# Runtime Objects

The “runtime” of Alembic involves the `EnvironmentContext` and `MigrationContext` objects. These are the objects that are in play once the `env.py` script is loaded up by a command and a migration operation proceeds.

## The Environment Context

The `EnvironmentContext` class provides most of the API used within an `env.py` script. Within `env.py`, the instantated `EnvironmentContext` is made available via a special proxy module called `alembic.context`. That is, you can import `alembic.context` like a regular Python module, and each name you call upon it is ultimately routed towards the current `EnvironmentContext` in use.

In particular, the key method used within `env.py` is `EnvironmentContext.configure()`, which establishes all the details about how the database will be accessed.

class `alembic.runtime.environment.``EnvironmentContext`(config, script, **kw)

  A configurational facade made available in an `env.py` script.

  The `EnvironmentContext` acts as a facade to the more nuts-and-bolts objects of `MigrationContext` as well as certain aspects of `Config`, within the context of the `env.py` script that is invoked by most Alembic commands.

  `EnvironmentContext` is normally instantiated when a command in `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 `EnvironmentContext` is available by importing this module.

  `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 `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 `MigrationContext.run_migrations()`, the `my_function()` function above will be called by the `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 `MigrationContext` and `ScriptDirectory` objects can be created and used directly. The `EnvironmentContext` object is only needed when you need to actually invoke the `env.py` module present in the migration environment.

  Construct a new `EnvironmentContext`.

  Parameters: | config – a `Config` instance. script – a `ScriptDirectory` instance. **kw – keyword options that will be ultimately passed along to the `MigrationContext` when `EnvironmentContext.configure()` is called.

  `begin_transaction`()

    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.

  `config` = None

    An instance of `Config` representing the configuration file contents as well as other variables set programmatically within it.

  `configure`(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 `MigrationContext` within this `EnvironmentContext` which will provide database connectivity and other configuration to a series of migration scripts.

    Many methods on `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 `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 `MigrationContext` is to be used in “online” mode.

    If the `is_offline_mode()` function returns `True`, then no connection is needed here. Otherwise, the `connection` parameter should be present as an instance of `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 `Connection` for which it was called is the one that will be operated upon by the next call to `run_migrations()`.

    General parameters:

    Parameters: | connection – a `Connection` to use for SQL execution in “online” mode. When present, is also used to determine the type of dialect in use. url – a string database url, or a `sqlalchemy.engine.url.URL` object. The type of dialect to be used will be derived from this if `connection` is not passed. 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. transactional_ddl – Force the usage of “transactional” DDL on or off; this otherwise defaults to whether or not the dialect in use supports it. transaction_per_migration – if True, nest each migration script in a transaction rather than the full series of migrations to run. New in version 0.6.5. 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 `Config` object. The value here overrides that of the `Config` object. output_encoding – when using `--sql` to generate SQL scripts, apply this encoding to the string output. 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 `Operations.inline_literal()`. Note the `literal_binds` flag is ignored on SQLAlchemy versions prior to 0.8 where this feature is not supported. New in version 0.7.6. See also `Operations.inline_literal()` starting_rev – Override the “starting revision” argument when using `--sql` mode. tag – a string tag for usage by custom `env.py` scripts. Set via the `--tag` option, can be overridden here. 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. version_table – The name of the Alembic version table. The default is `'alembic_version'`. 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:

