# HG changeset patch
# User cmlenz
# Date 1165838788 0
# Node ID 31dc3d18b39e622ef1eeef297478b42beec7763c
# Parent  a52452a65cd7f11b72088ff50a1ad09abcbf6851
0.3.x branch: add unit test for `Undefined` checks in template expressions.

diff --git a/genshi/eval.py b/genshi/eval.py
--- a/genshi/eval.py
+++ b/genshi/eval.py
@@ -61,6 +61,15 @@
     >>> data = dict(items=[1, 2, 3])
     >>> Expression('len(items)').evaluate(data)
     3
+    
+    Undefined variables can be accessed in expressions, and evaluate to a
+    non-truth value for tests. To check whether a particular variable is
+    defined, its type can be compared to the special builtin class `Undefined`:
+    
+    >>> Expression('foo').evaluate({})
+    undefined
+    >>> Expression('type(foo) is Undefined').evaluate({})
+    True
     """
     __slots__ = ['source', 'code']