Fixed QueryDict.setlistdefault.

It was broken by a seemingly innocuous change in MultiValueDict. Document the pitfall for now. This is fragile and should be considered for refactoring.

Author: aaugustin

django/utils/datastructures.py

@@ -339,15 +339,17 @@ def setlist(self, key, list_):
  def setdefault(self, key, default=None):
  if key not in self:
  self[key] = default
- return default
+ # Do not return default here because __setitem__() may store
+ # another value -- QueryDict.__setitem__() does. Look it up.
  return self[key]
 
  def setlistdefault(self, key, default_list=None):
  if key not in self:
  if default_list is None:
  default_list = []
  self.setlist(key, default_list)
- return default_list
+ # Do not return default_list here because setlist() may store
+ # another value -- QueryDict.setlist() does. Look it up.
  return self.getlist(key)
 
  def appendlist(self, key, value):