Alembic provides for the creation, management, and invocation of change management scripts for a relational database, using SQLAlchemy as the underlying engine. This tutorial will provide a full introduction to the theory and usage of this tool.
To begin, make sure Alembic is installed as described at Installation.
As stated in the linked document, it is usually preferable that Alembic is
installed in the same module / Python path as that of the target project,
usually using a Python virtual environment, so that when the
command is run, the Python script which is invoked by
alembic, namely your
env.py script, will have access to your application’s models.
This is not strictly necessary in all cases, however in the vast majority of
cases is usually preferred.
The tutorial below assumes the
alembic command line utility is present in
the local path and when invoked, will have access to the same Python module
environment as that of the target project.
The Migration Environment¶
Usage of Alembic starts with creation of the Migration Environment. This is a directory of scripts
that is specific to a particular application. The migration environment is created just once,
and is then maintained along with the application’s source code itself. The environment is
created using the
init command of Alembic, and is then customizable to suit the specific
needs of the application.
The structure of this environment, including some generated migration scripts, looks like:
yourproject/ alembic/ env.py README script.py.mako versions/ 3512b954651e_add_account.py 2b1ae634e5cd_add_order_id.py 3adcc9a56557_rename_username_field.py
The directory includes these directories/files:
yourproject- this is the root of your application’s source code, or some directory within it.
alembic- this directory lives within your application’s source tree and is the home of the migration environment. It can be named anything, and a project that uses multiple databases may even have more than one.
env.py- This is a Python script that is run whenever the alembic migration tool is invoked. At the very least, it contains instructions to configure and generate a SQLAlchemy engine, procure a connection from that engine along with a transaction, and then invoke the migration engine, using the connection as a source of database connectivity.
env.pyscript is part of the generated environment so that the way migrations run is entirely customizable. The exact specifics of how to connect are here, as well as the specifics of how the migration environment are invoked. The script can be modified so that multiple engines can be operated upon, custom arguments can be passed into the migration environment, application-specific libraries and models can be loaded in and made available.
Alembic includes a set of initialization templates which feature different varieties of
env.pyfor different use cases.
README- included with the various environment templates, should have something informative.
script.py.mako- This is a Mako template file which is used to generate new migration scripts. Whatever is here is used to generate new files within
versions/. This is scriptable so that the structure of each migration file can be controlled, including standard imports to be within each, as well as changes to the structure of the
downgrade()functions. For example, the
multidbenvironment allows for multiple functions to be generated using a naming scheme
versions/- This directory holds the individual version scripts. Users of other migration tools may notice that the files here don’t use ascending integers, and instead use a partial GUID approach. In Alembic, the ordering of version scripts is relative to directives within the scripts themselves, and it is theoretically possible to “splice” version files in between others, allowing migration sequences from different branches to be merged, albeit carefully by hand.
Creating an Environment¶
With a basic understanding of what the environment is, we can create one using
This will create an environment using the “generic” template:
$ cd /path/to/yourproject $ source /path/to/yourproject/.venv/bin/activate # assuming a local virtualenv $ alembic init alembic
Where above, the
init command was called to generate a migrations directory called
Creating directory /path/to/yourproject/alembic...done Creating directory /path/to/yourproject/alembic/versions...done Generating /path/to/yourproject/alembic.ini...done Generating /path/to/yourproject/alembic/env.py...done Generating /path/to/yourproject/alembic/README...done Generating /path/to/yourproject/alembic/script.py.mako...done Please edit configuration/connection/logging settings in '/path/to/yourproject/alembic.ini' before proceeding.
