API Details

This section describes some key functions used within the migration process, particularly those referenced within a migration environment’s env.py file.

Overview

The three main objects in use are the EnvironmentContext, MigrationContext, and Operations classes, pictured below.

_images/api_overview.png

An Alembic command begins by instantiating an EnvironmentContext object, then making it available via the alembic.context proxy module. The env.py script, representing a user-configurable migration environment, is then invoked. The env.py script is then responsible for calling upon the EnvironmentContext.configure(), whose job it is to create a MigrationContext object.

Before this method is called, there’s not yet any database connection or dialect-specific state set up. While many methods on EnvironmentContext are usable at this stage, those which require database access, or at least access to the kind of database dialect in use, are not. Once the EnvironmentContext.configure() method is called, the EnvironmentContext is said to be configured with database connectivity, available via a new MigrationContext object. The MigrationContext is associated with the EnvironmentContext object via the EnvironmentContext.get_context() method.

Finally, env.py calls upon the EnvironmentContext.run_migrations() method. Within this method, a new Operations object, which provides an API for individual database migration operations, is established within the alembic.op proxy module. The Operations object uses the MigrationContext object ultimately as a source of database connectivity, though in such a way that it does not care if the MigrationContext is talking to a real database or just writing out SQL to a file.

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.environment.EnvironmentContext(config, script, **kw)

Represent the state made available to an env.py script.

EnvironmentContext is normally instantiated by the commands present in the alembic.command module. From within an env.py script, the current EnvironmentContext is available via the alembic.context datamember.

EnvironmentContext is also 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()

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 = 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, 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

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

class alembic.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, 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.

The Operations Object

Within migration scripts, actual database migration operations are handled via an instance of Operations. See Operation Reference for an overview of this object.

Commands

Alembic commands are all represented by functions in the alembic.command package. They all accept the same style of usage, being sent the Config object as the first argument.

Commands can be run programmatically, by first constructing a Config object, as in:

from alembic.config import Config
from alembic import command
alembic_cfg = Config("/path/to/yourapp/alembic.ini")
command.upgrade(alembic_cfg, "head")

In many cases, and perhaps more often than not, an application will wish to call upon a series of Alembic commands and/or other features. It is usually a good idea to link multiple commands along a single connection and transaction, if feasible. This can be achieved using the Config.attributes dictionary in order to share a connection:

with engine.begin() as connection:
    alembic_cfg.attributes['connection'] = connection
    command.upgrade(alembic_cfg, "head")

This recipe requires that env.py consumes this connection argument; see the example in Sharing a Connection with a Series of Migration Commands and Environments for details.

To write small API functions that make direct use of database and script directory information, rather than just running one of the built-in commands, use the ScriptDirectory and MigrationContext classes directly.

alembic.command.branches(config, verbose=False)

Show current branch points

alembic.command.current(config, verbose=False, head_only=False)

Display the current revision for a database.

alembic.command.downgrade(config, revision, sql=False, tag=None)

Revert to a previous version.

alembic.command.heads(config, verbose=False, resolve_dependencies=False)

Show current available heads in the script directory

alembic.command.history(config, rev_range=None, verbose=False)

List changeset scripts in chronological order.

alembic.command.init(config, directory, template='generic')

Initialize a new scripts directory.

alembic.command.list_templates(config)

List available templates

alembic.command.merge(config, revisions, message=None, branch_label=None, rev_id=None)

Merge two revisions together. Creates a new migration file.

New in version 0.7.0.

See also

Working with Branches

alembic.command.revision(config, message=None, autogenerate=False, sql=False, head='head', splice=False, branch_label=None, version_path=None, rev_id=None)

Create a new revision file.

alembic.command.show(config, rev)

Show the revision(s) denoted by the given symbol.

alembic.command.stamp(config, revision, sql=False, tag=None)

‘stamp’ the revision table with the given revision; don’t run any migrations.

alembic.command.upgrade(config, revision, sql=False, tag=None)

Upgrade to a later version.

Configuration

The Config object represents the configuration passed to the Alembic environment. From an API usage perspective, it is needed for the following use cases:

  • to create a ScriptDirectory, which allows you to work with the actual script files in a migration environment
  • to create an EnvironmentContext, which allows you to actually run the env.py module within the migration environment
  • to programatically run any of the commands in the alembic.command module.

