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

.. currentmodule:: 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.

For details on how to add your own checks and integrate them with Django's
system checks, see the :doc:`System check topic guide </topics/checks>`.

API reference
=============

``CheckMessage``
----------------

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

The warnings and errors raised by system checks must be instances of
``CheckMessage``. An instance 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.

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, the
    hint can be omitted, or a value of ``None`` can be used.

``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 a ``__str__()`` 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 subclasses to make creating messages with common levels easier. When
using them you can omit the ``level`` argument because it is implied by the
class name.

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

.. _system-check-builtin-tags:

Builtin tags
============

Django's system checks are organized using the following tags:

* ``admin``: Checks of any admin site declarations.
* ``async_support``: Checks asynchronous-related configuration.
* ``caches``: Checks cache related configuration.
* ``compatibility``: Flags potential problems with version upgrades.
* ``database``: Checks database-related configuration issues. Database checks
  are not run by default because they do more than static code analysis as
  regular checks do. They are only run by the :djadmin:`migrate` command or if
  you specify configured database aliases using the ``--database`` option when
  calling the :djadmin:`check` command.
* ``files``: Checks files related configuration.
* ``models``: Checks of model, field, and manager definitions.
* ``security``: Checks security related configuration.
* ``signals``: Checks on signal declarations and handler registrations.
* ``sites``: Checks :mod:`django.contrib.sites` configuration.
* ``staticfiles``: Checks :mod:`django.contrib.staticfiles` configuration.
* ``templates``: Checks template related configuration.
* ``translation``: Checks translation related configuration.
* ``urls``: Checks URL configuration.

Some checks may be registered with multiple tags.

.. versionchanged:: 4.0

    The ``files`` tag was added.

Core system checks
==================

Asynchronous support
--------------------

The following checks verify your setup for :doc:`/topics/async`:

* **async.E001**: You should not set the :envvar:`DJANGO_ALLOW_ASYNC_UNSAFE`
  environment variable in deployment. This disables :ref:`async safety
  protection <async-safety>`.

Backwards compatibility
-----------------------

Compatibility checks warn of potential problems that might occur after
upgrading Django.

* **2_0.W001**: Your URL pattern ``<pattern>`` has a ``route`` that contains
  ``(?P<``, begins with a ``^``, or ends with a ``$``. This was likely an
  oversight when migrating from ``url()`` to :func:`~django.urls.path`.
* **4_0.E001**: As of Django 4.0, the values in the
  :setting:`CSRF_TRUSTED_ORIGINS` setting must start with a scheme (usually
  ``http://`` or ``https://``) but found ``<hostname>``.

Caches
------

The following checks verify that your :setting:`CACHES` setting is correctly
configured:

* **caches.E001**: You must define a ``'default'`` cache in your
  :setting:`CACHES` setting.
* **caches.W002**: Your ``<cache>`` configuration might expose your cache or
  lead to corruption of your data because its
  :setting:`LOCATION <CACHES-LOCATION>` matches/is inside/contains
  :setting:`MEDIA_ROOT`/:setting:`STATIC_ROOT`/:setting:`STATICFILES_DIRS`.
* **caches.W003**: Your ``<cache>`` cache :setting:`LOCATION <CACHES-LOCATION>`
  is relative. Use an absolute path instead.

