I'm sorry I need to revert some changes around updateWidgets. It just…

… breaks too many high level tests. I'm sure it would break for others too.

The way to change the widgets prefix would be to override updateWidgets:
super(MyForm, self).updateWidgets('myprefix')

CHANGES.txt

@@ -5,26 +5,22 @@ CHANGES
 2.10.0 (unreleased)
 -------------------
 
-- Initialize widgets in ``update`` step. The ``updateWidgets`` method
-  is now responsible only for actually updating the widgets.
+- The ``updateWidgets`` method has learned an argument
+  ``prefix`` which allows setting the prefix of the field widgets
+  adapter.
 
   This allows updating the common widgets prefix before the individual
   widgets are updated, useful for situations where neither a form, nor
   a widgets prefix is desired.
 
-  In addition, the ``updateWidgets`` method has learned an argument
-  ``prefix`` which allows setting the prefix of the field widgets
-  adapter.
-
 - Capitalize the messages 'no value' and 'select a value'. This change
-  has been applied also to the existing translations (where
-  applicable).
+  has been applied also to the existing translations (where applicable).
 
-- `TextLinesConverter`: do not ignore newlines at the end of the inputted
+- ``TextLinesConverter``: do not ignore newlines at the end of the inputted
   string, thus do not eat blank items
 
-- `TextLinesConverter`: toFieldValue, convert conversion exceptions to
-  `FormatterValidationError`, for cases like got a string instead of int
+- ``TextLinesConverter``: toFieldValue, convert conversion exceptions to
+  ``FormatterValidationError``, for cases like got a string instead of int
 
 2.9.0 (2012-09-17)
 ------------------

src/z3c/form/form.py

@@ -123,6 +123,8 @@ def getContent(self):
 
     def updateWidgets(self, prefix=None):
         '''See interfaces.IForm'''
+        self.widgets = zope.component.getMultiAdapter(
+            (self, self.request, self.getContent()), interfaces.IWidgets)
         if prefix is not None:
             self.widgets.prefix = prefix
         self.widgets.mode = self.mode
@@ -145,8 +147,6 @@ def extractData(self, setErrors=True):
 
     def update(self):
         '''See interfaces.IForm'''
-        self.widgets = zope.component.getMultiAdapter(
-            (self, self.request, self.getContent()), interfaces.IWidgets)
         self.updateWidgets()
 
     def render(self):

src/z3c/form/group.py

@@ -34,6 +34,8 @@ def __init__(self, context, request, parentForm):
 
     def updateWidgets(self, prefix=None):
         '''See interfaces.IForm'''
+        self.widgets = zope.component.getMultiAdapter(
+            (self, self.request, self.getContent()), interfaces.IWidgets)
         for attrName in ('mode', 'ignoreRequest', 'ignoreContext',
                          'ignoreReadonly'):
             value = getattr(self.parentForm.widgets, attrName)
@@ -44,7 +46,7 @@ def updateWidgets(self, prefix=None):
 
     def update(self):
         '''See interfaces.IForm'''
-        super(Group, self).update()
+        self.updateWidgets()
         groups = []
         for groupClass in self.groups:
             # only instantiate the groupClass if it hasn't already
@@ -121,10 +123,9 @@ def applyChanges(self, data):
 
         return changed
 
-    def updateWidgets(self, prefix=None):
+    def update(self):
         '''See interfaces.IForm'''
-        super(GroupForm, self).updateWidgets(prefix=prefix)
-
+        self.updateWidgets()
         groups = []
         for groupClass in self.groups:
             # only instantiate the groupClass if it hasn't already
@@ -136,3 +137,5 @@ def updateWidgets(self, prefix=None):
             group.update()
             groups.append(group)
         self.groups = tuple(groups)
+        self.updateActions()
+        self.actions.execute()