<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_cache_key(self, obj):
 """Derive cache key for given object."""

        if obj is not None:
            # Make sure that key is REALLY unique.
            return '{}-{}'.format(id(self), obj.pk)

        return "{}-None".format(id(self))