Alembic is a database migrations tool for SQLAlchemy, written by
sqlalchemy authors.
* Can emit ALTER statements to a database in order to change
the structure of tables and other constructs
* Provides a system whereby "migration scripts" may be constructed;
each script indicates a particular series of steps that can "upgrade" a
target database to a new version, and optionally a series of steps that can
"downgrade" similarly, doing the same steps in reverse.
* Allows the scripts to execute in some sequential manner.
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