diff --git a/src/pyquickhelper/sphinxext/sphinx_todoext_extension.py b/src/pyquickhelper/sphinxext/sphinx_todoext_extension.py
index 80e678726..f9000395e 100644
--- a/src/pyquickhelper/sphinxext/sphinx_todoext_extension.py
+++ b/src/pyquickhelper/sphinxext/sphinx_todoext_extension.py
@@ -20,6 +20,7 @@
 from sphinx.util.nodes import set_source_info, process_index_entry
 from sphinx import addnodes
 from ..texthelper.texts_language import TITLES
+from .sphinxext_helper import try_add_config_value
 
 
 class todoext_node(nodes.admonition):
@@ -493,7 +494,7 @@ def depart_todoextlist_node(self, node):
 
 def setup(app):
     """
-    setup for ``todoext`` (sphinx)
+    Setup for ``todoext`` (sphinx).
     """
     if hasattr(app, "add_mapping"):
         app.add_mapping('todoext', todoext_node)
@@ -501,7 +502,10 @@ def setup(app):
 
     app.add_config_value('todoext_include_todosext', False, 'html')
     app.add_config_value('todoext_link_only', False, 'html')
-    app.add_config_value('extlinks', {}, 'env')
+
+    # The following variable is shared with extension
+    # `todo <http://www.sphinx-doc.org/en/stable/ext/todo.html>`_.
+    try_add_config_value(app, 'extlinks', {}, 'env')
 
     app.add_node(todoextlist,
                  html=(visit_todoextlist_node, depart_todoextlist_node),
diff --git a/src/pyquickhelper/sphinxext/sphinxext_helper.py b/src/pyquickhelper/sphinxext/sphinxext_helper.py
new file mode 100644
index 000000000..0561fe311
--- /dev/null
+++ b/src/pyquickhelper/sphinxext/sphinxext_helper.py
@@ -0,0 +1,31 @@
+"""
+@file
+@brief Helpers for sphinx extensions.
+.. versionadded:: 1.5
+"""
+
+
+def try_add_config_value(app, name, default, rebuild, types_=()):
+    """
+    Add a variables in the config file if it does not have it yet.
+
+    @param      app         Sphinx application
+    @param      name        name of the variable
+    @param      default     default value
+    @param      rebuild     see below
+    @param      types_      expected types
+    @return                 True if added, False if already present.
+
+    Rebuild can be (source: `Sphinx <http://www.sphinx-doc.org/en/stable/extdev/appapi.html#sphinx.application.Sphinx.add_config_value>`_):
+
+    * 'env' if a change in the setting only takes effect when a document
+      is parsed - this means that the whole environment must be rebuilt.
+    * 'html' if a change in the setting needs a full rebuild of HTML documents.
+    * '' if a change in the setting will not need any special rebuild.
+
+    """
+    if name in app.config:
+        return False
+    help(app.add_config_value)
+    app.add_config_value(name, default, rebuild, types_)
+    return True