django/docs/howto/custom-management-commands.txt

========================================
Writing custom ``django-admin`` commands
========================================

.. module:: django.core.management

Applications can register their own actions with ``manage.py``. For example,
you might want to add a ``manage.py`` action for a Django app that you're
distributing. In this document, we will be building a custom ``closepoll``
command for the ``polls`` application from the
:doc:`tutorial</intro/tutorial01>`.

To do this, just add a ``management/commands`` directory to the application.
Django will register a ``manage.py`` command for each Python module in that
directory whose name doesn't begin with an underscore. For example::

    polls/
        __init__.py
        models.py
        management/
            __init__.py
            commands/
                __init__.py
                _private.py
                closepoll.py
        tests.py
        views.py

In this example, the ``closepoll`` command will be made available to any project
that includes the ``polls`` application in :setting:`INSTALLED_APPS`.

The ``_private.py`` module will not be available as a management command.

The ``closepoll.py`` module has only one requirement -- it must define a class
``Command`` that extends :class:`BaseCommand` or one of its
:ref:`subclasses<ref-basecommand-subclasses>`.

.. admonition:: Standalone scripts

    Custom management commands are especially useful for running standalone
    scripts or for scripts that are periodically executed from the UNIX crontab
    or from Windows scheduled tasks control panel.

To implement the command, edit ``polls/management/commands/closepoll.py`` to
look like this::

    from django.core.management.base import BaseCommand, CommandError
    from polls.models import Question as Poll

    class Command(BaseCommand):
        help = 'Closes the specified poll for voting'

        def add_arguments(self, parser):
            parser.add_argument('poll_id', nargs='+', type=int)

        def handle(self, *args, **options):
            for poll_id in options['poll_id']:
                try:
                    poll = Poll.objects.get(pk=poll_id)
                except Poll.DoesNotExist:
                    raise CommandError('Poll "%s" does not exist' % poll_id)

                poll.opened = False
                poll.save()

                self.stdout.write(self.style.SUCCESS('Successfully closed poll "%s"' % poll_id))

.. _management-commands-output:

.. note::
    When you are using management commands and wish to provide console
    output, you should write to ``self.stdout`` and ``self.stderr``,
    instead of printing to ``stdout`` and ``stderr`` directly. By
    using these proxies, it becomes much easier to test your custom
    command. Note also that you don't need to end messages with a newline
    character, it will be added automatically, unless you specify the ``ending``
    parameter::

        self.stdout.write("Unterminated line", ending='')

The new custom command can be called using ``python manage.py closepoll
<poll_id>``.

The ``handle()`` method takes one or more ``poll_ids`` and sets ``poll.opened``
to ``False`` for each one. If the user referenced any nonexistent polls, a
:exc:`CommandError` is raised. The ``poll.opened`` attribute does not exist in
the :doc:`tutorial</intro/tutorial01>` and was added to
``polls.models.Question`` for this example.

.. _custom-commands-options:

Accepting optional arguments
============================

The same ``closepoll`` could be easily modified to delete a given poll instead
of closing it by accepting additional command line options. These custom
options can be added in the :meth:`~BaseCommand.add_arguments` method like this::

    class Command(BaseCommand):
        def add_arguments(self, parser):
            # Positional arguments
            parser.add_argument('poll_id', nargs='+', type=int)

            # Named (optional) arguments
            parser.add_argument(
                '--delete',
                action='store_true',
                dest='delete',
                help='Delete poll instead of closing it',
            )

        def handle(self, *args, **options):
            # ...
            if options['delete']:
                poll.delete()
            # ...

The option (``delete`` in our example) is available in the options dict
parameter of the handle method. See the :py:mod:`argparse` Python documentation
for more about ``add_argument`` usage.

In addition to being able to add custom command line options, all
:doc:`management commands</ref/django-admin>` can accept some default options
such as :option:`--verbosity` and :option:`--traceback`.

.. _management-commands-and-locales:

Management commands and locales
===============================

By default, the :meth:`BaseCommand.execute` method deactivates translations
because some commands shipped with Django perform several tasks (for example,
user-facing content rendering and database population) that require a
project-neutral string language.