The Config is not needed for these cases:

class alembic.config.Config(file_=None, ini_section='alembic', output_buffer=None, stdout=<open file '<stdout>', mode 'w'>, cmd_opts=None, config_args=immutabledict({}), attributes=None)

Represent an Alembic configuration.

Within an env.py script, this is available via the EnvironmentContext.config attribute, which in turn is available at alembic.context:

from alembic import context

some_param = context.config.get_main_option("my option")

When invoking Alembic programatically, a new Config can be created by passing the name of an .ini file to the constructor:

from alembic.config import Config
alembic_cfg = Config("/path/to/yourapp/alembic.ini")

With a Config object, you can then run Alembic commands programmatically using the directives in alembic.command.

The Config object can also be constructed without a filename. Values can be set programmatically, and new sections will be created as needed:

from alembic.config import Config
alembic_cfg = Config()
alembic_cfg.set_main_option("script_location", "myapp:migrations")
alembic_cfg.set_main_option("url", "postgresql://foo/bar")
alembic_cfg.set_section_option("mysection", "foo", "bar")

For passing non-string values to environments, such as connections and engines, use the Config.attributes dictionary:

with engine.begin() as connection:
    alembic_cfg.attributes['connection'] = connection
    command.upgrade(alembic_cfg, "head")
Parameters:
  • file_ – name of the .ini file to open.
  • ini_section – name of the main Alembic section within the .ini file
  • output_buffer – optional file-like input buffer which will be passed to the MigrationContext - used to redirect the output of “offline generation” when using Alembic programmatically.
  • stdout –

    buffer where the “print” output of commands will be sent. Defaults to sys.stdout.

    New in version 0.4.

  • config_args –

    A dictionary of keys and values that will be used for substitution in the alembic config file. The dictionary as given is copied to a new one, stored locally as the attribute .config_args. When the Config.file_config attribute is first invoked, the replacement variable here will be added to this dictionary before the dictionary is passed to SafeConfigParser() to parse the .ini file.

    New in version 0.7.0.

  • attributes –

    optional dictionary of arbitrary Python keys/values, which will be populated into the Config.attributes dictionary.

    New in version 0.7.5.

    See also

    Sharing a Connection with a Series of Migration Commands and Environments

Construct a new Config

attributes

A Python dictionary for storage of additional state.

This is a utility dictionary which can include not just strings but engines, connections, schema objects, or anything else. Use this to pass objects into an env.py script, such as passing a Connection when calling commands from alembic.command programmatically.

New in version 0.7.5.

See also

Sharing a Connection with a Series of Migration Commands and Environments

Config.attributes

cmd_opts = None

The command-line options passed to the alembic script.

Within an env.py script this can be accessed via the EnvironmentContext.config attribute.

New in version 0.6.0.

See also

EnvironmentContext.get_x_argument()

config_file_name = None

Filesystem path to the .ini file in use.

config_ini_section = None

Name of the config file section to read basic configuration from. Defaults to alembic, that is the [alembic] section of the .ini file. This value is modified using the -n/--name option to the Alembic runnier.

file_config

Return the underlying ConfigParser object.

Direct access to the .ini file is available here, though the Config.get_section() and Config.get_main_option() methods provide a possibly simpler interface.

get_main_option(name, default=None)

Return an option from the ‘main’ section of the .ini file.

This defaults to being a key from the [alembic] section, unless the -n/--name flag were used to indicate a different section.

get_section(name)

Return all the configuration options from a given .ini file section as a dictionary.

get_section_option(section, name, default=None)

Return an option from the given section of the .ini file.

get_template_directory()

Return the directory where Alembic setup templates are found.

This method is used by the alembic init and list_templates commands.

print_stdout(text, *arg)

Render a message to standard out.

set_main_option(name, value)

Set an option programmatically within the ‘main’ section.

This overrides whatever was in the .ini file.

set_section_option(section, name, value)

Set an option programmatically within the given section.

The section is created if it doesn’t exist already. The value here will override whatever was in the .ini file.

alembic.config.main(argv=None, prog=None, **kwargs)

The console runner function for Alembic.

Script Directory

The ScriptDirectory object provides programmatic access to the Alembic version files present in the filesystem.

class alembic.script.Script(module, rev_id, path)

Represent a single revision file in a versions/ directory.

