======================
System check framework
======================

.. module:: django.core.checks

The system check framework is a set of static checks for validating Django
projects. It detects common problems and provides hints for how to fix them.
The framework is extensible so you can easily add your own checks.

Checks can be triggered explicitly via the :djadmin:`check` command. Checks are
triggered implicitly before most commands, including :djadmin:`runserver` and
:djadmin:`migrate`. For performance reasons, checks are not run as part of the
WSGI stack that is used in deployment. If you need to run system checks on your
deployment server, trigger them explicitly using :djadmin:`check`.

Serious errors will prevent Django commands (such as :djadmin:`runserver`) from
running at all. Minor problems are reported to the console. If you have inspected
the cause of a warning and are happy to ignore it, you can hide specific warnings
using the :setting:`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
:doc:`System check reference </ref/checks>`.

Writing your own checks
=======================

The framework is flexible and allows you to write functions that perform
any other kind of check you may require. The following is an example stub
check function::

    from django.core.checks import Error, register

    @register()
    def example_check(app_configs, **kwargs):
        errors = []
        # ... your check logic here
        if check_failed:
            errors.append(
                Error(
                    'an error',
                    hint='A hint.',
                    obj=checked_object,
                    id='myapp.E001',
                )
            )
        return errors

The check function *must* accept an ``app_configs`` argument; this argument is
the list of applications that should be inspected. If ``None``, the check must
be run on *all* installed apps in the project.

The check will receive a ``databases`` keyword argument. This is a list of
database aliases whose connections may be used to inspect database level
configuration. If ``databases`` is ``None``, the check must not use any
database connections.

The ``**kwargs`` argument is required for future expansion.

Messages
--------

The function must return a list of messages. If no problems are found as a result
of the check, the check function must return an empty list.

The warnings and errors raised by the check method must be instances of
:class:`~django.core.checks.CheckMessage`. An instance of
:class:`~django.core.checks.CheckMessage` encapsulates a single reportable
error or warning. It also provides context and hints applicable to the
message, and a unique identifier that is used for filtering purposes.

The concept is very similar to messages from the :doc:`message framework
</ref/contrib/messages>` or the :doc:`logging framework </topics/logging>`.
Messages are tagged with a ``level`` indicating the severity of the message.

There are also shortcuts to make creating messages with common levels easier.
When using these classes you can omit the ``level`` argument because it is
implied by the class name.

* :class:`Debug`
* :class:`Info`
* :class:`Warning`
* :class:`Error`
* :class:`Critical`

.. _registering-labeling-checks:

Registering and labeling checks
-------------------------------

Lastly, your check function must be registered explicitly with system check
registry. Checks should be registered in a file that's loaded when your
application is loaded; for example, in the :meth:`AppConfig.ready()
<django.apps.AppConfig.ready>` method.

.. function:: register(*tags)(function)

You can pass as many tags to ``register`` as you want in order to label your
check. Tagging checks is useful since it allows you to run only a certain
group of checks. For example, to register a compatibility check, you would
make the following call::

    from django.core.checks import register, Tags

    @register(Tags.compatibility)
    def my_check(app_configs, **kwargs):
        # ... perform compatibility checks and collect errors
        return errors

You can register "deployment checks" that are only relevant to a production
settings file like this::

    @register(Tags.security, deploy=True)
    def my_check(app_configs, **kwargs):
        ...

These checks will only be run if the :option:`check --deploy` option is used.

You can also use ``register`` as a function rather than a decorator by
passing a callable object (usually a function) as the first argument
to ``register``.

The code below is equivalent to the code above::

    def my_check(app_configs, **kwargs):
        ...
    register(my_check, Tags.security, deploy=True)

.. _field-checking:

Field, model, manager, and database checks
------------------------------------------

In some cases, you won't need to register your check function -- you can
piggyback on an existing registration.

Fields, models, model managers, and database backends all implement a
``check()`` method that is already registered with the check framework. If you
want to add extra checks, you can extend the implementation on the base class,
perform any extra checks you need, and append any messages to those generated
by the base class. It's recommended that you delegate each check to separate
methods.

Consider an example where you are implementing a custom field named
``RangedIntegerField``. This field adds ``min`` and ``max`` arguments to the
constructor of ``IntegerField``. You may want to add a check to ensure that users
provide a min value that is less than or equal to the max value. The following
code snippet shows how you can implement this check::

    from django.core import checks
    from django.db import models

    class RangedIntegerField(models.IntegerField):
        def __init__(self, min=None, max=None, **kwargs):
            super().__init__(**kwargs)
            self.min = min
            self.max = max

        def check(self, **kwargs):
            # Call the superclass
            errors = super().check(**kwargs)

            # Do some custom checks and add messages to `errors`:
            errors.extend(self._check_min_max_values(**kwargs))

            # Return all errors and warnings
            return errors

        def _check_min_max_values(self, **kwargs):
            if (self.min is not None and
                    self.max is not None and
                    self.min > self.max):
                return [
                    checks.Error(
                        'min greater than max.',
                        hint='Decrease min or increase max.',
                        obj=self,
                        id='myapp.E001',
                    )
                ]
            # When no error, return an empty list
            return []

If you wanted to add checks to a model manager, you would take the same
approach on your subclass of :class:`~django.db.models.Manager`.

If you want to add a check to a model class, the approach is *almost* the same:
the only difference is that the check is a classmethod, not an instance method::

    class MyModel(models.Model):
        @classmethod
        def check(cls, **kwargs):
            errors = super().check(**kwargs)
            # ... your own checks ...
            return errors

Writing tests
-------------

Messages are comparable. That allows you to easily write tests::

    from django.core.checks import Error
    errors = checked_object.check()
    expected_errors = [
        Error(
            'an error',
            hint='A hint.',
            obj=checked_object,
            id='myapp.E001',
        )
    ]
    self.assertEqual(errors, expected_errors)
