<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def logo_url(self):
 """Get URL to collection logo. :returns: Path to community logo. :rtype: str """

        if self.logo_ext:
            return '/api/files/{bucket}/{key}'.format(
                bucket=current_app.config['COMMUNITIES_BUCKET_UUID'],
                key='{0}/logo.{1}'.format(self.id, self.logo_ext),
            )
        return None