django/docs/topics/checks.txt

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

.. versionadded:: 1.7

.. 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, the checks are not performed if
:setting:`DEBUG` is set to ``False``.

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 register

    @register()
    def example_check(app_configs, **kwargs):
        errors = []
        # ... your check logic here
        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 ``**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.

.. class:: CheckMessage(level, msg, hint, obj=None, id=None)

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.

Constructor arguments are:

``level``
    The severity of the message. Use one of the
    predefined values: ``DEBUG``, ``INFO``, ``WARNING``, ``ERROR``,
    ``CRITICAL``. If the level is greater or equal to ``ERROR``, then Django
    will prevent management commands from executing. Messages with
    level lower than ``ERROR`` (i.e. warnings) are reported to the console,
    but can be silenced.

``msg``
    A short (less than 80 characters) string describing the problem. The string
    should *not* contain newlines.

``hint``
    A single-line string providing a hint for fixing the problem. If no hint
    can be provided, or the hint is self-evident from the error message, a
    value of ``None`` can be used::

        Error('error message') # Will not work.
        Error('error message', None) # Good
        Error('error message', hint=None) # Better

``obj``
    Optional. An object providing context for the message (for example, the
    model where the problem was discovered). The object should be a model, field,
    or manager or any other object that defines ``__str__`` method (on
    Python 2 you need to define ``__unicode__`` method). The method is used while
    reporting all messages and its result precedes the message.

``id``
    Optional string. A unique identifier for the issue. Identifiers should
    follow the pattern ``applabel.X001``, where ``X`` is one of the letters
    ``CEWID``, indicating the message severity (``C`` for criticals,
    ``E`` for errors and so). The number can be allocated by the application,
    but should be unique within that application.

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

.. class:: Debug(msg, hint, obj=None, id=None)
.. class:: Info(msg, hint, obj=None, id=None)
.. class:: Warning(msg, hint, obj=None, id=None)
.. class:: Error(msg, hint, obj=None, id=None)
.. class:: Critical(msg, hint, obj=None, id=None)

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=None,
            obj=checked_object,
            id='myapp.E001',
        )
    ]
    self.assertEqual(errors, expected_errors)

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

Lastly, your check function must be registered explicitly with system check
registry.

.. 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

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

.. _field-checking:

Field, Model, and Manager checks
--------------------------------

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

Fields, models, and model managers 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 the you delegate each check to a 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(RangedIntegerField, self).__init__(**kwargs)
            self.min = min
            self.max = max

        def check(self, **kwargs):
            # Call the superclass
            errors = super(RangedIntegerField, self).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(MyModel, cls).check(**kwargs)
            # ... your own checks ...
            return errors