begin_transaction

begin_transaction(_per_migration: bool = False) → Union[alembic.runtime.migration._ProxyTransaction, AbstractContextManager]

Begin a logical transaction for migration operations.

This method is used within an env.py script to demarcate where the outer “transaction” for a series of migrations begins. Example:

def run_migrations_online():
    connectable = create_engine(...)

    with connectable.connect() as connection:
        context.configure(
            connection=connection, target_metadata=target_metadata
        )

        with context.begin_transaction():
            context.run_migrations()

Above, MigrationContext.begin_transaction() is used to demarcate where the outer logical transaction occurs around the MigrationContext.run_migrations() operation.

A “Logical” transaction means that the operation may or may not correspond to a real database transaction. If the target database supports transactional DDL (or EnvironmentContext.configure.transactional_ddl is true), the EnvironmentContext.configure.transaction_per_migration flag is not set, and the migration is against a real database connection (as opposed to using “offline” --sql mode), a real transaction will be started. If --sql mode is in effect, the operation would instead correspond to a string such as “BEGIN” being emitted to the string output.

The returned object is a Python context manager that should only be used in the context of a with: statement as indicated above. The object has no other guaranteed API features present.

See also:

• MigrationContext.autocommit_block()