If, for some reason, your custom management command needs to use a fixed locale,
you should manually activate and deactivate it in your
:meth:`~BaseCommand.handle` method using the functions provided by the I18N
support code::

    from django.core.management.base import BaseCommand, CommandError
    from django.utils import translation

    class Command(BaseCommand):
        ...

        def handle(self, *args, **options):

            # Activate a fixed locale, e.g. Russian
            translation.activate('ru')

            # Or you can activate the LANGUAGE_CODE # chosen in the settings:
            from django.conf import settings
            translation.activate(settings.LANGUAGE_CODE)

            # Your command logic here
            ...

            translation.deactivate()

Another need might be that your command simply should use the locale set in
settings and Django should be kept from deactivating it. You can achieve
it by using the :data:`BaseCommand.leave_locale_alone` option.

When working on the scenarios described above though, take into account that
system management commands typically have to be very careful about running in
non-uniform locales, so you might need to:

* Make sure the :setting:`USE_I18N` setting is always ``True`` when running
  the command (this is a good example of the potential problems stemming
  from a dynamic runtime environment that Django commands avoid offhand by
  deactivating translations).

* Review the code of your command and the code it calls for behavioral
  differences when locales are changed and evaluate its impact on
  predictable behavior of your command.

Testing
=======

Information on how to test custom management commands can be found in the
:ref:`testing docs <topics-testing-management-commands>`.

Overriding commands
===================

Django registers the built-in commands and then searches for commands in
:setting:`INSTALLED_APPS` in reverse. During the search, if a command name
duplicates an already registered command, the newly discovered command
overrides the first.

In other words, to override a command, the new command must have the same name
and its app must be before the overridden command's app in
:setting:`INSTALLED_APPS`.

Management commands from third-party apps that have been unintentionally
overridden can be made available under a new name by creating a new command in
one of your project's apps (ordered before the third-party app in
:setting:`INSTALLED_APPS`) which imports the ``Command`` of the overridden
command.

Command objects
===============

.. class:: BaseCommand

The base class from which all management commands ultimately derive.

Use this class if you want access to all of the mechanisms which
parse the command-line arguments and work out what code to call in
response; if you don't need to change any of that behavior,
consider using one of its :ref:`subclasses<ref-basecommand-subclasses>`.

Subclassing the :class:`BaseCommand` class requires that you implement the
:meth:`~BaseCommand.handle` method.

Attributes
----------

All attributes can be set in your derived class and can be used in
:class:`BaseCommand`’s :ref:`subclasses<ref-basecommand-subclasses>`.

.. attribute:: BaseCommand.help

    A short description of the command, which will be printed in the
    help message when the user runs the command
    ``python manage.py help <command>``.

.. attribute:: BaseCommand.missing_args_message

    If your command defines mandatory positional arguments, you can customize
    the message error returned in the case of missing arguments. The default is
    output by :py:mod:`argparse` ("too few arguments").

.. attribute:: BaseCommand.output_transaction

    A boolean indicating whether the command outputs SQL statements; if
    ``True``, the output will automatically be wrapped with ``BEGIN;`` and
    ``COMMIT;``. Default value is ``False``.

.. attribute:: BaseCommand.requires_migrations_checks

    A boolean; if ``True``, the command prints a warning if the set of
    migrations on disk don't match the migrations in the database. A warning
    doesn't prevent the command from executing. Default value is ``False``.

.. attribute:: BaseCommand.requires_system_checks

    A boolean; if ``True``, the entire Django project will be checked for
    potential problems prior to executing the command. Default value is ``True``.

.. attribute:: BaseCommand.leave_locale_alone

    A boolean indicating whether the locale set in settings should be preserved
    during the execution of the command instead of being forcibly set to 'en-us'.

    Default value is ``False``.

    Make sure you know what you are doing if you decide to change the value of
    this option in your custom command if it creates database content that
    is locale-sensitive and such content shouldn't contain any translations
    (like it happens e.g. with :mod:`django.contrib.auth` permissions) as
    making the locale differ from the de facto default 'en-us' might cause
    unintended effects. See the `Management commands and locales`_ section
    above for further details.