The Script instance is returned by methods such as ScriptDirectory.iterate_revisions().

doc

Return the docstring given in the script.

longdoc

Return the docstring given in the script.

module = None

The Python module representing the actual script itself.

path = None

Filesystem path of the script.

class alembic.script.ScriptDirectory(dir, file_template='%(rev)s_%(slug)s', truncate_slug_length=40, version_locations=None, sourceless=False, output_encoding='utf-8')

Provides operations upon an Alembic script directory.

This object is useful to get information as to current revisions, most notably being able to get at the “head” revision, for schemes that want to test if the current revision in the database is the most recent:

from alembic.script import ScriptDirectory
from alembic.config import Config
config = Config()
config.set_main_option("script_location", "myapp:migrations")
script = ScriptDirectory.from_config(config)

head_revision = script.get_current_head()
as_revision_number(id_)

Convert a symbolic revision, i.e. ‘head’ or ‘base’, into an actual revision number.

classmethod from_config(config)

Produce a new ScriptDirectory given a Config instance.

The Config need only have the script_location key present.

generate_revision(revid, message, head=None, refresh=False, splice=False, branch_labels=None, version_path=None, depends_on=None, **kw)

Generate a new revision file.

This runs the script.py.mako template, given template arguments, and creates a new file.

Parameters:
  • revid – String revision id. Typically this comes from alembic.util.rev_id().
  • message – the revision message, the one passed by the -m argument to the revision command.
  • head –

    the head revision to generate against. Defaults to the current “head” if no branches are present, else raises an exception.

    New in version 0.7.0.

  • splice – if True, allow the “head” version to not be an actual head; otherwise, the selected head must be a head (e.g. endpoint) revision.
  • refresh – deprecated.
get_base()

Return the “base” revision as a string.

This is the revision number of the script that has a down_revision of None.

If the script directory has multiple bases, an error is raised; ScriptDirectory.get_bases() should be preferred.

get_bases()

return all “base” revisions as strings.

This is the revision number of all scripts that have a down_revision of None.

New in version 0.7.0.

get_current_head()

Return the current head revision.

If the script directory has multiple heads due to branching, an error is raised; ScriptDirectory.get_heads() should be preferred.

Returns:a string revision number.

See also

ScriptDirectory.get_heads()

get_heads()

Return all “versioned head” revisions as strings.

This is normally a list of length one, unless branches are present. The ScriptDirectory.get_current_head() method can be used normally when a script directory has only one head.

Returns:a tuple of string revision numbers.
get_revision(id_)

Return the Script instance with the given rev id.

See also

ScriptDirectory.get_revisions()

get_revisions(id_)

Return the Script instance with the given rev identifier, symbolic name, or sequence of identifiers.

New in version 0.7.0.

iterate_revisions(upper, lower)

Iterate through script revisions, starting at the given upper revision identifier and ending at the lower.

The traversal uses strictly the down_revision marker inside each migration script, so it is a requirement that upper >= lower, else you’ll get nothing back.

The iterator yields Script objects.

See also

RevisionMap.iterate_revisions()

run_env()

Run the script environment.

This basically runs the env.py script present in the migration environment. It is called exclusively by the command functions in alembic.command.

walk_revisions(base='base', head='heads')

Iterate through all revisions.

Parameters:
  • base – the base revision, or “base” to start from the empty revision.
  • head –

    the head revision; defaults to “heads” to indicate all head revisions. May also be “head” to indicate a single head revision.

    Changed in version 0.7.0: the “head” identifier now refers to the head of a non-branched repository only; use “heads” to refer to the set of all head branches simultaneously.

Revision

The RevisionMap object serves as the basis for revision management, used exclusively by ScriptDirectory.

class alembic.revision.Revision(revision, down_revision, dependencies=None, branch_labels=None)

Base class for revisioned objects.

The Revision class is the base of the more public-facing Script object, which represents a migration script. The mechanics of revision management and traversal are encapsulated within Revision, while Script applies this logic to Python files in a version directory.

branch_labels = None

Optional string/tuple of symbolic names to apply to this revision’s branch

dependencies = None

Additional revisions which this revision is dependent on.

From a migration standpoint, these dependencies are added to the down_revision to form the full iteration. However, the separation of down_revision from “dependencies” is to assist in navigating a history that contains many branches, typically a multi-root scenario.

down_revision = None

The down_revision identifier(s) within the migration script.

Note that the total set of “down” revisions is down_revision + dependencies.

is_base

Return True if this Revision is a ‘base’ revision.

is_branch_point

Return True if this Script is a branch point.

