When a template renderer isn't supported, provide a better error mess…

…age.

Closes pecan#81

pecan/core.py

@@ -412,6 +412,11 @@ def render(self, template, namespace):
                 renderer_name,
                 self.template_path
             )
+            if renderer is None:
+                raise RuntimeError(
+                    'support for "%s" was not found; ' % renderer_name +
+                    'supported template engines are %s' % self.renderers.keys()
+                )
         else:
             renderer = self.renderers.get(
                 self.default_renderer,

pecan/templating.py

@@ -285,3 +285,6 @@ def get(self, name, template_path):
             else:
                 self._renderers[name] = cls(template_path, self.extra_vars)
         return self._renderers[name]
+
+    def keys(self, *args, **kwargs):
+        return self._renderer_classes.keys(*args, **kwargs)

pecan/tests/test_base.py

@@ -1898,6 +1898,23 @@ def badtemplate(self):
                     break
         assert error_msg is not None
 
+    def test_renderer_not_found(self):
+
+        class RootController(object):
+            @expose('mako3:mako.html')
+            def index(self, name='Jonathan'):
+                return dict(name=name)
+
+        app = TestApp(
+            Pecan(RootController(), template_path=self.template_path)
+        )
+        try:
+            r = app.get('/')
+        except Exception as e:
+            expected = e
+
+        assert 'support for "mako3" was not found;' in str(expected)
+
     def test_json(self):
         try:
             from simplejson import loads