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djangofrdoc / howto / custom-management-commands.txt

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

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.
Each Python module in that directory will be auto-discovered and registered as
a command that can be executed as an action when you run ``manage.py``::

    polls/
        __init__.py
        models.py
        management/
            __init__.py
            commands/
                __init__.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 ``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:

.. code-block:: python

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

    class Command(BaseCommand):
        args = '<poll_id poll_id ...>'
        help = 'Closes the specified poll for voting'

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

                poll.opened = False
                poll.save()

                self.stdout.write('Successfully closed poll "%s"\n' % poll_id)

.. 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.

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

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

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
must be added to :attr:`~BaseCommand.option_list` like this:

.. code-block:: python

    from optparse import make_option

    class Command(BaseCommand):
        option_list = BaseCommand.option_list + (
            make_option('--delete',
                action='store_true',
                dest='delete',
                default=False,
                help='Delete poll instead of closing it'),
            )
        # ...

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 :djadminopt:`--verbosity` and :djadminopt:`--traceback`.

.. admonition:: Management commands and locales

    The :meth:`BaseCommand.execute` method sets the hardcoded ``en-us`` locale
    because the commands shipped with Django perform several tasks
    (for example, user-facing content rendering and database population) that
    require a system-neutral string language (for which we use ``en-us``).

    If your custom management command uses another locale, you should manually
    activate and deactivate it in your :meth:`~BaseCommand.handle` or
    :meth:`~NoArgsCommand.handle_noargs` method using the functions provided by
    the I18N support code:

    .. code-block:: python

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

        class Command(BaseCommand):
            ...
            self.can_import_settings = True

            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()

    Take into account though, that system management commands typically have to
    be very careful about running in non-uniform locales, so:

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

    * 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.

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.args

  A string listing the arguments accepted by the command,
  suitable for use in help messages; e.g., a command which takes
  a list of application names might set this to '<appname
  appname ...>'.

.. attribute:: BaseCommand.can_import_settings

  A boolean indicating whether the command needs to be able to
  import Django settings; if ``True``, ``execute()`` will verify
  that this is possible before proceeding. Default value is
  ``True``.

.. 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.option_list

  This is the list of ``optparse`` options which will be fed
  into the command's ``OptionParser`` for parsing 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_model_validation

  A boolean; if ``True``, validation of installed models will be
  performed prior to executing the command. Default value is
  ``True``. To validate an individual application's models
  rather than all applications' models, call
  :meth:`~BaseCommand.validate` from :meth:`~BaseCommand.handle`.

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__``.

  .. code-block:: python

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

.. method:: BaseCommand.get_version()

    Return 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)

    Try to execute this command, performing model validation if
    needed (as controlled by the attribute
    :attr:`requires_model_validation`). If the command raises a
    :class:`CommandError`, intercept it and print it sensibly to
    stderr.

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

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

.. _ref-basecommand-subclasses:

BaseCommand subclasses
----------------------

.. class:: AppCommand

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

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

.. method:: AppCommand.handle_app(app, **options)

    Perform the command's actions for ``app``, which will be the
    Python module corresponding to an application name 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.

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

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

.. class:: NoArgsCommand

A command which takes no arguments on the command line.

Rather than implementing :meth:`~BaseCommand.handle`, subclasses must implement
:meth:`~NoArgsCommand.handle_noargs`; :meth:`~BaseCommand.handle` itself is
overridden to ensure no arguments are passed to the command.

.. method:: NoArgsCommand.handle_noargs(**options)

    Perform this command's actions

.. _ref-command-exceptions:

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

.. class:: CommandError

Exception class indicating a problem while executing a management
command.

If this exception is raised during the execution of a management
command, 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.