<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def create_basic_url(self):
 """ Create URL prefix for API calls based on type of repo. Repo may be forked and may be in namespace. That makes total 4 different types of URL. :return: """

        if self.username is None:
            if self.namespace is None:
                request_url = "{}/api/0/{}/".format(
                    self.instance, self.repo)
            else:
                request_url = "{}/api/0/{}/{}/".format(
                    self.instance, self.namespace, self.repo)
        else:
            if self.namespace is None:
                request_url = "{}/api/0/fork/{}/{}/".format(
                    self.instance, self.username, self.repo)
            else:
                request_url = "{}/api/0/fork/{}/{}/{}/".format(
                    self.instance, self.username, self.namespace, self.repo)
        return request_url