begin_transaction

begin_transaction() → Union[_ProxyTransaction, AbstractContextManager]

Return a context manager that will enclose an operation within a “transaction”, as defined by the environment’s offline and transactional DDL settings.

e.g.:

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

begin_transaction() is intended to “do the right thing” regardless of calling context:

• If is_transactional_ddl() is False, returns a “do nothing” context manager which otherwise produces no transactional state or directives.
• If is_offline_mode() is True, returns a context manager that will invoke the DefaultImpl.emit_begin() and DefaultImpl.emit_commit() methods, which will produce the string directives BEGIN and COMMIT on the output stream, as rendered by the target backend (e.g. SQL Server would emit BEGIN TRANSACTION).
• Otherwise, calls sqlalchemy.engine.Connection.begin() on the current online connection, which returns a sqlalchemy.engine.Transaction object. This object demarcates a real transaction and is itself a context manager, which will roll back if an exception is raised.

Note: that a custom env.py script which has more specific transactional needs can of course manipulate the Connection directly to produce transactional state in “online” mode.