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Edit File: environment.py
from __future__ import annotations from typing import Callable from typing import ContextManager from typing import Dict from typing import List from typing import Optional from typing import overload from typing import TextIO from typing import Tuple from typing import TYPE_CHECKING from typing import Union from .migration import MigrationContext from .. import util from ..operations import Operations if TYPE_CHECKING: from typing import Literal from sqlalchemy.engine.base import Connection from sqlalchemy.sql.schema import MetaData from .migration import _ProxyTransaction from ..config import Config from ..script.base import ScriptDirectory _RevNumber = Optional[Union[str, Tuple[str, ...]]] class EnvironmentContext(util.ModuleClsProxy): """A configurational facade made available in an ``env.py`` script. The :class:`.EnvironmentContext` acts as a *facade* to the more nuts-and-bolts objects of :class:`.MigrationContext` as well as certain aspects of :class:`.Config`, within the context of the ``env.py`` script that is invoked by most Alembic commands. :class:`.EnvironmentContext` is normally instantiated when a command in :mod:`alembic.command` is run. It then makes itself available in the ``alembic.context`` module for the scope of the command. From within an ``env.py`` script, the current :class:`.EnvironmentContext` is available by importing this module. :class:`.EnvironmentContext` also supports programmatic usage. At this level, it acts as a Python context manager, that is, is intended to be used using the ``with:`` statement. A typical use of :class:`.EnvironmentContext`:: from alembic.config import Config from alembic.script import ScriptDirectory config = Config() config.set_main_option("script_location", "myapp:migrations") script = ScriptDirectory.from_config(config) def my_function(rev, context): '''do something with revision "rev", which will be the current database revision, and "context", which is the MigrationContext that the env.py will create''' with EnvironmentContext( config, script, fn = my_function, as_sql = False, starting_rev = 'base', destination_rev = 'head', tag = "sometag" ): script.run_env() The above script will invoke the ``env.py`` script within the migration environment. If and when ``env.py`` calls :meth:`.MigrationContext.run_migrations`, the ``my_function()`` function above will be called by the :class:`.MigrationContext`, given the context itself as well as the current revision in the database. .. note:: For most API usages other than full blown invocation of migration scripts, the :class:`.MigrationContext` and :class:`.ScriptDirectory` objects can be created and used directly. The :class:`.EnvironmentContext` object is *only* needed when you need to actually invoke the ``env.py`` module present in the migration environment. """ _migration_context: Optional["MigrationContext"] = None config: "Config" = None # type:ignore[assignment] """An instance of :class:`.Config` representing the configuration file contents as well as other variables set programmatically within it.""" script: "ScriptDirectory" = None # type:ignore[assignment] """An instance of :class:`.ScriptDirectory` which provides programmatic access to version files within the ``versions/`` directory. """ def __init__( self, config: "Config", script: "ScriptDirectory", **kw ) -> None: r"""Construct a new :class:`.EnvironmentContext`. :param config: a :class:`.Config` instance. :param script: a :class:`.ScriptDirectory` instance. :param \**kw: keyword options that will be ultimately passed along to the :class:`.MigrationContext` when :meth:`.EnvironmentContext.configure` is called. """ self.config = config self.script = script self.context_opts = kw def __enter__(self) -> "EnvironmentContext": """Establish a context which provides a :class:`.EnvironmentContext` object to env.py scripts. The :class:`.EnvironmentContext` will be made available as ``from alembic import context``. """ self._install_proxy() return self def __exit__(self, *arg, **kw) -> None: self._remove_proxy() def is_offline_mode(self) -> bool: """Return True if the current migrations environment is running in "offline mode". This is ``True`` or ``False`` depending on the ``--sql`` flag passed. This function does not require that the :class:`.MigrationContext` has been configured. """ return self.context_opts.get("as_sql", False) def is_transactional_ddl(self): """Return True if the context is configured to expect a transactional DDL capable backend. This defaults to the type of database in use, and can be overridden by the ``transactional_ddl`` argument to :meth:`.configure` This function requires that a :class:`.MigrationContext` has first been made available via :meth:`.configure`. """ return self.get_context().impl.transactional_ddl def requires_connection(self) -> bool: return not self.is_offline_mode() def get_head_revision(self) -> _RevNumber: """Return the hex identifier of the 'head' script revision. If the script directory has multiple heads, this method raises a :class:`.CommandError`; :meth:`.EnvironmentContext.get_head_revisions` should