A branchpoint is defined as a Script which is referred to by more than one succeeding Script, that is more than one Script has a down_revision identifier pointing here.

is_head

Return True if this Revision is a ‘head’ revision.

This is determined based on whether any other Script within the ScriptDirectory refers to this Script. Multiple heads can be present.

is_merge_point

Return True if this Script is a merge point.

nextrev = frozenset([])

following revisions, based on down_revision only.

revision = None

The string revision number.

class alembic.revision.RevisionMap(generator)

Maintains a map of Revision objects.

RevisionMap is used by ScriptDirectory to maintain and traverse the collection of Script objects, which are themselves instances of Revision.

Construct a new RevisionMap.

Parameters:generator – a zero-arg callable that will generate an iterable of Revision instances to be used. These are typically Script subclasses within regular Alembic use.
add_revision(revision, _replace=False)

add a single revision to an existing map.

This method is for single-revision use cases, it’s not appropriate for fully populating an entire revision map.

bases

All “base” revisions as strings.

These are revisions that have a down_revision of None, or empty tuple.

Returns:a tuple of string revision numbers.
get_current_head(branch_label=None)

Return the current head revision.

If the script directory has multiple heads due to branching, an error is raised; ScriptDirectory.get_heads() should be preferred.

Parameters:branch_label – optional branch name which will limit the heads considered to those which include that branch_label.
Returns:a string revision number.

See also

ScriptDirectory.get_heads()

get_revision(id_)

Return the Revision instance with the given rev id.

If a symbolic name such as “head” or “base” is given, resolves the identifier into the current head or base revision. If the symbolic name refers to multiples, MultipleHeads is raised.

Supports partial identifiers, where the given identifier is matched against all identifiers that start with the given characters; if there is exactly one match, that determines the full revision.

get_revisions(id_)

Return the Revision instances with the given rev id or identifiers.

May be given a single identifier, a sequence of identifiers, or the special symbols “head” or “base”. The result is a tuple of one or more identifiers, or an empty tuple in the case of “base”.

In the cases where ‘head’, ‘heads’ is requested and the revision map is empty, returns an empty tuple.

Supports partial identifiers, where the given identifier is matched against all identifiers that start with the given characters; if there is exactly one match, that determines the full revision.

heads

All “head” revisions as strings.

This is normally a tuple of length one, unless unmerged branches are present.

Returns:a tuple of string revision numbers.
iterate_revisions(upper, lower, implicit_base=False, inclusive=False, assert_relative_length=True)

Iterate through script revisions, starting at the given upper revision identifier and ending at the lower.

The traversal uses strictly the down_revision marker inside each migration script, so it is a requirement that upper >= lower, else you’ll get nothing back.

The iterator yields Revision objects.

Autogeneration

Alembic 0.3 introduces a small portion of the autogeneration system as a public API.

alembic.autogenerate.compare_metadata(context, metadata)

Compare a database schema to that given in a MetaData instance.

The database connection is presented in the context of a MigrationContext object, which provides database connectivity as well as optional comparison functions to use for datatypes and server defaults - see the “autogenerate” arguments at EnvironmentContext.configure() for details on these.

The return format is a list of “diff” directives, each representing individual differences:

from alembic.migration import MigrationContext
from alembic.autogenerate import compare_metadata
from sqlalchemy.schema import SchemaItem
from sqlalchemy.types import TypeEngine
from sqlalchemy import (create_engine, MetaData, Column,
        Integer, String, Table)
import pprint

engine = create_engine("sqlite://")

engine.execute('''
    create table foo (
        id integer not null primary key,
        old_data varchar,
        x integer
    )''')

engine.execute('''
    create table bar (
        data varchar
    )''')

metadata = MetaData()
Table('foo', metadata,
    Column('id', Integer, primary_key=True),
    Column('data', Integer),
    Column('x', Integer, nullable=False)
)
Table('bat', metadata,
    Column('info', String)
)

mc = MigrationContext.configure(engine.connect())

diff = compare_metadata(mc, metadata)
pprint.pprint(diff, indent=2, width=20)

Output:

[ ( 'add_table',
    Table('bat', MetaData(bind=None),
        Column('info', String(), table=<bat>), schema=None)),
  ( 'remove_table',
    Table(u'bar', MetaData(bind=None),
        Column(u'data', VARCHAR(), table=<bar>), schema=None)),
  ( 'add_column',
    None,
    'foo',
    Column('data', Integer(), table=<foo>)),
  ( 'remove_column',
    None,
    'foo',
    Column(u'old_data', VARCHAR(), table=None)),
  [ ( 'modify_nullable',
      None,
      'foo',
      u'x',
      { 'existing_server_default': None,
        'existing_type': INTEGER()},
      True,
      False)]]
Parameters:

DDL Internals

These are some of the constructs used to generate migration instructions. The APIs here build off of the sqlalchemy.schema.DDLElement and sqlalchemy.ext.compiler systems.