Alembic also includes other environment templates. These can be listed out using the
$ alembic list_templates Available templates: generic - Generic single-database configuration. multidb - Rudimentary multi-database configuration. pylons - Configuration that reads from a Pylons project environment. Templates are used via the 'init' command, e.g.: alembic init --template pylons ./scripts
Editing the .ini File¶
Alembic placed a file
alembic.ini into the current directory. This is a file that the
script looks for when invoked. This file can be anywhere, either in the same directory
from which the
alembic script will normally be invoked, or if in a different directory, can
be specified by using the
--config option to the
The file generated with the “generic” configuration looks like:
# A generic, single database configuration. [alembic] # path to migration scripts script_location = alembic # template used to generate migration files # file_template = %%(rev)s_%%(slug)s # sys.path path, will be prepended to sys.path if present. # defaults to the current working directory. # (new in 1.5.5) prepend_sys_path = . # timezone to use when rendering the date # within the migration file as well as the filename. # string value is passed to dateutil.tz.gettz() # leave blank for localtime # timezone = # max length of characters to apply to the # "slug" field # truncate_slug_length = 40 # set to 'true' to run the environment during # the 'revision' command, regardless of autogenerate # revision_environment = false # set to 'true' to allow .pyc and .pyo files without # a source .py file to be detected as revisions in the # versions/ directory # sourceless = false # version location specification; this defaults # to alembic/versions. When using multiple version # directories, initial revisions must be specified with --version-path # version_locations = %(here)s/bar %(here)s/bat alembic/versions # the output encoding used when revision files # are written from script.py.mako # output_encoding = utf-8 sqlalchemy.url = driver://user:pass@localhost/dbname # post_write_hooks defines scripts or Python functions that are run # on newly generated revision scripts. See the documentation for further # detail and examples # format using "black" - use the console_scripts runner, # against the "black" entrypoint # hooks = black # black.type = console_scripts # black.entrypoint = black # black.options = -l 79 REVISION_SCRIPT_FILENAME # Logging configuration [loggers] keys = root,sqlalchemy,alembic [handlers] keys = console [formatters] keys = generic [logger_root] level = WARN handlers = console qualname = [logger_sqlalchemy] level = WARN handlers = qualname = sqlalchemy.engine [logger_alembic] level = INFO handlers = qualname = alembic [handler_console] class = StreamHandler args = (sys.stderr,) level = NOTSET formatter = generic [formatter_generic] format = %(levelname)-5.5s [%(name)s] %(message)s datefmt = %H:%M:%S
The file is read using Python’s
ConfigParser.SafeConfigParser object. The
%(here)s variable is provided as a substitution variable, which
can be used to produce absolute pathnames to directories and files, as we do above
with the path to the Alembic script location.
This file contains the following features:
[alembic]- this is the section read by Alembic to determine configuration. Alembic itself does not directly read any other areas of the file. The name “alembic” can be customized using the
--namecommandline flag; see Run Multiple Alembic Environments from one .ini file for a basic example of this.
script_location- this is the location of the Alembic environment. It is normally specified as a filesystem location, either relative or absolute. If the location is a relative path, it’s interpreted as relative to the current directory.
This is the only key required by Alembic in all cases. The generation of the .ini file by the command
alembic init alembicautomatically placed the directory name
alembichere. The special variable
%(here)scan also be used, as in
For support of applications that package themselves into .egg files, the value can also be specified as a package resource, in which case
resource_filename()is used to find the file (new in 0.2.2). Any non-absolute URI which contains colons is interpreted here as a resource name, rather than a straight filename.
file_template- this is the naming scheme used to generate new migration files. The value present is the default, so is commented out. Tokens available include:
%%(rev)s- revision id
%%(slug)s- a truncated string derived from the revision message
%%(second).2d- components of the create date, by default
timezoneconfiguration option is also used.
timezone- an optional timezone name (e.g.
EST5EDT, etc.) that will be applied to the timestamp which renders inside the migration file’s comment as well as within the filename. If
timezoneis specified, the create date object is no longer derived from
datetime.datetime.now()and is instead generated as:
datetime.datetime.utcnow().replace( tzinfo=dateutil.tz.tzutc() ).astimezone( dateutil.tz.gettz(<timezone>) )
truncate_slug_length- defaults to 40, the max number of characters to include in the “slug” field.
sqlalchemy.url- A URL to connect to the database via SQLAlchemy. This configuration value is only used if the
env.pyfile calls upon them; in the “generic” template, the call to
run_migrations_offline()function and the call to
run_migrations_online()function are where this key is referenced. If the SQLAlchemy URL should come from some other source, such as from environment variables or a global registry, or if the migration environment makes use of multiple database URLs, the developer is encouraged to alter the
env.pyfile to use whatever methods are appropriate in order to acquire the database URL or URLs.
