Add setdefault() to HttpResponse

[Robert Coup]

HttpResponse acts like a dictionary with respect to headers. It's a common middleware pattern to set headers unless they've been explicitly set by the view. eg.

class MyMiddleware(object):
  def process_response(self, request, response):
    if not 'X-Test' in response:
      response['X-Test'] = 12345
    return response

Having a setdefault() implementation would simplify this (not much in the trivial one-header case, but for example CORS middleware where you're setting a number of headers)

class MyMiddleware(object):
  def process_response(self, request, response):
    response.setdefault('X-Test', 12345)
    return response

[Tim Graham <timograham@…>]

In 059c9ab24c41e1460fd8b7af65ea8d5f80f1aa82:

Fixed #23977 -- Added setdefault() method to HttpResponse

[Дилян Палаузов]

HttpResponseBase.setdefault should return a value in order to be consistent with dict.setdefault and what users expect from a function called setdefault on an objects, that behaves like dict.

[Tim Graham]

Please open a new ticket rather then reopening an existing one that's been fixed for three years. You can provide patches as attachments rather than as comments.