For programmatic usage of Alembic’s migration directives, the easiest route is to use the higher level functions given by alembic.operations.

class alembic.ddl.base.AddColumn(name, column, schema=None)
class alembic.ddl.base.AlterColumn(name, column_name, schema=None, existing_type=None, existing_nullable=None, existing_server_default=None)
class alembic.ddl.base.AlterTable(table_name, schema=None)

Represent an ALTER TABLE statement.

Only the string name and optional schema name of the table is required, not a full Table object.

class alembic.ddl.base.ColumnDefault(name, column_name, default, **kw)
class alembic.ddl.base.ColumnName(name, column_name, newname, **kw)
class alembic.ddl.base.ColumnNullable(name, column_name, nullable, **kw)
class alembic.ddl.base.ColumnType(name, column_name, type_, **kw)
class alembic.ddl.base.DropColumn(name, column, schema=None)
class alembic.ddl.base.RenameTable(old_table_name, new_table_name, schema=None)
alembic.ddl.base.add_column(compiler, column, **kw)
alembic.ddl.base.alter_column(compiler, name)
alembic.ddl.base.alter_table(compiler, name, schema)
alembic.ddl.base.drop_column(compiler, name)
alembic.ddl.base.format_column_name(compiler, name)
alembic.ddl.base.format_server_default(compiler, default)
alembic.ddl.base.format_table_name(compiler, name, schema)
alembic.ddl.base.format_type(compiler, type_)
alembic.ddl.base.quote_dotted(name, quote)

quote the elements of a dotted name

alembic.ddl.base.visit_add_column(element, compiler, **kw)
alembic.ddl.base.visit_column_default(element, compiler, **kw)
alembic.ddl.base.visit_column_name(element, compiler, **kw)
alembic.ddl.base.visit_column_nullable(element, compiler, **kw)
alembic.ddl.base.visit_column_type(element, compiler, **kw)
alembic.ddl.base.visit_drop_column(element, compiler, **kw)
alembic.ddl.base.visit_rename_table(element, compiler, **kw)
class alembic.ddl.impl.DefaultImpl(dialect, connection, as_sql, transactional_ddl, output_buffer, context_opts)

Provide the entrypoint for major migration operations, including database-specific behavioral variances.

While individual SQL/DDL constructs already provide for database-specific implementations, variances here allow for entirely different sequences of operations to take place for a particular migration, such as SQL Server’s special ‘IDENTITY INSERT’ step for bulk inserts.

add_column(table_name, column, schema=None)
add_constraint(const)
alter_column(table_name, column_name, nullable=None, server_default=False, name=None, type_=None, schema=None, autoincrement=None, existing_type=None, existing_server_default=None, existing_nullable=None, existing_autoincrement=None)
autogen_column_reflect(inspector, table, column_info)

A hook that is attached to the ‘column_reflect’ event for when a Table is reflected from the database during the autogenerate process.

Dialects can elect to modify the information gathered here.

bind
bulk_insert(table, rows, multiinsert=True)
command_terminator = ';'
compare_server_default(inspector_column, metadata_column, rendered_metadata_default, rendered_inspector_default)
compare_type(inspector_column, metadata_column)
correct_for_autogen_constraints(conn_uniques, conn_indexes, metadata_unique_constraints, metadata_indexes)
create_index(index)
create_table(table)
drop_column(table_name, column, schema=None, **kw)
drop_constraint(const)
drop_index(index)
drop_table(table)
emit_begin()

Emit the string BEGIN, or the backend-specific equivalent, on the current connection context.

This is used in offline mode and typically via EnvironmentContext.begin_transaction().

emit_commit()

Emit the string COMMIT, or the backend-specific equivalent, on the current connection context.

