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 tutorial.

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 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 BaseCommand or one of its subclasses.

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 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"' % 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. 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 zero or more poll_ids and sets poll.opened to False for each one. If the user referenced any nonexistent polls, a CommandError is raised. The poll.opened attribute does not exist in the tutorial 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 option_list like this:

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'),
        )

    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 optparse Python documentation for more about make_option usage.

In addition to being able to add custom command line options, all management commands can accept some default options such as --verbosity and --traceback.

Management commands and locales

The 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 handle() or handle_noargs() 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):
    ...
    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 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 subclasses.

Subclassing the BaseCommand class requires that you implement the handle() method.

Attributes

All attributes can be set in your derived class and can be used in BaseCommand‘s subclasses.

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 ...>’.

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.

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>.

BaseCommand.option_list

This is the list of optparse options which will be fed into the command’s OptionParser for parsing arguments.

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.

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 validate() from handle().

Methods

BaseCommand has a few methods that can be overridden but only the handle() method must be implemented.

Implementing a constructor in a subclass

If you implement __init__ in your subclass of BaseCommand, you must call BaseCommand‘s __init__.

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

BaseCommand.execute(*args, **options)

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

Calling a management command in your code

execute() should not be called directly from your code to execute a command. Use call_command instead.

BaseCommand.handle(*args, **options)

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

BaseCommand.validate(app=None, display_num_errors=False)

Validates the given app, raising CommandError for any errors.

If app is None, then all installed apps are validated.

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 handle(), subclasses must implement handle_app(), which will be called once for each application.

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 handle(), subclasses must implement handle_label(), which will be called once for each label.

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 handle(), subclasses must implement handle_noargs(); handle() itself is overridden to ensure no arguments are passed to the command.

NoArgsCommand.handle_noargs(**options)

Perform this command’s actions

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 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 call_command, it’s up to you to catch the exception when needed.