diff -r 30602138e7f7 -r ac40004d7c73 src/pyams_content/shared/view/interfaces.py
--- a/src/pyams_content/shared/view/interfaces.py	Mon Jan 07 11:36:09 2019 +0100
+++ b/src/pyams_content/shared/view/interfaces.py	Tue Jan 08 14:02:06 2019 +0100
@@ -168,7 +168,13 @@
     """View query extension interface"""
 
     def get_params(self, context, request=None):
-        """Add params to catalog query"""
+        """Add params to catalog query
+
+        This method may return an iterator.
+        If defined settings are such that no result can be found (for example, if the view is
+        defined to use context themes but context doesn't have any), method can yield a "None"
+        value which will cancel query execution.
+        """
 
 
 class IViewQueryEsParamsExtension(IViewQueryExtension):