Database
--------

MySQL and MariaDB
~~~~~~~~~~~~~~~~~

If you're using MySQL or MariaDB, the following checks will be performed:

* **mysql.E001**: MySQL/MariaDB does not allow unique ``CharField``\s to have a
  ``max_length`` > 255. *This check was changed to* ``mysql.W003`` *in Django
  3.1 as the real maximum size depends on many factors.*
* **mysql.W002**: MySQL/MariaDB Strict Mode is not set for database connection
  ``<alias>``. See also :ref:`mysql-sql-mode`.
* **mysql.W003**: MySQL/MariaDB may not allow unique ``CharField``\s to have a
  ``max_length`` > 255.

Managing files
--------------

.. versionadded:: 4.0

The following checks verify your setup for :doc:`/topics/files`:

* **files.E001**: The :setting:`FILE_UPLOAD_TEMP_DIR` setting refers to the
  nonexistent directory ``<path>``.

Model fields
------------

* **fields.E001**: Field names must not end with an underscore.
* **fields.E002**: Field names must not contain ``"__"``.
* **fields.E003**: ``pk`` is a reserved word that cannot be used as a field
  name.
* **fields.E004**: ``choices`` must be an iterable (e.g., a list or tuple).
* **fields.E005**: ``choices`` must be an iterable containing ``(actual value,
  human readable name)`` tuples.
* **fields.E006**: ``db_index`` must be ``None``, ``True`` or ``False``.
* **fields.E007**: Primary keys must not have ``null=True``.
* **fields.E008**: All ``validators`` must be callable.
* **fields.E009**: ``max_length`` is too small to fit the longest value in
  ``choices`` (``<count>`` characters).
* **fields.E010**: ``<field>`` default should be a callable instead of an
  instance so that it's not shared between all field instances.
* **fields.E100**: ``AutoField``\s must set primary_key=True.
* **fields.E110**: ``BooleanField``\s do not accept null values. *This check
  appeared before support for null values was added in Django 2.1.*
* **fields.E120**: ``CharField``\s must define a ``max_length`` attribute.
* **fields.E121**: ``max_length`` must be a positive integer.
* **fields.W122**: ``max_length`` is ignored when used with
  ``<integer field type>``.
* **fields.E130**: ``DecimalField``\s must define a ``decimal_places`` attribute.
* **fields.E131**: ``decimal_places`` must be a non-negative integer.
* **fields.E132**: ``DecimalField``\s must define a ``max_digits`` attribute.
* **fields.E133**: ``max_digits`` must be a positive integer.
* **fields.E134**: ``max_digits`` must be greater or equal to ``decimal_places``.
* **fields.E140**: ``FilePathField``\s must have either ``allow_files`` or
  ``allow_folders`` set to True.
* **fields.E150**: ``GenericIPAddressField``\s cannot have ``blank=True`` if
  ``null=False``, as blank values are stored as nulls.
* **fields.E160**: The options ``auto_now``, ``auto_now_add``, and ``default``
  are mutually exclusive. Only one of these options may be present.
* **fields.W161**: Fixed default value provided.
* **fields.W162**: ``<database>`` does not support a database index on
  ``<field data type>`` columns.
* **fields.E170**: ``BinaryField``’s ``default`` cannot be a string. Use bytes
  content instead.
* **fields.E180**: ``<database>`` does not support ``JSONField``\s.
* **fields.E190**: ``<database>`` does not support a database collation on
  ``<field_type>``\s.
* **fields.E900**: ``IPAddressField`` has been removed except for support in
  historical migrations.
* **fields.W900**: ``IPAddressField`` has been deprecated. Support for it
  (except in historical migrations) will be removed in Django 1.9. *This check
  appeared in Django 1.7 and 1.8*.
* **fields.W901**: ``CommaSeparatedIntegerField`` has been deprecated. Support
  for it (except in historical migrations) will be removed in Django 2.0. *This
  check appeared in Django 1.10 and 1.11*.
* **fields.E901**: ``CommaSeparatedIntegerField`` is removed except for support
  in historical migrations.
* **fields.W902**: ``FloatRangeField`` is deprecated and will be removed in
  Django 3.1. *This check appeared in Django 2.2 and 3.0*.
* **fields.W903**: ``NullBooleanField`` is deprecated. Support for it (except
  in historical migrations) will be removed in Django 4.0. *This check appeared
  in Django 3.1 and 3.2*.
* **fields.E903**: ``NullBooleanField`` is removed except for support in
  historical migrations.
* **fields.W904**: ``django.contrib.postgres.fields.JSONField`` is deprecated.
  Support for it (except in historical migrations) will be removed in Django
  4.0. *This check appeared in Django 3.1 and 3.2*.
* **fields.E904**: ``django.contrib.postgres.fields.JSONField`` is removed
  except for support in historical migrations.

File fields
~~~~~~~~~~~

* **fields.E200**: ``unique`` is not a valid argument for a ``FileField``.
  *This check is removed in Django 1.11*.
* **fields.E201**: ``primary_key`` is not a valid argument for a ``FileField``.
* **fields.E202**: ``FileField``’s ``upload_to`` argument must be a relative
  path, not an absolute path.
* **fields.E210**: Cannot use ``ImageField`` because Pillow is not installed.

