def index(request):
    if 'q' in request.POST:
	q = request.POST['q']
	return render_to_response('index.html', q)
    else:
	return render_to_response('index.html', {'q': 'error'} )

{{ q }}
<form action="" method="POST" /> {% csrf_token %}
<input type="text" name="q" />
<input type="submit">
</form>

def index(request):
    q=request.POST.get('q', 'error')
    args={'q': q}
    args.update(csrf(request))
    return render_to_response('index.html', args)

return render_to_response('index.html', q)

render_to_response(template[, dictionary][, context_instance][, mimetype])