revision_environment- this is a flag which when set to the value ‘true’, will indicate that the migration environment script
env.pyshould be run unconditionally when generating new revision files, as well as when running the
sourceless- when set to ‘true’, revision files that only exist as .pyc or .pyo files in the versions directory will be used as versions, allowing “sourceless” versioning folders. When left at the default of ‘false’, only .py files are consumed as version files.
version_locations- an optional list of revision file locations, to allow revisions to exist in multiple directories simultaneously. See Working with Multiple Bases for examples.
output_encoding- the encoding to use when Alembic writes the
script.py.makofile into a new migration file. Defaults to
[formatter_*]- these sections are all part of Python’s standard logging configuration, the mechanics of which are documented at Configuration File Format. As is the case with the database connection, these directives are used directly as the result of the
logging.config.fileConfig()call present in the
env.pyscript, which you’re free to modify.
For starting up with just a single database and the generic configuration, setting up the SQLAlchemy URL is all that’s needed:
sqlalchemy.url = postgresql://scott:tiger@localhost/test
Create a Migration Script¶
With the environment in place we can create a new revision, using
$ alembic revision -m "create account table" Generating /path/to/yourproject/alembic/versions/1975ea83b712_create_accoun t_table.py...done
A new file
1975ea83b712_create_account_table.py is generated. Looking inside the file:
"""create account table Revision ID: 1975ea83b712 Revises: Create Date: 2011-11-08 11:40:27.089406 """ # revision identifiers, used by Alembic. revision = '1975ea83b712' down_revision = None branch_labels = None from alembic import op import sqlalchemy as sa def upgrade(): pass def downgrade(): pass
The file contains some header information, identifiers for the current revision
and a “downgrade” revision, an import of basic Alembic directives,
downgrade() functions. Our
job here is to populate the
downgrade() functions with directives that
will apply a set of changes to our database. Typically,
upgrade() is required
downgrade() is only needed if down-revision capability is desired, though it’s
probably a good idea.
Another thing to notice is the
down_revision variable. This is how Alembic
knows the correct order in which to apply migrations. When we create the next revision,
the new file’s
down_revision identifier would point to this one:
# revision identifiers, used by Alembic. revision = 'ae1027a6acf' down_revision = '1975ea83b712'
Every time Alembic runs an operation against the
versions/ directory, it reads all
the files in, and composes a list based on how the
down_revision identifiers link together,
None representing the first file. In theory, if a
migration environment had thousands of migrations, this could begin to add some latency to
startup, but in practice a project should probably prune old migrations anyway
(see the section Building an Up to Date Database from Scratch for a description on how to do this, while maintaining
the ability to build the current database fully).
We can then add some directives to our script, suppose adding a new table
def upgrade(): op.create_table( 'account', sa.Column('id', sa.Integer, primary_key=True), sa.Column('name', sa.String(50), nullable=False), sa.Column('description', sa.Unicode(200)), ) def downgrade(): op.drop_table('account')
drop_table() are Alembic directives. Alembic provides
all the basic database migration operations via these directives, which are designed to be as simple and
minimalistic as possible;
there’s no reliance upon existing table metadata for most of these directives. They draw upon
a global “context” that indicates how to get at a database connection (if any; migrations can
dump SQL/DDL directives to files as well) in order to invoke the command. This global
context is set up, like everything else, in the
An overview of all Alembic directives is at Operation Reference.
Running our First Migration¶
We now want to run our migration. Assuming our database is totally clean, it’s as
yet unversioned. The
alembic upgrade command will run upgrade operations, proceeding
from the current database revision, in this example
None, to the given target revision.
We can specify
1975ea83b712 as the revision we’d like to upgrade to, but it’s easier
in most cases just to tell it “the most recent”, in this case
$ alembic upgrade head INFO [alembic.context] Context class PostgresqlContext. INFO [alembic.context] Will assume transactional DDL. INFO [alembic.context] Running upgrade None -> 1975ea83b712
Wow that rocked! Note that the information we see on the screen is the result of the
logging configuration set up in
alembic.ini - logging the
alembic stream to the
console (standard error, specifically).
