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Diffstat (limited to 'alembic/runtime/environment.py')
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diff --git a/alembic/runtime/environment.py b/alembic/runtime/environment.py new file mode 100644 index 0000000..3b04fea --- /dev/null +++ b/alembic/runtime/environment.py @@ -0,0 +1,875 @@ +from ..operations import Operations +from .migration import MigrationContext +from .. import util + + +class EnvironmentContext(util.ModuleClsProxy): + + """Represent the state made available to an ``env.py`` script. + + :class:`.EnvironmentContext` is normally instantiated + by the commands present in the :mod:`alembic.command` + module. From within an ``env.py`` script, the current + :class:`.EnvironmentContext` is available via the + ``alembic.context`` datamember. + + :class:`.EnvironmentContext` is also 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 = None + + config = None + """An instance of :class:`.Config` representing the + configuration file contents as well as other variables + set programmatically within it.""" + + script = None + """An instance of :class:`.ScriptDirectory` which provides + programmatic access to version files within the ``versions/`` + directory. + + """ + + def __init__(self, config, script, **kw): + """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): + """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): + self._remove_proxy() + + def is_offline_mode(self): + """Return True if the current migrations environment + is running in "offline mode". + + This is ``True`` or ``False`` depending + on the 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): + return not self.is_offline_mode() + + def get_head_revision(self): + """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): + """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. + + .. versionadded:: 0.7.0 + + """ + return self.script.as_revision_number("heads") + + def get_starting_revision_argument(self): + """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): + """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): + """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) + + def get_x_argument(self, as_dictionary=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. + + .. versionadded:: 0.6.0 + + .. 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=None, + url=None, + dialect_name=None, + transactional_ddl=None, + transaction_per_migration=False, + output_buffer=None, + starting_rev=None, + tag=None, + template_args=None, + render_as_batch=False, + target_metadata=None, + include_symbol=None, + include_object=None, + include_schemas=False, + process_revision_directives=None, + compare_type=False, + compare_server_default=False, + render_item=None, + literal_binds=False, + upgrade_token="upgrades", + downgrade_token="downgrades", + alembic_module_prefix="op.", + sqlalchemy_module_prefix="sa.", + user_module_prefix=None, + **kw + ): + """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 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. + + .. versionadded:: 0.6.5 + + :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. + + .. versionadded:: 0.7.6 + + .. 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. + + Parameters specific to the autogenerate feature, when + ``alembic revision`` is run with the ``--autogenerate`` feature: + + :param target_metadata: a :class:`sqlalchemy.schema.MetaData` + object that + will be consulted during autogeneration. The tables present + 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_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"`` + + .. versionadded:: 0.7.0 Support for indexes and unique constraints + within the + :paramref:`~.EnvironmentContext.configure.include_object` hook. + + .. versionadded:: 0.7.1 Support for foreign keys within the + :paramref:`~.EnvironmentContext.configure.include_object` hook. + + * ``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 + ) + + :paramref:`.EnvironmentContext.configure.include_object` can also + be used to filter on specific schemas to include or omit, when + the :paramref:`.EnvironmentContext.configure.include_schemas` + flag is set to ``True``. The :attr:`.Table.schema` attribute + on each :class:`.Table` object reflected will indicate the name of the + schema from which the :class:`.Table` originates. + + .. versionadded:: 0.6.0 + + .. seealso:: + + :paramref:`.EnvironmentContext.configure.include_schemas` + + :param include_symbol: A callable function which, given a table name + and schema name (may be ``None``), returns ``True`` or ``False``, + indicating if the given table should be considered in the + autogenerate sweep. + + .. deprecated:: 0.6.0 + :paramref:`.EnvironmentContext.configure.include_symbol` + is superceded by the more generic + :paramref:`.EnvironmentContext.configure.include_object` + parameter. + + E.g.:: + + def include_symbol(tablename, schema): + return tablename not in ("skip_table_one", "skip_table_two") + + context.configure( + # ... + include_symbol = include_symbol + ) + + .. seealso:: + + :paramref:`.EnvironmentContext.configure.include_schemas` + + :paramref:`.EnvironmentContext.configure.include_object` + + :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. + + .. versionadded:: 0.7.0 + + .. 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_object` + option to specify a callable which + can filter the tables/schemas that get included. + + .. seealso:: + + :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. + + .. versionchanged:: 0.7.0 + :paramref:`.EnvironmentContext.configure.user_module_prefix` + no longer defaults to the value of + :paramref:`.EnvironmentContext.configure.sqlalchemy_module_prefix` + when left at ``None``; the ``__module__`` attribute is now used. + + .. versionadded:: 0.6.3 added + :paramref:`.EnvironmentContext.configure.user_module_prefix` + + .. 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``. + + + .. versionadded:: 0.8.0 + + .. seealso:: + + :ref:`customizing_revision` + + + 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_symbol'] = include_symbol + 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 + + 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, + opts=opts + ) + + def run_migrations(self, **kw): + """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`. + + """ + 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): + """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): + """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 |