Related fields
~~~~~~~~~~~~~~

* **fields.E300**: Field defines a relation with model ``<model>``, which is
  either not installed, or is abstract.
* **fields.E301**: Field defines a relation with the model
  ``<app_label>.<model>`` which has been swapped out.
* **fields.E302**: Reverse accessor ``<related model>.<accessor name>`` for
  ``<app_label>.<model>.<field name>`` clashes with field name
  ``<app_label>.<model>.<field name>``.
* **fields.E303**: Reverse query name for ``<app_label>.<model>.<field name>``
  clashes with field name ``<app_label>.<model>.<field name>``.
* **fields.E304**: Reverse accessor ``<related model>.<accessor name>`` for
  ``<app_label>.<model>.<field name>`` clashes with reverse accessor for
  ``<app_label>.<model>.<field name>``.
* **fields.E305**: Reverse query name for ``<app_label>.<model>.<field name>``
  clashes with reverse query name for ``<app_label>.<model>.<field name>``.
* **fields.E306**: The name ``<name>`` is invalid ``related_name`` for field
  ``<model>.<field name>``.
* **fields.E307**: The field ``<app label>.<model>.<field name>`` was declared
  with a lazy reference to ``<app label>.<model>``, but app ``<app label>``
  isn't installed or doesn't provide model ``<model>``.
* **fields.E308**: Reverse query name ``<related query name>`` must not end
  with an underscore.
* **fields.E309**: Reverse query name ``<related query name>`` must not contain
  ``'__'``.
* **fields.E310**: No subset of the fields ``<field1>``, ``<field2>``, ... on
  model ``<model>`` is unique.
* **fields.E311**: ``<model>.<field name>`` must be unique because it is
  referenced by a ``ForeignKey``.
* **fields.E312**: The ``to_field`` ``<field name>`` doesn't exist on the
  related model ``<app label>.<model>``.
* **fields.E320**: Field specifies ``on_delete=SET_NULL``, but cannot be null.
* **fields.E321**: The field specifies ``on_delete=SET_DEFAULT``, but has no
  default value.
* **fields.E330**: ``ManyToManyField``\s cannot be unique.
* **fields.E331**: Field specifies a many-to-many relation through model
  ``<model>``, which has not been installed.
* **fields.E332**: Many-to-many fields with intermediate tables must not be
  symmetrical. *This check appeared before Django 3.0.*
* **fields.E333**: The model is used as an intermediate model by ``<model>``,
  but it has more than two foreign keys to ``<model>``, which is ambiguous.
  You must specify which two foreign keys Django should use via the
  ``through_fields`` keyword argument.
* **fields.E334**: The model is used as an intermediate model by ``<model>``,
  but it has more than one foreign key from ``<model>``, which is ambiguous.
  You must specify which foreign key Django should use via the
  ``through_fields`` keyword argument.
* **fields.E335**: The model is used as an intermediate model by ``<model>``,
  but it has more than one foreign key to ``<model>``, which is ambiguous.
  You must specify which foreign key Django should use via the
  ``through_fields`` keyword argument.
* **fields.E336**: The model is used as an intermediary model by ``<model>``,
  but it does not have foreign key to ``<model>`` or ``<model>``.
* **fields.E337**: Field specifies ``through_fields`` but does not provide the
  names of the two link fields that should be used for the relation through
  ``<model>``.
* **fields.E338**: The intermediary model ``<through model>`` has no field
  ``<field name>``.
* **fields.E339**: ``<model>.<field name>`` is not a foreign key to ``<model>``.
* **fields.E340**: The field's intermediary table ``<table name>`` clashes with
  the table name of ``<model>``/``<model>.<field name>``.
* **fields.W340**: ``null`` has no effect on ``ManyToManyField``.
* **fields.W341**: ``ManyToManyField`` does not support ``validators``.
* **fields.W342**: Setting ``unique=True`` on a ``ForeignKey`` has the same
  effect as using a ``OneToOneField``.
* **fields.W343**: ``limit_choices_to`` has no effect on ``ManyToManyField``
  with a ``through`` model. *This check appeared before Django 4.0.*
* **fields.W344**: The field's intermediary table ``<table name>`` clashes with
  the table name of ``<model>``/``<model>.<field name>``.
* **fields.W345**: ``related_name`` has no effect on ``ManyToManyField`` with a
  symmetrical relationship, e.g. to "self".

Models
------

* **models.E001**: ``<swappable>`` is not of the form ``app_label.app_name``.
* **models.E002**: ``<SETTING>`` references ``<model>``, which has not been
  installed, or is abstract.
* **models.E003**: The model has two identical many-to-many relations through
  the intermediate model ``<app_label>.<model>``.
* **models.E004**: ``id`` can only be used as a field name if the field also
  sets ``primary_key=True``.
* **models.E005**: The field ``<field name>`` from parent model ``<model>``
  clashes with the field ``<field name>`` from parent model ``<model>``.
* **models.E006**: The field ``<field name>`` clashes with the field
  ``<field name>`` from model ``<model>``.
* **models.E007**: Field ``<field name>`` has column name ``<column name>``
  that is used by another field.
* **models.E008**: ``index_together`` must be a list or tuple.
* **models.E009**: All ``index_together`` elements must be lists or tuples.
* **models.E010**: ``unique_together`` must be a list or tuple.