The process which occurred here included that Alembic first checked if the database had
a table called
alembic_version, and if not, created it. It looks in this table
for the current version, if any, and then calculates the path from this version to
the version requested, in this case
head, which is known to be
It then invokes the
upgrade() method in each file to get to the target revision.
Running our Second Migration¶
Let’s do another one so we have some things to play with. We again create a revision file:
$ alembic revision -m "Add a column" Generating /path/to/yourapp/alembic/versions/ae1027a6acf_add_a_column.py... done
Let’s edit this file and add a new column to the
"""Add a column Revision ID: ae1027a6acf Revises: 1975ea83b712 Create Date: 2011-11-08 12:37:36.714947 """ # revision identifiers, used by Alembic. revision = 'ae1027a6acf' down_revision = '1975ea83b712' from alembic import op import sqlalchemy as sa def upgrade(): op.add_column('account', sa.Column('last_transaction_date', sa.DateTime)) def downgrade(): op.drop_column('account', 'last_transaction_date')
Running again to
$ alembic upgrade head INFO [alembic.context] Context class PostgresqlContext. INFO [alembic.context] Will assume transactional DDL. INFO [alembic.context] Running upgrade 1975ea83b712 -> ae1027a6acf
We’ve now added the
last_transaction_date column to the database.
Partial Revision Identifiers¶
Any time we need to refer to a revision number explicitly, we have the option to use a partial number. As long as this number uniquely identifies the version, it may be used in any command in any place that version numbers are accepted:
$ alembic upgrade ae1
Above, we use
ae1 to refer to revision
Alembic will stop and let you know if more than one version starts with
Relative Migration Identifiers¶
Relative upgrades/downgrades are also supported. To move two versions from the current, a decimal value “+N” can be supplied:
$ alembic upgrade +2
Negative values are accepted for downgrades:
$ alembic downgrade -1
Relative identifiers may also be in terms of a specific revision. For example,
to upgrade to revision
ae1027a6acf plus two additional steps:
$ alembic upgrade ae10+2
With a few revisions present we can get some information about the state of things.
First we can view the current revision:
$ alembic current INFO [alembic.context] Context class PostgresqlContext. INFO [alembic.context] Will assume transactional DDL. Current revision for postgresql://scott:XXXXX@localhost/test: 1975ea83b712 -> ae1027a6acf (head), Add a column
head is displayed only if the revision identifier for this database matches the head revision.
We can also view history with
alembic history; the
(accepted by several commands, including
branches) will show us full information about each revision:
$ alembic history --verbose Rev: ae1027a6acf (head) Parent: 1975ea83b712 Path: /path/to/yourproject/alembic/versions/ae1027a6acf_add_a_column.py add a column Revision ID: ae1027a6acf Revises: 1975ea83b712 Create Date: 2014-11-20 13:02:54.849677 Rev: 1975ea83b712 Parent: <base> Path: /path/to/yourproject/alembic/versions/1975ea83b712_add_account_table.py create account table Revision ID: 1975ea83b712 Revises: Create Date: 2014-11-20 13:02:46.257104
Viewing History Ranges¶
-r option to
alembic history, we can also view various slices
of history. The
-r argument accepts an argument
either may be a revision number, symbols like
current to specify the current revision(s), as well as negative
relative ranges for
[start] and positive relative ranges for
$ alembic history -r1975ea:ae1027
A relative range starting from three revs ago up to current migration, which will invoke the migration environment against the database to get the current migration:
$ alembic history -r-3:current
View all revisions from 1975 to the head:
$ alembic history -r1975ea:
We can illustrate a downgrade back to nothing, by calling
alembic downgrade back
to the beginning, which in Alembic is called
$ alembic downgrade base INFO [alembic.context] Context class PostgresqlContext. INFO [alembic.context] Will assume transactional DDL. INFO [alembic.context] Running downgrade ae1027a6acf -> 1975ea83b712 INFO [alembic.context] Running downgrade 1975ea83b712 -> None
Back to nothing - and up again:
$ alembic upgrade head INFO [alembic.context] Context class PostgresqlContext. INFO [alembic.context] Will assume transactional DDL. INFO [alembic.context] Running upgrade None -> 1975ea83b712 INFO [alembic.context] Running upgrade 1975ea83b712 -> ae1027a6acf