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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 _get_revision(self, revision):
 """ For git backend we always return integer here. This way we ensure that changset's revision attribute would become integer. """


        is_null = lambda o: len(o) == revision.count('0')

        try:
            self.revisions[0]
        except (KeyError, IndexError):
            raise EmptyRepositoryError("There are no changesets yet")

        if revision in (None, '', 'tip', 'HEAD', 'head', -1):
            return self.revisions[-1]

        is_bstr = isinstance(revision, (str, unicode))
        if ((is_bstr and revision.isdigit() and len(revision) < 12)
            or isinstance(revision, int) or is_null(revision)):
            try:
                revision = self.revisions[int(revision)]
            except Exception:
                raise ChangesetDoesNotExistError("Revision %s does not exist "
                    "for this repository" % (revision))

        elif is_bstr:
            # get by branch/tag name
            _ref_revision = self._parsed_refs.get(revision)
            if _ref_revision:  # and _ref_revision[1] in ['H', 'RH', 'T']:
                return _ref_revision[0]

            _tags_shas = self.tags.values()
            # maybe it's a tag ? we don't have them in self.revisions
            if revision in _tags_shas:
                return _tags_shas[_tags_shas.index(revision)]

            elif not SHA_PATTERN.match(revision) or revision not in self.revisions:
                raise ChangesetDoesNotExistError("Revision %s does not exist "
                    "for this repository" % (revision))

        # Ensure we return full id
        if not SHA_PATTERN.match(str(revision)):
            raise ChangesetDoesNotExistError("Given revision %s not recognized"
                % revision)
        return revision