<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def verify_checksum(file_id, pessimistic=False, chunk_size=None, throws=True, checksum_kwargs=None):
 """Verify checksum of a file instance. :param file_id: The file ID. """

    f = FileInstance.query.get(uuid.UUID(file_id))

    # Anything might happen during the task, so being pessimistic and marking
    # the file as unchecked is a reasonable precaution
    if pessimistic:
        f.clear_last_check()
        db.session.commit()
    f.verify_checksum(
        progress_callback=progress_updater, chunk_size=chunk_size,
        throws=throws, checksum_kwargs=checksum_kwargs)
    db.session.commit()