def post(self):

        username = self.request.get('username')
        password = self.request.get('password')
        password2 = self.request.get('password2')
        mail = self.request.get('mail')
        passwords_match = (password == password2)

        if  valid_username(username) and valid_pw(password) and passwords_match and valid_mail(mail):
            self.redirect("/welcome")
        else:
            if not passwords_match:
                self.write_form()