Add a .ready() method to the panel API

This method will be called for all installed panels from the
DebugToolbarConfig.ready() method during Django initialization to
support initialization that needs to happen unconditionally for the
panel regardless of whether it is enabled for a particular request.

Author: living180

debug_toolbar/apps.py

@@ -17,9 +17,10 @@ class DebugToolbarConfig(AppConfig):
     def ready(self):
         from debug_toolbar.toolbar import DebugToolbar
 
-        # Import the panels when the app is ready. This allows panels
-        # like CachePanel to enable the instrumentation immediately.
-        DebugToolbar.get_panel_classes()
+        # Import the panels when the app is ready and call their ready() methods.  This
+        # allows panels like CachePanel to enable their instrumentation immediately.
+        for cls in DebugToolbar.get_panel_classes():
+            cls.ready()
 
 
 def check_template_config(config):

debug_toolbar/panels/__init__.py

@@ -114,6 +114,19 @@ def scripts(self):
         """
         return []
 
+    # Panel early initialization
+
+    @classmethod
+    def ready(cls):
+        """
+        Perform early initialization for the panel.
+
+        This should only include initialization or instrumentation that needs to
+        be done unconditionally for the panel regardless of whether it is
+        enabled for a particular request.  It should be idempotent.
+        """
+        pass
+
     # URLs for panel-specific views
 
     @classmethod

docs/panels.rst

@@ -352,6 +352,8 @@ unauthorized access. There is no public CSS API at this time.
 
     .. autoattribute:: debug_toolbar.panels.Panel.scripts
 
+    .. automethod:: debug_toolbar.panels.Panel.ready
+
     .. automethod:: debug_toolbar.panels.Panel.get_urls
 
     .. automethod:: debug_toolbar.panels.Panel.enable_instrumentation