The Migration Context
The MigrationContext handles the actual work to be performed against a database backend as migration operations proceed. It is generally not exposed to the end-user, except when the on_version_apply callback hook is used.
class alembic.runtime.migration.MigrationContext(dialect: Dialect, connection: Optional[Connection], opts: Dict[str, Any], environment_context: Optional[EnvironmentContext] = None)
Represent the database state made available to a migration script.
MigrationContext is the front end to an actual database connection, or alternatively a string output stream given a particular database dialect, from an Alembic perspective.
When inside the env.py script, the MigrationContext is available via the EnvironmentContext.get_context() method, which is available at alembic.context:
# from within env.py script
from alembic import context
migration_context = context.get_context()
For usage outside of an env.py script, such as for utility routines that want to check the current version in the database, the MigrationContext.configure() method to create new MigrationContext objects. For example, to get at the current revision in the database using MigrationContext.get_current_revision():
# in any application, outside of an env.py script
from alembic.migration import MigrationContext
from sqlalchemy import create_engine
engine = create_engine("postgresql://mydatabase")
conn = engine.connect()
context = MigrationContext.configure(conn)
current_rev = context.get_current_revision()
The above context can also be used to produce Alembic migration operations with an Operations instance:
# in any application, outside of the normal Alembic environment
from alembic.operations import Operations
op = Operations(context)
op.alter_column("mytable", "somecolumn", nullable=True)