<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def timestampFormat(self, timestampFormat):
 """ Setter to _timestampFormat. Formatting string for conversion of timestamps to QtCore.QDateTime Raises: AssertionError: if timestampFormat is not of type unicode. Args: timestampFormat (unicode):
 assign timestampFormat to _timestampFormat. Formatting string for conversion of timestamps to QtCore.QDateTime. Used in data method. """

        if not isinstance(timestampFormat, str):
            raise TypeError('not of type unicode')
        #assert isinstance(timestampFormat, unicode) or timestampFormat.__class__.__name__ == "DateFormat", "not of type unicode"
        self._timestampFormat = timestampFormat