<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def flavor_extra_set(request, flavor_id, metadata):
 """Set the flavor extra spec keys."""

    flavor = _nova.novaclient(request).flavors.get(flavor_id)
    if (not metadata):  # not a way to delete keys
        return None
    return flavor.set_keys(metadata)