<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_event_sort_key(self, event_type):
 """Return the process sort key"""

        # Process sort depending on alert type
        if event_type.startswith("MEM"):
            # Sort TOP process by memory_percent
            ret = 'memory_percent'
        elif event_type.startswith("CPU_IOWAIT"):
            # Sort TOP process by io_counters (only for Linux OS)
            ret = 'io_counters'
        else:
            # Default sort is...
            ret = 'cpu_percent'
        return ret