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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 find_by_id(self, submission_id):
 """Finds submission by ID. Args: submission_id: ID of the submission Returns: SubmissionDescriptor with information about submission or None if submission is not found. """

    return self._attacks.get(
        submission_id,
        self._defenses.get(
            submission_id,
            self._targeted_attacks.get(submission_id, None)))