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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def is_valid_callsign(self, callsign, timestamp=timestamp_now):
 """ Checks if a callsign is valid Args: callsign (str):
 Amateur Radio callsign timestamp (datetime, optional):
 datetime in UTC (tzinfo=pytz.UTC) Returns: bool: True / False Example: The following checks if "DH1TW" is a valid callsign True """

        try:
            if self.get_all(callsign, timestamp):
                return True
        except KeyError:
            return False