    - Parameters: | target_metadata – a `sqlalchemy.schema.MetaData` object that will be consulted during autogeneration. The tables present will be compared against what is locally available on the target `Connection` to produce candidate upgrade/downgrade operations. 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 Comparing Types for an example as well as information on other type comparison options. See also Comparing Types `EnvironmentContext.configure.compare_server_default` 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 `sqlalchemy.engine.reflection.Inspector.get_columns()`, whereas `metadata_column` is a `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. See also `EnvironmentContext.configure.compare_type` 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 `SchemaItem` object such as a `Table`, `Column`, `Index` `UniqueConstraint`, or `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"` New in version 0.7.0: Support for indexes and unique constraints within the `include_object` hook. New in version 0.7.1: Support for foreign keys within the `include_object` hook. `reflected`: `True` if the given object was produced based on table reflection, `False` if it’s from a local `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
          )
    - `EnvironmentContext.configure.include_object` can also be used to filter on specific schemas to include or omit, when the `EnvironmentContext.configure.include_schemas` flag is set to `True`. The `Table.schema` attribute on each `Table` object reflected will indicate the name of the schema from which the `Table` originates. New in version 0.6.0. See also `EnvironmentContext.configure.include_schemas` 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 since version 0.6.0: `EnvironmentContext.configure.include_symbol` is superceded by the more generic `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
          )
    - See also `EnvironmentContext.configure.include_schemas` `EnvironmentContext.configure.include_object` 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. New in version 0.7.0. See also Running “Batch” Migrations for SQLite and Other Databases include_schemas – If True, autogenerate will scan across all schemas located by the SQLAlchemy `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 `EnvironmentContext.configure.include_object` option to specify a callable which can filter the tables/schemas that get included. See also `EnvironmentContext.configure.include_object` 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"`. See also Affecting the Rendering of Types Themselves 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`. 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`. alembic_module_prefix – When autogenerate refers to Alembic `alembic.operations` constructs, this prefix will be used (i.e. `op.create_table`) Defaults to “`op.`”. Can be `None` to indicate no prefix. sqlalchemy_module_prefix – When autogenerate refers to SQLAlchemy `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()`. user_module_prefix – When autogenerate refers to a SQLAlchemy type (e.g. `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. Changed in version 0.7.0: `EnvironmentContext.configure.user_module_prefix` no longer defaults to the value of `EnvironmentContext.configure.sqlalchemy_module_prefix` when left at `None`; the `__module__` attribute is now used. New in version 0.6.3: added `EnvironmentContext.configure.user_module_prefix` See also Controlling the Module Prefix 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 `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 Customizing Revision Generation shows an example of doing this. The `context` parameter is the `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 `UpgradeOps` and `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 `Rewriter` object. This is a helper object that assists in the production of autogenerate-stream rewriter functions. New in version 0.8.0. Changed in version 0.8.1: - The `EnvironmentContext.configure.process_revision_directives` hook can append op directives into `UpgradeOps` and `DowngradeOps` which will be rendered in Python regardless of whether the `--autogenerate` option is in use or not; the `revision_environment` configuration variable should be set to “true” in the config to enable this. See also Customizing Revision Generation Fine-Grained Autogenerate Generation with Rewriters

    Parameters specific to individual backends:

    Parameters: | 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. 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.

  `execute`(sql, execution_options=None)

    Execute the given SQL using the current change context.

    The behavior of `execute()` is the same as that of `Operations.execute()`. Please see that function’s documentation for full detail including caveats and limitations.

    This function requires that a `MigrationContext` has first been made available via `configure()`.

  `get_bind`()

    Return the current ‘bind’.

    In “online” mode, this is the `sqlalchemy.engine.Connection` currently being used to emit SQL to the database.

    This function requires that a `MigrationContext` has first been made available via `configure()`.

  `get_context`()

    Return the current `MigrationContext` object.

    If `EnvironmentContext.configure()` has not been called yet, raises an exception.

  `get_head_revision`()

    Return the hex identifier of the ‘head’ script revision.

    If the script directory has multiple heads, this method raises a `CommandError`; `EnvironmentContext.get_head_revisions()` should be preferred.

    This function does not require that the `MigrationContext` has been configured.

    See also

    `EnvironmentContext.get_head_revisions()`

  `get_head_revisions`()

    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 `MigrationContext` has been configured.

    New in version 0.7.0.

  `get_revision_argument`()

    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 `MigrationContext` has been configured.

  `get_starting_revision_argument`()

    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 `MigrationContext` has been configured.

  `get_tag_argument`()

    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 `MigrationContext` has been configured.

    See also

    `EnvironmentContext.get_x_argument()` - a newer and more open ended system of extending `env.py` scripts via the command line.

  `get_x_argument`(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 `MigrationContext` has been configured.

    New in version 0.6.0.

