before_render_template signal can override render template.

flask/templating.py

@@ -101,8 +101,19 @@ class DispatchingJinjaLoader(BaseLoader):
 
 
 def _render(template, context, app):
-    """Renders the template and fires the signal"""
-    before_render_template.send(app, template=template, context=context)
+    """Renders the template and fires signals.
+
+    It is possible to override render template in the before_render_template signal.
+    It can be done only if exactly one receiver and it return not None result."""
+
+    brt_resp = before_render_template.send(app, template=template, context=context)
+    
+    if len(brt_resp) == 1:
+        first_resp = brt_resp[0]
+
+        if len(first_resp) == 2 and first_resp[1] is not None:
+            return first_resp[1]
+
     rv = template.render(context)
     template_rendered.send(app, template=template, context=context)
     return rv

tests/test_signals.py

@@ -67,6 +67,22 @@ def test_before_render_template():
         assert context['whiskey'] == 43
         assert rv.data == b'<h1>43</h1>'
 
+def test_before_render_template_signal_not_None_result_render_skip_render_template():
+    app = flask.Flask(__name__)
+
+    @app.route('/')
+    def index():
+        return flask.render_template('simple_template.html', whiskey=42)
+
+    recorded = []
+
+    def record(sender, template, context):
+        recorded.append((template, context))
+        return 'Not template string'
+
+    with flask.before_render_template.connected_to(record):
+        rv = app.test_client().get('/')
+        assert rv.data == 'Not template string'
 
 def test_request_signals():
     app = flask.Flask(__name__)