be preferred. This function does not require that the :class:`.MigrationContext` has been configured. .. seealso:: :meth:`.EnvironmentContext.get_head_revisions` """ return self.script.as_revision_number("head") def get_head_revisions(self) -> _RevNumber: """Return the hex identifier of the 'heads' script revision(s). This returns a tuple containing the version number of all heads in the script directory. This function does not require that the :class:`.MigrationContext` has been configured. """ return self.script.as_revision_number("heads") def get_starting_revision_argument(self) -> _RevNumber: """Return the 'starting revision' argument, if the revision was passed using ``start:end``. This is only meaningful in "offline" mode. Returns ``None`` if no value is available or was configured. This function does not require that the :class:`.MigrationContext` has been configured. """ if self._migration_context is not None: return self.script.as_revision_number( self.get_context()._start_from_rev ) elif "starting_rev" in self.context_opts: return self.script.as_revision_number( self.context_opts["starting_rev"] ) else: # this should raise only in the case that a command # is being run where the "starting rev" is never applicable; # this is to catch scripts which rely upon this in # non-sql mode or similar raise util.CommandError( "No starting revision argument is available." ) def get_revision_argument(self) -> _RevNumber: """Get the 'destination' revision argument. This is typically the argument passed to the ``upgrade`` or ``downgrade`` command. If it was specified as ``head``, the actual version number is returned; if specified as ``base``, ``None`` is returned. This function does not require that the :class:`.MigrationContext` has been configured. """ return self.script.as_revision_number( self.context_opts["destination_rev"] ) def get_tag_argument(self) -> Optional[str]: """Return the value passed for the ``--tag`` argument, if any. The ``--tag`` argument is not used directly by Alembic, but is available for custom ``env.py`` configurations that wish to use it; particularly for offline generation scripts that wish to generate tagged filenames. This function does not require that the :class:`.MigrationContext` has been configured. .. seealso:: :meth:`.EnvironmentContext.get_x_argument` - a newer and more open ended system of extending ``env.py`` scripts via the command line. """ return self.context_opts.get("tag", None) @overload def get_x_argument( # type:ignore[misc] self, as_dictionary: "Literal[False]" = ... ) -> List[str]: ... @overload def get_x_argument( # type:ignore[misc] self, as_dictionary: "Literal[True]" = ... ) -> Dict[str, str]: ... def get_x_argument(self, as_dictionary: bool = False): """Return the value(s) passed for the ``-x`` argument, if any. The ``-x`` argument is an open ended flag that allows any user-defined value or values to be passed on the command line, then available here for consumption by a custom ``env.py`` script. The return value is a list, returned directly from the ``argparse`` structure. If ``as_dictionary=True`` is passed, the ``x`` arguments are parsed using ``key=value`` format into a dictionary that is then returned. For example, to support passing a database URL on the command line, the standard ``env.py`` script can be modified like this:: cmd_line_url = context.get_x_argument( as_dictionary=True).get('dbname') if cmd_line_url: engine = create_engine(cmd_line_url) else: engine = engine_from_config( config.get_section(config.config_ini_section), prefix='sqlalchemy.', poolclass=pool.NullPool) This then takes effect by running the ``alembic`` script as:: alembic -x dbname=postgresql://user:pass@host/dbname upgrade head This function does not require that the :class:`.MigrationContext` has been configured. .. seealso:: :meth:`.EnvironmentContext.get_tag_argument` :attr:`.Config.cmd_opts` """ if self.config.cmd_opts is not None: value = self.config.cmd_opts.x or [] else: value = [] if as_dictionary: value = dict(arg.split("=", 1) for arg in value) return value def configure( self, connection: Optional["Connection"] = None, url: Optional[str] = None, dialect_name: Optional[str] = None, dialect_opts: Optional[dict] = None, transactional_ddl: Optional[bool] = None, transaction_per_migration: bool = False, output_buffer: Optional[TextIO] = None, starting_rev: Optional[str] = None, tag: Optional[str] = None, template_args: Optional[dict] = None, render_as_batch: bool = False, target_metadata: Optional["MetaData"] = None, include_name: Optional[Callable] = None, include_object: Optional[Callable] = None, include_schemas: bool = False, process_revision_directives: Optional[Callable] = None, compare_type: bool = False, compare_server_default: bool = False, render_item: Optional[Callable] = None, literal_binds: bool = False, upgrade_token: str = "upgrades", downgrade_token: str = "downgrades", alembic_module_prefix: str = "op.", sqlalchemy_module_prefix: str = "sa.", user_module_prefix: Optional[str] = None, on_version_apply: Optional[Callable] = None, **kw, ) -> None: """Configure a :class:`.MigrationContext` within this :class:`.EnvironmentContext` which