This is used in offline mode and typically via EnvironmentContext.begin_transaction().

execute(sql, execution_options=None)
classmethod get_by_dialect(dialect)
prep_table_for_batch(table)

perform any operations needed on a table before a new one is created to replace it in batch mode.

the PG dialect uses this to drop constraints on the table before the new one uses those same names.

rename_table(old_table_name, new_table_name, schema=None)
requires_recreate_in_batch(batch_op)

Return True if the given BatchOperationsImpl would need the table to be recreated and copied in order to proceed.

Normally, only returns True on SQLite when operations other than add_column are present.

start_migrations()

A hook called when EnvironmentContext.run_migrations() is called.

Implementations can set up per-migration-run state here.

static_output(text)
transactional_ddl = False
class alembic.ddl.impl.ImplMeta(classname, bases, dict_)

MySQL

class alembic.ddl.mysql.MySQLAlterDefault(name, column_name, default, schema=None)

Bases: alembic.ddl.base.AlterColumn

class alembic.ddl.mysql.MySQLChangeColumn(name, column_name, schema=None, newname=None, type_=None, nullable=None, default=False, autoincrement=None)

Bases: alembic.ddl.base.AlterColumn

class alembic.ddl.mysql.MySQLImpl(dialect, connection, as_sql, transactional_ddl, output_buffer, context_opts)

Bases: alembic.ddl.impl.DefaultImpl

alter_column(table_name, column_name, nullable=None, server_default=False, name=None, type_=None, schema=None, autoincrement=None, existing_type=None, existing_server_default=None, existing_nullable=None, existing_autoincrement=None)
compare_server_default(inspector_column, metadata_column, rendered_metadata_default, rendered_inspector_default)
correct_for_autogen_constraints(conn_unique_constraints, conn_indexes, metadata_unique_constraints, metadata_indexes)
transactional_ddl = False
class alembic.ddl.mysql.MySQLModifyColumn(name, column_name, schema=None, newname=None, type_=None, nullable=None, default=False, autoincrement=None)

Bases: alembic.ddl.mysql.MySQLChangeColumn

MS-SQL

class alembic.ddl.mssql.MSSQLImpl(*arg, **kw)

Bases: alembic.ddl.impl.DefaultImpl

alter_column(table_name, column_name, nullable=None, server_default=False, name=None, type_=None, schema=None, autoincrement=None, existing_type=None, existing_server_default=None, existing_nullable=None, existing_autoincrement=None)
batch_separator = 'GO'
bulk_insert(table, rows, **kw)
drop_column(table_name, column, **kw)
emit_begin()
emit_commit()
transactional_ddl = True
alembic.ddl.mssql.mssql_add_column(compiler, column, **kw)
alembic.ddl.mssql.visit_add_column(element, compiler, **kw)
alembic.ddl.mssql.visit_column_default(element, compiler, **kw)
alembic.ddl.mssql.visit_column_nullable(element, compiler, **kw)
alembic.ddl.mssql.visit_column_type(element, compiler, **kw)
alembic.ddl.mssql.visit_rename_column(element, compiler, **kw)
alembic.ddl.mssql.visit_rename_table(element, compiler, **kw)

Postgresql

class alembic.ddl.postgresql.PostgresqlImpl(dialect, connection, as_sql, transactional_ddl, output_buffer, context_opts)

Bases: alembic.ddl.impl.DefaultImpl

autogen_column_reflect(inspector, table, column_info)
compare_server_default(inspector_column, metadata_column, rendered_metadata_default, rendered_inspector_default)
correct_for_autogen_constraints(conn_unique_constraints, conn_indexes, metadata_unique_constraints, metadata_indexes)
prep_table_for_batch(table)
transactional_ddl = True
alembic.ddl.postgresql.visit_rename_table(element, compiler, **kw)

SQLite

class alembic.ddl.sqlite.SQLiteImpl(dialect, connection, as_sql, transactional_ddl, output_buffer, context_opts)

Bases: alembic.ddl.impl.DefaultImpl

add_constraint(const)
compare_server_default(inspector_column, metadata_column, rendered_metadata_default, rendered_inspector_default)
correct_for_autogen_constraints(conn_unique_constraints, conn_indexes, metadata_unique_constraints, metadata_indexes)
drop_constraint(const)
requires_recreate_in_batch(batch_op)

Return True if the given BatchOperationsImpl would need the table to be recreated and copied in order to proceed.

Normally, only returns True on SQLite when operations other than add_column are present.

transactional_ddl = False

SQLite supports transactional DDL, but pysqlite does not: see: http://bugs.python.org/issue10740