Refs django-commons#910 - After Django introduced template based widget rendering, the number of context values which need prettifying has increased drastically, which can (given enough template contexts to render) cause pages using the template panel to become essentially unresponsive due to the sheer amount of data.

Instead, when first seeing a new context layer, attempt to render it as a string (a key) and store that + the pretty output (the value) into a dictionary, and when subsequently seeing the same string (ie: individual iterations of a Select widget) key, use the existing pretty output instead of re-rendering it.
This cuts the number of pformat calls at a minimum by half, and ultimately (far) more than that due to re-use of the same one where possible.

Author: kezabelle

debug_toolbar/panels/templates/panel.py

@@ -69,6 +69,11 @@ class TemplatesPanel(Panel):
     def __init__(self, *args, **kwargs):
         super(TemplatesPanel, self).__init__(*args, **kwargs)
         self.templates = []
+        # Refs GitHub issue #910
+        # Holds a collection of unique context layers, keyed by a string 
+        # version of them, with the value holding the `pformat` output 
+        # of the original dictionary. See _store_template_info.
+        self.pformats = {}
 
     def _store_template_info(self, sender, **kwargs):
         template, context = kwargs['template'], kwargs['context']
@@ -104,7 +109,7 @@ def _store_template_info(self, sender, **kwargs):
                     else:
                         try:
                             recording(False)
-                            pformat(value)  # this MAY trigger a db query
+                            force_text(value)  # this MAY trigger a db query
                         except SQLQueryTriggered:
                             temp_layer[key] = '<<triggers database query>>'
                         except UnicodeEncodeError:
@@ -115,12 +120,28 @@ def _store_template_info(self, sender, **kwargs):
                             temp_layer[key] = value
                         finally:
                             recording(True)
+            # Refs GitHub issue #910
+            # After Django introduced template based form widget rendering, 
+            # djdt has to collect and format far more contexts, many of which
+            # are duplicates, and don't need formatting if we've already seen
+            # the exact same context.
+            # We sort the `temp_layer` dictionary as a 2-tuple to ensure that
+            # ordering is consistent, before simply converting it to a string
+            # representation to ensure UnicodeEncodeError can be raised as it
+            # was previously.
+            # If the stringification succeeded, and we've not seen the key
+            # before, pformat it. If we've seen it before, we should be able
+            # to just re-use it.
             try:
-                context_list.append(pformat(temp_layer))
+                forced = force_text(sorted(temp_layer.items()))
             except UnicodeEncodeError:
-                pass
+                continue
+            else:
+                if forced not in self.pformats:
+                    self.pformats[forced] = force_text(pformat(temp_layer))
+                context_list.append(self.pformats[forced])
 
-        kwargs['context'] = [force_text(item) for item in context_list]
+        kwargs['context'] = context_list
         kwargs['context_processors'] = getattr(context, 'context_processors', None)
         self.templates.append(kwargs)