why request.path return None?

　　　in django, sometimes you want to pass value of current page path , you will do this follows:

　　a. <form action="./?next={{request.path}}" method="post">...</form>

　　b.<form action="" method="post">

　　　<input type="hidden" name="next" value="{{request.path}}"/>

　　　</form>

　　c.<a href="xxx/?next={{request.path}}">xx</a>

　　however, perheps you find out that the value of 'next' is None in views.py. so you could think that request is None. But my code is that:

def index(request):
    xxxx
    return render_to_response('index.html',{},RequestContext(request))

　　through my test. if you should apparently pass request, it will work ok. now the modified code is as follow:

def index(request):
    xxxx
    return render_to_response('index.html',{'request':request},RequestContext(request))

　　and i don't know why. if anyone konws,pleasure tell me the answer.