The system check framework is a set of static checks for validating Django\nprojects. It detects common problems and provides hints for how to fix them.\nThe framework is extensible so you can easily add your own checks.
\nChecks can be triggered explicitly via the check command. Checks are\ntriggered implicitly before most commands, including runserver and\nmigrate. For performance reasons, checks are not run as part of the\nWSGI stack that is used in deployment. If you need to run system checks on your\ndeployment server, trigger them explicitly using check.
Serious errors will prevent Django commands (such as runserver) from\nrunning at all. Minor problems are reported to the console. If you have inspected\nthe cause of a warning and are happy to ignore it, you can hide specific warnings\nusing the SILENCED_SYSTEM_CHECKS setting in your project settings file.
A full list of all checks that can be raised by Django can be found in the\nSystem check reference.
\nThe framework is flexible and allows you to write functions that perform\nany other kind of check you may require. The following is an example stub\ncheck function:
\nfrom django.core.checks import Error, register\n\n@register()\ndef example_check(app_configs, **kwargs):\n errors = []\n # ... your check logic here\n if check_failed:\n errors.append(\n Error(\n 'an error',\n hint='A hint.',\n obj=checked_object,\n id='myapp.E001',\n )\n )\n return errors\nThe check function must accept an app_configs argument; this argument is\nthe list of applications that should be inspected. If None, the check must be\nrun on all installed apps in the project. The **kwargs argument is required\nfor future expansion.
The function must return a list of messages. If no problems are found as a result\nof the check, the check function must return an empty list.
\nThe warnings and errors raised by the check method must be instances of\nCheckMessage. An instance of\nCheckMessage encapsulates a single reportable\nerror or warning. It also provides context and hints applicable to the\nmessage, and a unique identifier that is used for filtering purposes.
The concept is very similar to messages from the message framework or the logging framework.\nMessages are tagged with a level indicating the severity of the message.
There are also shortcuts to make creating messages with common levels easier.\nWhen using these classes you can omit the level argument because it is\nimplied by the class name.
Lastly, your check function must be registered explicitly with system check\nregistry. Checks should be registered in a file that's loaded when your\napplication is loaded; for example, in the AppConfig.ready() method.
You can pass as many tags to register as you want in order to label your\ncheck. Tagging checks is useful since it allows you to run only a certain\ngroup of checks. For example, to register a compatibility check, you would\nmake the following call:
from django.core.checks import register, Tags\n\n@register(Tags.compatibility)\ndef my_check(app_configs, **kwargs):\n # ... perform compatibility checks and collect errors\n return errors\nYou can register "deployment checks" that are only relevant to a production\nsettings file like this:
\n@register(Tags.security, deploy=True)\ndef my_check(app_configs, **kwargs):\n ...\nThese checks will only be run if the check --deploy option is used.
You can also use register as a function rather than a decorator by\npassing a callable object (usually a function) as the first argument\nto register.
The code below is equivalent to the code above:
\ndef my_check(app_configs, **kwargs):\n ...\nregister(my_check, Tags.security, deploy=True)\nIn some cases, you won't need to register your check function -- you can\npiggyback on an existing registration.
\nFields, models, model managers, and database backends all implement a\ncheck() method that is already registered with the check framework. If you\nwant to add extra checks, you can extend the implementation on the base class,\nperform any extra checks you need, and append any messages to those generated\nby the base class. It's recommended that you delegate each check to separate\nmethods.
Consider an example where you are implementing a custom field named\nRangedIntegerField. This field adds min and max arguments to the\nconstructor of IntegerField. You may want to add a check to ensure that users\nprovide a min value that is less than or equal to the max value. The following\ncode snippet shows how you can implement this check:
from django.core import checks\nfrom django.db import models\n\nclass RangedIntegerField(models.IntegerField):\n def __init__(self, min=None, max=None, **kwargs):\n super(RangedIntegerField, self).__init__(**kwargs)\n self.min = min\n self.max = max\n\n def check(self, **kwargs):\n # Call the superclass\n errors = super(RangedIntegerField, self).check(**kwargs)\n\n # Do some custom checks and add messages to `errors`:\n errors.extend(self._check_min_max_values(**kwargs))\n\n # Return all errors and warnings\n return errors\n\n def _check_min_max_values(self, **kwargs):\n if (self.min is not None and\n self.max is not None and\n self.min > self.max):\n return [\n checks.Error(\n 'min greater than max.',\n hint='Decrease min or increase max.',\n obj=self,\n id='myapp.E001',\n )\n ]\n # When no error, return an empty list\n return []\nIf you wanted to add checks to a model manager, you would take the same\napproach on your subclass of Manager.
If you want to add a check to a model class, the approach is almost the same:\nthe only difference is that the check is a classmethod, not an instance method:
\nclass MyModel(models.Model):\n @classmethod\n def check(cls, **kwargs):\n errors = super(MyModel, cls).check(**kwargs)\n # ... your own checks ...\n return errors\nMessages are comparable. That allows you to easily write tests:
\nfrom django.core.checks import Error\nerrors = checked_object.check()\nexpected_errors = [\n Error(\n 'an error',\n hint='A hint.',\n obj=checked_object,\n id='myapp.E001',\n )\n]\nself.assertEqual(errors, expected_errors)\n