Report a more useful error if an expression fails.

2 files changed, 14 insertions, 4 deletions

diff --git a/coverage/templite.py b/coverage/templite.py
index c2e8981..c42e380 100644
--- a/coverage/templite.py
+++ b/coverage/templite.py
@@ -11,6 +11,11 @@ class TempliteSyntaxError(ValueError):
     pass
 
 
+class TempliteValueError(ValueError):
+    """Raised when an expression won't evaluate in a template."""
+    pass
+
+
 class CodeBuilder(object):
     """Build source code conveniently."""
 
@@ -250,7 +255,12 @@ class Templite(object):
             try:
                 value = getattr(value, dot)
             except AttributeError:
-                value = value[dot]
+                try:
+                    value = value[dot]
+                except (TypeError, KeyError):
+                    raise TempliteValueError(
+                        "Couldn't evaluate %r.%s" % (value, dot)
+                    )
             if callable(value):
                 value = value()
         return value
diff --git a/tests/test_templite.py b/tests/test_templite.py
index aca848a..2b64e4e 100644
--- a/tests/test_templite.py
+++ b/tests/test_templite.py
@@ -3,7 +3,7 @@
 
 import re
 
-from coverage.templite import Templite, TempliteSyntaxError
+from coverage.templite import Templite, TempliteSyntaxError, TempliteValueError
 
 from tests.coveragetest import CoverageTest
 
@@ -244,8 +244,8 @@ class TempliteTest(CoverageTest):
 
     def test_exception_during_evaluation(self):
         # TypeError: Couldn't evaluate {{ foo.bar.baz }}:
-        # 'NoneType' object is unsubscriptable
-        with self.assertRaises(TypeError):
+        msg = "Couldn't evaluate None.bar"
+        with self.assertRaisesRegex(TempliteValueError, msg):
             self.try_render(
                 "Hey {{foo.bar.baz}} there", {'foo': None}, "Hey ??? there"
             )