will provide database connectivity and other configuration to a series of migration scripts. Many methods on :class:`.EnvironmentContext` require that this method has been called in order to function, as they ultimately need to have database access or at least access to the dialect in use. Those which do are documented as such. The important thing needed by :meth:`.configure` is a means to determine what kind of database dialect is in use. An actual connection to that database is needed only if the :class:`.MigrationContext` is to be used in "online" mode. If the :meth:`.is_offline_mode` function returns ``True``, then no connection is needed here. Otherwise, the ``connection`` parameter should be present as an instance of :class:`sqlalchemy.engine.Connection`. This function is typically called from the ``env.py`` script within a migration environment. It can be called multiple times for an invocation. The most recent :class:`~sqlalchemy.engine.Connection` for which it was called is the one that will be operated upon by the next call to :meth:`.run_migrations`. General parameters: :param connection: a :class:`~sqlalchemy.engine.Connection` to use for SQL execution in "online" mode. When present, is also used to determine the type of dialect in use. :param url: a string database url, or a :class:`sqlalchemy.engine.url.URL` object. The type of dialect to be used will be derived from this if ``connection`` is not passed. :param dialect_name: string name of a dialect, such as "postgresql", "mssql", etc. The type of dialect to be used will be derived from this if ``connection`` and ``url`` are not passed. :param dialect_opts: dictionary of options to be passed to dialect constructor. .. versionadded:: 1.0.12 :param transactional_ddl: Force the usage of "transactional" DDL on or off; this otherwise defaults to whether or not the dialect in use supports it. :param transaction_per_migration: if True, nest each migration script in a transaction rather than the full series of migrations to run. :param output_buffer: a file-like object that will be used for textual output when the ``--sql`` option is used to generate SQL scripts. Defaults to ``sys.stdout`` if not passed here and also not present on the :class:`.Config` object. The value here overrides that of the :class:`.Config` object. :param output_encoding: when using ``--sql`` to generate SQL scripts, apply this encoding to the string output. :param literal_binds: when using ``--sql`` to generate SQL scripts, pass through the ``literal_binds`` flag to the compiler so that any literal values that would ordinarily be bound parameters are converted to plain strings. .. warning:: Dialects can typically only handle simple datatypes like strings and numbers for auto-literal generation. Datatypes like dates, intervals, and others may still require manual formatting, typically using :meth:`.Operations.inline_literal`. .. note:: the ``literal_binds`` flag is ignored on SQLAlchemy versions prior to 0.8 where this feature is not supported. .. seealso:: :meth:`.Operations.inline_literal` :param starting_rev: Override the "starting revision" argument when using ``--sql`` mode. :param tag: a string tag for usage by custom ``env.py`` scripts. Set via the ``--tag`` option, can be overridden here. :param template_args: dictionary of template arguments which will be added to the template argument environment when running the "revision" command. Note that the script environment is only run within the "revision" command if the --autogenerate option is used, or if the option "revision_environment=true" is present in the alembic.ini file. :param version_table: The name of the Alembic version table. The default is ``'alembic_version'``. :param version_table_schema: Optional schema to place version table within. :param version_table_pk: boolean, whether the Alembic version table should use a primary key constraint for the "value" column; this only takes effect when the table is first created. Defaults to True; setting to False should not be necessary and is here for backwards compatibility reasons. :param on_version_apply: a callable or collection of callables to be run for each migration step. The callables will be run in the order they are given, once for each migration step, after the respective operation has been applied but before its transaction is finalized. Each callable accepts no positional arguments and the following keyword arguments: * ``ctx``: the :class:`.MigrationContext` running the migration, * ``step``: a :class:`.MigrationInfo` representing the step currently being applied, * ``heads``: a collection of version strings representing the current heads, * ``run_args``: the ``**kwargs`` passed to :meth:`.run_migrations`. Parameters specific to the autogenerate feature, when ``alembic revision`` is run with the ``--autogenerate`` feature: :param target_metadata: a :class:`sqlalchemy.schema.MetaData` object, or a sequence of :class:`~sqlalchemy.schema.MetaData` objects, that will be consulted during autogeneration. The tables present in each :class:`~sqlalchemy.schema.MetaData` will be compared against what is locally available on the target :class:`~sqlalchemy.engine.Connection` to produce candidate upgrade/downgrade operations. :param compare_type: Indicates type comparison behavior during an autogenerate operation. Defaults to ``False`` which disables type comparison. Set to ``True`` to turn on default type comparison, which has varied accuracy depending on backend. See :ref:`compare_types` for an example as well as information on other type comparison options. .. seealso:: :ref:`compare_types` :paramref:`.EnvironmentContext.configure.compare_server_default` :param compare_server_default: Indicates server default comparison behavior during an autogenerate operation. Defaults to ``False`` which disables server default comparison. Set to ``True`` to turn on server default comparison, which has varied accuracy depending on backend. To customize server default comparison behavior, a callable may be specified which can filter server default comparisons during an autogenerate operation. defaults during an autogenerate operation. The format of this callable is:: def my_compare_server_default(context, inspected_column, metadata_column, inspected_default, metadata_default, rendered_metadata_default): # return True if the defaults are different, # False if not, or None to allow the default implementation # to compare these defaults return None context.configure( # ... compare_server_default = my_compare_server_default ) ``inspected_column`` is a dictionary structure as returned by :meth:`sqlalchemy.engine.reflection.Inspector.get_columns`, whereas ``metadata_column`` is a :class:`sqlalchemy.schema.Column` from the local model environment. A return value of ``None`` indicates to allow default server default comparison to proceed. Note that some backends such as Postgresql actually execute the two defaults on the database side to compare for equivalence. .. seealso:: :paramref:`.EnvironmentContext.configure.compare_type` :param include_name: A callable function which is given the chance to return ``True`` or ``False`` for any database reflected object based on its name, including database schema names when the :paramref:`.EnvironmentContext.configure.include_schemas` flag is set to ``True``. The function accepts the following positional arguments: * ``name``: the name of the object, such as schema name or table name. Will be ``None`` when indicating the default schema name of the database connection. * ``type``: a string describing the type of object; currently ``"schema"``, ``"table"``, ``"column"``, ``"index"``, ``"unique_constraint"``, or ``"foreign_key_constraint"`` * ``parent_names``: a dictionary of "parent" object names, that are relative to the name being given. Keys in this dictionary may include: ``"schema_name"``, ``"table_name"``. E.g.:: def include_name(name, type_, parent_names): if type_ == "schema": return name in ["schema_one", "schema_two"] else: return True context.configure( # ... include_schemas = True, include_name = include_name ) .. versionadded:: 1.5 .. seealso:: :ref:`autogenerate_include_hooks` :paramref:`.EnvironmentContext.configure.include_object` :paramref:`.EnvironmentContext.configure.include_schemas` :param include_object: A callable function which is given the chance to return ``True`` or ``False`` for any object, indicating if the given object should be considered in the autogenerate sweep. The function accepts the following positional arguments: * ``object``: a :class:`~sqlalchemy.schema.SchemaItem` object such as a :class:`~sqlalchemy.schema.Table`, :class:`~sqlalchemy.schema.Column`, :class:`~sqlalchemy.schema.Index` :class:`~sqlalchemy.schema.UniqueConstraint`, or :class:`~sqlalchemy.schema.ForeignKeyConstraint` object * ``name``: the name of the object. This is typically available via ``object.name``. * ``type``: a string describing the type of object; currently ``"table"``, ``"column"``, ``"index"``, ``"unique_constraint"``, or ``"foreign_key_constraint"`` * ``reflected``: ``True`` if the given object was produced based on table reflection, ``False`` if it's from a local :class:`.MetaData` object. * ``compare_to``: the object being compared against, if available, else ``None``. E.g.:: def include_object(object, name, type_, reflected, compare_to): if (type_ == "column" and not reflected and object.info.get("skip_autogenerate", False)): return False else: return True context.configure( # ... include_object = include_object ) For the use case of omitting specific schemas from a target database when :paramref:`.EnvironmentContext.configure.include_schemas` is set to ``True``, the :attr:`~sqlalchemy.schema.Table.schema` attribute can be checked for each :class:`~sqlalchemy.schema.Table` object passed to the hook, however it is much more efficient to filter on schemas before reflection of objects takes place using the :paramref:`.EnvironmentContext.configure.include_name` hook. .. seealso:: :ref:`autogenerate_include_hooks` :paramref:`.EnvironmentContext.configure.include_name` :paramref:`.EnvironmentContext.configure.include_schemas` :param render_as_batch: if True, commands which alter elements within a table will be placed under a ``with batch_alter_table():`` directive, so that batch