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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 by_release(cls, session, package_name, version):
 """ Get release files for a given package name and for a given version. :param session: SQLAlchemy session :type session: :class:`sqlalchemy.Session` :param package_name: package name :type package_name: unicode :param version: version :type version: unicode :return: release files :rtype: generator of :class:`pyshop.models.ReleaseFile` """

        return cls.find(session,
                        join=(Release, Package),
                        where=(Package.name == package_name,
                               Release.version == version,
                               ))