<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def get_site_url():
 """Returns a URL for current site. :rtype: str|unicode """

    site_url = getattr(_THREAD_LOCAL, _THREAD_SITE_URL, None)
    if site_url is None:
        site_url = SITE_URL or get_site_url_()
        setattr(_THREAD_LOCAL, _THREAD_SITE_URL, site_url)

    return site_url