     'c'
@@ -219,15 +219,15 @@ Functions
 
 You can define functions which you'd like the expresssions to have access to:
 
-.. code-block:: python
+.. code-block:: pycon
 
     >>> simple_eval("double(21)", functions={"double": lambda x:x*2})
     42
 
 You can define "real" functions to pass in rather than lambdas, of course too,
 and even re-name them so that expressions can be shorter
 
-.. code-block:: python
+.. code-block:: pycon
 
     >>> def double(x):
             return x * 2
@@ -252,7 +252,7 @@ are provided in the ``DEFAULT_FUNCTIONS`` dict:
 If you want to provide a list of functions, but want to keep these as well,
 then you can do a normal python ``.copy()`` & ``.update``:
 
-.. code-block:: python
+.. code-block:: pycon
 
     >>> my_functions = simpleeval.DEFAULT_FUNCTIONS.copy()
     >>> my_functions.update(
@@ -267,15 +267,15 @@ Names
 Sometimes it's useful to have variables available, which in python terminology
 are called 'names'.
 
-.. code-block:: python
+.. code-block:: pycon
 
     >>> simple_eval("a + b", names={"a": 11, "b": 100})
     111
 
 You can also hand the handling of names over to a function, if you prefer:
 
 
-.. code-block:: python
+.. code-block:: pycon
 
     >>> def name_handler(node):
             return ord(node.id[0].lower(a))-96
@@ -284,9 +284,34 @@ 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.
+
+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 an "I can't find that name!", then it should raise a ``simpleeval.NameNotDefined`` 
+exception. Eg:
 
-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).
+.. code-block:: pycon
+
+   >>> 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
 ---------------------------
@@ -296,7 +321,7 @@ evaluations, you can create a SimpleEval object, and pass it expressions each
 time (which should be a bit quicker, and certainly more convenient for some use
 cases):
 
-.. code-block:: python
+.. code-block:: pycon
 
     >>> s = SimpleEval()
 
@@ -375,7 +400,7 @@ comprehensions.
 Since the primary intention of this library is short expressions - an extra 'sweetener' is
 enabled by default.  You can access a dict (or similar's) keys using the .attr syntax:
 
-.. code-block:: python
+.. code-block:: pycon
 
     >>>  simple_eval("foo.bar", names={"foo": {"bar": 42}})
     42

simpleeval.py

@@ -554,24 +554,30 @@ 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

@@ -987,6 +987,36 @@ 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."""