migrations will take place. .. seealso:: :ref:`batch_migrations` :param include_schemas: If True, autogenerate will scan across all schemas located by the SQLAlchemy :meth:`~sqlalchemy.engine.reflection.Inspector.get_schema_names` method, and include all differences in tables found across all those schemas. When using this option, you may want to also use the :paramref:`.EnvironmentContext.configure.include_name` parameter to specify a callable which can filter the tables/schemas that get included. .. seealso:: :ref:`autogenerate_include_hooks` :paramref:`.EnvironmentContext.configure.include_name` :paramref:`.EnvironmentContext.configure.include_object` :param render_item: Callable that can be used to override how any schema item, i.e. column, constraint, type, etc., is rendered for autogenerate. The callable receives a string describing the type of object, the object, and the autogen context. If it returns False, the default rendering method will be used. If it returns None, the item will not be rendered in the context of a Table construct, that is, can be used to skip columns or constraints within op.create_table():: def my_render_column(type_, col, autogen_context): if type_ == "column" and isinstance(col, MySpecialCol): return repr(col) else: return False context.configure( # ... render_item = my_render_column ) Available values for the type string include: ``"column"``, ``"primary_key"``, ``"foreign_key"``, ``"unique"``, ``"check"``, ``"type"``, ``"server_default"``. .. seealso:: :ref:`autogen_render_types` :param upgrade_token: When autogenerate completes, the text of the candidate upgrade operations will be present in this template variable when ``script.py.mako`` is rendered. Defaults to ``upgrades``. :param downgrade_token: When autogenerate completes, the text of the candidate downgrade operations will be present in this template variable when ``script.py.mako`` is rendered. Defaults to ``downgrades``. :param alembic_module_prefix: When autogenerate refers to Alembic :mod:`alembic.operations` constructs, this prefix will be used (i.e. ``op.create_table``) Defaults to "``op.``". Can be ``None`` to indicate no prefix. :param sqlalchemy_module_prefix: When autogenerate refers to SQLAlchemy :class:`~sqlalchemy.schema.Column` or type classes, this prefix will be used (i.e. ``sa.Column("somename", sa.Integer)``) Defaults to "``sa.``". Can be ``None`` to indicate no prefix. Note that when dialect-specific types are rendered, autogenerate will render them using the dialect module name, i.e. ``mssql.BIT()``, ``postgresql.UUID()``. :param user_module_prefix: When autogenerate refers to a SQLAlchemy type (e.g. :class:`.TypeEngine`) where the module name is not under the ``sqlalchemy`` namespace, this prefix will be used within autogenerate. If left at its default of ``None``, the ``__module__`` attribute of the type is used to render the import module. It's a good practice to set this and to have all custom types be available from a fixed module space, in order to future-proof migration files against reorganizations in modules. .. seealso:: :ref:`autogen_module_prefix` :param process_revision_directives: a callable function that will be passed a structure representing the end result of an autogenerate or plain "revision" operation, which can be manipulated to affect how the ``alembic revision`` command ultimately outputs new revision scripts. The structure of the callable is:: def process_revision_directives(context, revision, directives): pass The ``directives`` parameter is a Python list containing a single :class:`.MigrationScript` directive, which represents the revision file to be generated. This list as well as its contents may be freely modified to produce any set of commands. The section :ref:`customizing_revision` shows an example of doing this. The ``context`` parameter is the :class:`.MigrationContext` in use, and ``revision`` is a tuple of revision identifiers representing the current revision of the database. The callable is invoked at all times when the ``--autogenerate`` option is passed to ``alembic revision``. If ``--autogenerate`` is not passed, the callable is invoked only if the ``revision_environment`` variable is set to True in the Alembic configuration, in which case the given ``directives`` collection will contain empty :class:`.UpgradeOps` and :class:`.DowngradeOps` collections for ``.upgrade_ops`` and ``.downgrade_ops``. The ``--autogenerate`` option itself can be inferred by inspecting ``context.config.cmd_opts.autogenerate``. The callable function may optionally be an instance of a :class:`.Rewriter` object. This is a helper object that assists in the production of autogenerate-stream rewriter functions. .. seealso:: :ref:`customizing_revision` :ref:`autogen_rewriter` :paramref:`.command.revision.process_revision_directives` Parameters specific to individual backends: :param mssql_batch_separator: The "batch separator" which will be placed between each statement when generating offline SQL Server migrations. Defaults to ``GO``. Note this is in addition to the customary semicolon ``;`` at the end of each statement; SQL Server considers the "batch separator" to