    See also

    `EnvironmentContext.get_tag_argument()`

    `Config.cmd_opts`

  `is_offline_mode`()

    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 `MigrationContext` has been configured.

  `is_transactional_ddl`()

    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 `configure()`

    This function requires that a `MigrationContext` has first been made available via `configure()`.

  `run_migrations`(**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 `MigrationContext` has first been made available via `configure()`.

  `script` = None

    An instance of `ScriptDirectory` which provides programmatic access to version files within the `versions/` directory.

  `static_output`(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.

## 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.

class `alembic.runtime.migration.``MigrationContext`(dialect, connection, opts, environment_context=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)

  `bind`

    Return the current “bind”.

    In online mode, this is an instance of `sqlalchemy.engine.Connection`, and is suitable for ad-hoc execution of any kind of usage described in SQL Expression Language Tutorial as well as for usage with the `sqlalchemy.schema.Table.create()` and `sqlalchemy.schema.MetaData.create_all()` methods of `Table`, `MetaData`.

    Note that when “standard output” mode is enabled, this bind will be a “mock” connection handler that cannot return results and is only appropriate for a very limited subset of commands.

  `config`

    Return the `Config` used by the current environment, if any.

    New in version 0.6.6.

  classmethod `configure`(connection=None, url=None, dialect_name=None, dialect=None, environment_context=None, opts=None)

    Create a new `MigrationContext`.

    This is a factory method usually called by `EnvironmentContext.configure()`.

    Parameters: | connection – a `Connection` to use for SQL execution in “online” mode. When present, is also used to determine the type of dialect in use. url – a string database url, or a `sqlalchemy.engine.url.URL` object. The type of dialect to be used will be derived from this if `connection` is not passed. 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. opts – dictionary of options. Most other options accepted by `EnvironmentContext.configure()` are passed via this dictionary.

  `execute`(sql, execution_options=None)

    Execute a SQL construct or string statement.

    The underlying execution mechanics are used, that is if this is “offline mode” the SQL is written to the output buffer, otherwise the SQL is emitted on the current SQLAlchemy connection.

  `get_current_heads`()

    Return a tuple of the current ‘head versions’ that are represented in the target database.

    For a migration stream without branches, this will be a single value, synonymous with that of `MigrationContext.get_current_revision()`. However when multiple unmerged branches exist within the target database, the returned tuple will contain a value for each head.

    If this `MigrationContext` was configured in “offline” mode, that is with `as_sql=True`, the `starting_rev` parameter is returned in a one-length tuple.

    If no version table is present, or if there are no revisions present, an empty tuple is returned.

    New in version 0.7.0.

  `get_current_revision`()

    Return the current revision, usually that which is present in the `alembic_version` table in the database.

    This method intends to be used only for a migration stream that does not contain unmerged branches in the target database; if there are multiple branches present, an exception is raised. The `MigrationContext.get_current_heads()` should be preferred over this method going forward in order to be compatible with branch migration support.

    If this `MigrationContext` was configured in “offline” mode, that is with `as_sql=True`, the `starting_rev` parameter is returned instead, if any.

  `run_migrations`(**kw)

    Run the migration scripts established for this `MigrationContext`, if any.

    The commands in `alembic.command` will set up a function that is ultimately passed to the `MigrationContext` as the `fn` argument. This function represents the “work” that will be done when `MigrationContext.run_migrations()` is called, typically from within the `env.py` script of the migration environment. The “work function” then provides an iterable of version callables and other version information which in the case of the `upgrade` or `downgrade` commands are the list of version scripts to invoke. Other commands yield nothing, in the case that a command wants to run some other operation against the database such as the `current` or `stamp` commands.

    Parameters: | **kw – keyword arguments here will be passed to each migration callable, that is the `upgrade()` or `downgrade()` method within revision scripts.

  `stamp`(script_directory, revision)

    Stamp the version table with a specific revision.

    This method calculates those branches to which the given revision can apply, and updates those branches as though they were migrated towards that revision (either up or down). If no current branches include the revision, it is added as a new branch head.

    New in version 0.7.0.