运行django出错，object has no attribute 'has_key'，是什么问题？

from django.http import HttpResponse
text = """<form method="post" action="/add/">
    <input type="text" name="a" value="%d"> + <input type="text" name="b" value="%d">
    <input type="submit" value="="> <input type="text" value="%d">
</form>"""
def index(request):
    if request.POST.has_key('a'):
        a = int(request.POST['a'])
        b = int(request.POST['b'])
    else:
        a = 0
        b = 0
    return HttpResponse(text % (a, b, a + b))