denote the end of an individual statement execution, and cannot group certain dependent operations in one step. :param oracle_batch_separator: The "batch separator" which will be placed between each statement when generating offline Oracle migrations. Defaults to ``/``. Oracle doesn't add a semicolon between statements like most other backends. """ opts = self.context_opts if transactional_ddl is not None: opts["transactional_ddl"] = transactional_ddl if output_buffer is not None: opts["output_buffer"] = output_buffer elif self.config.output_buffer is not None: opts["output_buffer"] = self.config.output_buffer if starting_rev: opts["starting_rev"] = starting_rev if tag: opts["tag"] = tag if template_args and "template_args" in opts: opts["template_args"].update(template_args) opts["transaction_per_migration"] = transaction_per_migration opts["target_metadata"] = target_metadata opts["include_name"] = include_name opts["include_object"] = include_object opts["include_schemas"] = include_schemas opts["render_as_batch"] = render_as_batch opts["upgrade_token"] = upgrade_token opts["downgrade_token"] = downgrade_token opts["sqlalchemy_module_prefix"] = sqlalchemy_module_prefix opts["alembic_module_prefix"] = alembic_module_prefix opts["user_module_prefix"] = user_module_prefix opts["literal_binds"] = literal_binds opts["process_revision_directives"] = process_revision_directives opts["on_version_apply"] = util.to_tuple(on_version_apply, default=()) if render_item is not None: opts["render_item"] = render_item if compare_type is not None: opts["compare_type"] = compare_type if compare_server_default is not None: opts["compare_server_default"] = compare_server_default opts["script"] = self.script opts.update(kw) self._migration_context = MigrationContext.configure( connection=connection, url=url, dialect_name=dialect_name, environment_context=self, dialect_opts=dialect_opts, opts=opts, ) def run_migrations(self, **kw) -> None: """Run migrations as determined by the current command line configuration as well as versioning information present (or not) in the current database connection (if one is present). The function accepts optional ``**kw`` arguments. If these are passed, they are sent directly to the ``upgrade()`` and ``downgrade()`` functions within each target revision file. By modifying the ``script.py.mako`` file so that the ``upgrade()`` and ``downgrade()`` functions accept arguments, parameters can be passed here so that contextual information, usually information to identify a particular database in use, can be passed from a custom ``env.py`` script to the migration functions. This function requires that a :class:`.MigrationContext` has first been made available via :meth:`.configure`. """ assert self._migration_context is not None with Operations.context(self._migration_context): self.get_context().run_migrations(**kw) def execute(self, sql, execution_options=None): """Execute the given SQL using the current change context. The behavior of :meth:`.execute` is the same as that of :meth:`.Operations.execute`. Please see that function's documentation for full detail including caveats and limitations. This function requires that a :class:`.MigrationContext` has first been made available via :meth:`.configure`. """ self.get_context().execute(sql, execution_options=execution_options) def static_output(self, text): """Emit text directly to the "offline" SQL stream. Typically this is for emitting comments that start with --. The statement is not treated as a SQL execution, no ; or batch separator is added, etc. """ self.get_context().impl.static_output(text) def begin_transaction( self, ) -> Union["_ProxyTransaction", ContextManager]: """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() :meth:`.begin_transaction` is intended to "do the right thing" regardless of calling context: * If :meth:`.is_transactional_ddl` is ``False``, returns a "do nothing" context manager which otherwise produces no transactional state or directives. * If :meth:`.is_offline_mode` is ``True``, returns a context manager that will invoke the :meth:`.DefaultImpl.emit_begin` and :meth:`.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 :meth:`sqlalchemy.engine.Connection.begin` on the current online connection, which returns a :class:`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 :class:`~sqlalchemy.engine.Connection` directly to produce transactional state in "online" mode. """ return self.get_context().begin_transaction() def get_context(self) -> "MigrationContext": """Return the current :class:`.MigrationContext` object. If :meth:`.EnvironmentContext.configure` has not been called yet, raises an exception. """ if self._migration_context is None: raise Exception("No context has been configured yet.") return self._migration_context def get_bind(self): """Return the current 'bind'. In "online" mode, this is the :class:`sqlalchemy.engine.Connection` currently being used to emit SQL to the database. This function requires that a :class:`.MigrationContext` has first been made available via :meth:`.configure`. """ return self.get_context().bind def get_impl(self): return self.get_context().impl