.. attribute:: BaseCommand.style

    An instance attribute that helps create colored output when writing to
    ``stdout`` or ``stderr``. For example::

        self.stdout.write(self.style.SUCCESS('...'))

    See :ref:`syntax-coloring` to learn how to modify the color palette and to
    see the available styles (use uppercased versions of the "roles" described
    in that section).

    If you pass the :option:`--no-color` option when running your command, all
    ``self.style()`` calls will return the original string uncolored.

Methods
-------

:class:`BaseCommand` has a few methods that can be overridden but only
the :meth:`~BaseCommand.handle` method must be implemented.

.. admonition:: Implementing a constructor in a subclass

    If you implement ``__init__`` in your subclass of :class:`BaseCommand`,
    you must call :class:`BaseCommand`’s ``__init__``::

        class Command(BaseCommand):
            def __init__(self, *args, **kwargs):
                super().__init__(*args, **kwargs)
                # ...

.. method:: BaseCommand.add_arguments(parser)

    Entry point to add parser arguments to handle command line arguments passed
    to the command. Custom commands should override this method to add both
    positional and optional arguments accepted by the command. Calling
    ``super()`` is not needed when directly subclassing ``BaseCommand``.

.. method:: BaseCommand.get_version()

    Returns the Django version, which should be correct for all built-in Django
    commands. User-supplied commands can override this method to return their
    own version.

.. method:: BaseCommand.execute(*args, **options)

    Tries to execute this command, performing system checks if needed (as
    controlled by the :attr:`requires_system_checks` attribute). If the command
    raises a :exc:`CommandError`, it's intercepted and printed to stderr.

.. admonition:: Calling a management command in your code

    ``execute()`` should not be called directly from your code to execute a
    command. Use :func:`~django.core.management.call_command` instead.

.. method:: BaseCommand.handle(*args, **options)

    The actual logic of the command. Subclasses must implement this method.

    It may return a string which will be printed to ``stdout`` (wrapped
    by ``BEGIN;`` and ``COMMIT;`` if :attr:`output_transaction` is ``True``).

.. method:: BaseCommand.check(app_configs=None, tags=None, display_num_errors=False)

    Uses the system check framework to inspect the entire Django project for
    potential problems. Serious problems are raised as a :exc:`CommandError`;
    warnings are output to stderr; minor notifications are output to stdout.

    If ``app_configs`` and ``tags`` are both ``None``, all system checks are
    performed. ``tags`` can be a list of check tags, like ``compatibility`` or
    ``models``.

.. _ref-basecommand-subclasses:

``BaseCommand`` subclasses
--------------------------

.. class:: AppCommand

A management command which takes one or more installed application labels as
arguments, and does something with each of them.

Rather than implementing :meth:`~BaseCommand.handle`, subclasses must
implement :meth:`~AppCommand.handle_app_config`, which will be called once for
each application.

.. method:: AppCommand.handle_app_config(app_config, **options)

    Perform the command's actions for ``app_config``, which will be an
    :class:`~django.apps.AppConfig` instance corresponding to an application
    label given on the command line.

.. class:: LabelCommand

A management command which takes one or more arbitrary arguments (labels) on
the command line, and does something with each of them.

Rather than implementing :meth:`~BaseCommand.handle`, subclasses must implement
:meth:`~LabelCommand.handle_label`, which will be called once for each label.

.. attribute:: LabelCommand.label

    A string describing the arbitrary arguments passed to the command. The
    string is used in the usage text and error messages of the command.
    Defaults to ``'label'``.

.. method:: LabelCommand.handle_label(label, **options)

    Perform the command's actions for ``label``, which will be the string as
    given on the command line.

Command exceptions
------------------

.. exception:: CommandError

Exception class indicating a problem while executing a management command.

If this exception is raised during the execution of a management command from a
command line console, it will be caught and turned into a nicely-printed error
message to the appropriate output stream (i.e., stderr); as a result, raising
this exception (with a sensible description of the error) is the preferred way
to indicate that something has gone wrong in the execution of a command.

If a management command is called from code through
:func:`~django.core.management.call_command`, it's up to you to catch the
exception when needed.