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: Config, script: ScriptDirectory, **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:
begin_transaction() → '_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()

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

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: Config = None

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

configure(connection: 'Connection' | None = None, url: str | None = None, dialect_name: str | None = None, dialect_opts: dict | None = None, transactional_ddl: bool | None = None, transaction_per_migration: bool = False, output_buffer: TextIO | None = None, starting_rev: str | None = None, tag: str | None = None, template_args: dict | None = None, render_as_batch: bool = False, target_metadata: 'MetaData' | None = None, include_name: Callable | None = None, include_object: Callable | None = None, include_schemas: bool = False, process_revision_directives: Callable | None = None, compare_type: bool = False, compare_server_default: bool = False, render_item: Callable | None = 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: str | None = None, on_version_apply: Callable | None = None, **kw) → None

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.

  • dialect_opts –

    dictionary of options to be passed to dialect constructor.

    New in version 1.0.12.

  • 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.

  • 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.

    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.

  • 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.

  • 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 MigrationContext running the migration,

    • step: a MigrationInfo representing the step currently being applied,

    • heads: a collection of version strings representing the current heads,

    • run_args: the **kwargs passed to run_migrations().

Parameters specific to the autogenerate feature, when alembic revision is run with the --autogenerate feature:

Parameters:

  • target_metadata – a sqlalchemy.schema.MetaData object, or a sequence of MetaData objects, that will be consulted during autogeneration. The tables present in each MetaData 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_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 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
)

    New in version 1.5.

    See also

    Controlling What to be Autogenerated

    EnvironmentContext.configure.include_object

    EnvironmentContext.configure.include_schemas

  • 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"

    • 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
)

    For the use case of omitting specific schemas from a target database when EnvironmentContext.configure.include_schemas is set to True, the schema attribute can be checked for each Table object passed to the hook, however it is much more efficient to filter on schemas before reflection of objects takes place using the EnvironmentContext.configure.include_name hook.

    See also

    Controlling What to be Autogenerated

    EnvironmentContext.configure.include_name

    EnvironmentContext.configure.include_schemas

  • 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.

    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_name parameter to specify a callable which can filter the tables/schemas that get included.

    See also

    Controlling What to be Autogenerated

    EnvironmentContext.configure.include_name

    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.

    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.

    See also

    Customizing Revision Generation

    Fine-Grained Autogenerate Generation with Rewriters

    command.revision.process_revision_directives

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() → MigrationContext

Return the current MigrationContext object.

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

get_head_revision() → str | Tuple[str, ...] | None

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() → str | Tuple[str, ...] | None

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.

get_revision_argument() → str | Tuple[str, ...] | None

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() → str | Tuple[str, ...] | None

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() → str | None

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: Literal[False] = False) → List[str]
get_x_argument(as_dictionary: Literal[True] = False) → Dict[str, str]

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.

See also

EnvironmentContext.get_tag_argument()

Config.cmd_opts

is_offline_mode() → 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 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) → 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 MigrationContext has first been made available via configure().

script: ScriptDirectory = 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, except when the on_version_apply callback hook is used.

class alembic.runtime.migration.MigrationContext(dialect: Dialect, connection: 'Connection' | None, opts: Dict[str, Any], environment_context: 'EnvironmentContext' | None = 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)
autocommit_block() → Iterator[None]

Enter an “autocommit” block, for databases that support AUTOCOMMIT isolation levels.

This special directive is intended to support the occasional database DDL or system operation that specifically has to be run outside of any kind of transaction block. The PostgreSQL database platform is the most common target for this style of operation, as many of its DDL operations must be run outside of transaction blocks, even though the database overall supports transactional DDL.

The method is used as a context manager within a migration script, by calling on Operations.get_context() to retrieve the MigrationContext, then invoking MigrationContext.autocommit_block() using the with: statement:

def upgrade():
    with op.get_context().autocommit_block():
        op.execute("ALTER TYPE mood ADD VALUE 'soso'")

Above, a PostgreSQL “ALTER TYPE..ADD VALUE” directive is emitted, which must be run outside of a transaction block at the database level. The MigrationContext.autocommit_block() method makes use of the SQLAlchemy AUTOCOMMIT isolation level setting, which against the psycogp2 DBAPI corresponds to the connection.autocommit setting, to ensure that the database driver is not inside of a DBAPI level transaction block.

Warning

As is necessary, the database transaction preceding the block is unconditionally committed. This means that the run of migrations preceding the operation will be committed, before the overall migration operation is complete.

It is recommended that when an application includes migrations with “autocommit” blocks, that EnvironmentContext.transaction_per_migration be used so that the calling environment is tuned to expect short per-file migrations whether or not one of them has an autocommit block.

New in version 1.2.0.

begin_transaction(_per_migration: bool = False) → _ProxyTransaction | ContextManager

Begin a logical transaction for migration operations.

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

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

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

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

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

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

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

See also

MigrationContext.autocommit_block()

property bind: 'Connection' | None

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 sqlexpression_toplevel 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.

property config: 'Config' | None

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

classmethod configure(connection: 'Connection' | None = None, url: str | None = None, dialect_name: str | None = None, dialect: 'Dialect' | None = None, environment_context: 'EnvironmentContext' | None = None, dialect_opts: Dict[str, str] | None = None, opts: Any | None = None) → MigrationContext

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: str, execution_options: None = None) → 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() → Tuple[str, ...]

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.

get_current_revision() → str | None

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) → None

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: ScriptDirectory, revision: str) → None

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.