<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def keys_values(self):
 """Key values might be a list or not, always return a list."""

        keys_values = []
        for value in self.keys.values():
            if isinstance(value, list):
                keys_values += value
            elif isinstance(value, basestring) and not value.startswith('-'):
                # Avoid -99. Should be fixed in geozones
                keys_values.append(value)
            elif isinstance(value, int) and value >= 0:
                # Avoid -99. Should be fixed in geozones
                keys_values.append(str(value))
        return keys_values