<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _find_keys(self):
 """ Looks for the property keys in the filter string :return: A list of property keys """

        formatter = string.Formatter()
        return [
            val[1] for val in formatter.parse(self._original_filter) if val[1]
        ]