* **models.E011**: All ``unique_together`` elements must be lists or tuples.
* **models.E012**: ``constraints/indexes/index_together/unique_together``
  refers to the nonexistent field ``<field name>``.
* **models.E013**: ``constraints/indexes/index_together/unique_together``
  refers to a ``ManyToManyField`` ``<field name>``, but ``ManyToManyField``\s
  are not supported for that option.
* **models.E014**: ``ordering`` must be a tuple or list (even if you want to
  order by only one field).
* **models.E015**: ``ordering`` refers to the nonexistent field, related field,
  or lookup ``<field name>``.
* **models.E016**: ``constraints/indexes/index_together/unique_together``
  refers to field ``<field_name>`` which is not local to model ``<model>``.
* **models.E017**: Proxy model ``<model>`` contains model fields.
* **models.E018**: Autogenerated column name too long for field ``<field>``.
  Maximum length is ``<maximum length>`` for database ``<alias>``.
* **models.E019**: Autogenerated column name too long for M2M field
  ``<M2M field>``. Maximum length is ``<maximum length>`` for database
  ``<alias>``.
* **models.E020**: The ``<model>.check()`` class method is currently overridden.
* **models.E021**: ``ordering`` and ``order_with_respect_to`` cannot be used
  together.
* **models.E022**: ``<function>`` contains a lazy reference to
  ``<app label>.<model>``, but app ``<app label>`` isn't installed or
  doesn't provide model ``<model>``.
* **models.E023**: The model name ``<model>`` cannot start or end with an
  underscore as it collides with the query lookup syntax.
* **models.E024**: The model name ``<model>`` cannot contain double underscores
  as it collides with the query lookup syntax.
* **models.E025**: The property ``<property name>`` clashes with a related
  field accessor.
* **models.E026**: The model cannot have more than one field with
  ``primary_key=True``.
* **models.W027**: ``<database>`` does not support check constraints.
* **models.E028**: ``db_table`` ``<db_table>`` is used by multiple models:
  ``<model list>``.
* **models.E029**: index name ``<index>`` is not unique for model ``<model>``.
* **models.E030**: index name ``<index>`` is not unique among models:
  ``<model list>``.
* **models.E031**: constraint name ``<constraint>`` is not unique for model
  ``<model>``.
* **models.E032**: constraint name ``<constraint>`` is not unique among
  models: ``<model list>``.
* **models.E033**: The index name ``<index>`` cannot start with an underscore
  or a number.
* **models.E034**: The index name ``<index>`` cannot be longer than
  ``<max_length>`` characters.
* **models.W035**: ``db_table`` ``<db_table>`` is used by multiple models:
  ``<model list>``.
* **models.W036**: ``<database>`` does not support unique constraints with
  conditions.
* **models.W037**: ``<database>`` does not support indexes with conditions.
* **models.W038**: ``<database>`` does not support deferrable unique
  constraints.
* **models.W039**: ``<database>`` does not support unique constraints with
  non-key columns.
* **models.W040**: ``<database>`` does not support indexes with non-key
  columns.
* **models.E041**: ``constraints`` refers to the joined field ``<field name>``.
* **models.W042**: Auto-created primary key used when not defining a primary
  key type, by default ``django.db.models.AutoField``.
* **models.W043**: ``<database>`` does not support indexes on expressions.
* **models.W044**: ``<database>`` does not support unique constraints on
  expressions.

Security
--------

The security checks do not make your site secure. They do not audit code, do
intrusion detection, or do anything particularly complex. Rather, they help
perform an automated, low-hanging-fruit checklist, that can help you to improve
your site's security.

Some of these checks may not be appropriate for your particular deployment
configuration. For instance, if you do your HTTP to HTTPS redirection in a load
balancer, it'd be irritating to be constantly warned about not having enabled
:setting:`SECURE_SSL_REDIRECT`. Use :setting:`SILENCED_SYSTEM_CHECKS` to
silence unneeded checks.

The following checks are run if you use the :option:`check --deploy` option:

* **security.W001**: You do not have
  :class:`django.middleware.security.SecurityMiddleware` in your
  :setting:`MIDDLEWARE` so the :setting:`SECURE_HSTS_SECONDS`,
  :setting:`SECURE_CONTENT_TYPE_NOSNIFF`, :setting:`SECURE_REFERRER_POLICY`,
  :setting:`SECURE_CROSS_ORIGIN_OPENER_POLICY`, and
  :setting:`SECURE_SSL_REDIRECT` settings will have no effect.
* **security.W002**: You do not have
  :class:`django.middleware.clickjacking.XFrameOptionsMiddleware` in your
  :setting:`MIDDLEWARE`, so your pages will not be served with an
  ``'x-frame-options'`` header. Unless there is a good reason for your
  site to be served in a frame, you should consider enabling this
  header to help prevent clickjacking attacks.
* **security.W003**: You don't appear to be using Django's built-in cross-site
  request forgery protection via the middleware
  (:class:`django.middleware.csrf.CsrfViewMiddleware` is not in your
  :setting:`MIDDLEWARE`). Enabling the middleware is the safest
  approach to ensure you don't leave any holes.
* **security.W004**: You have not set a value for the
  :setting:`SECURE_HSTS_SECONDS` setting. If your entire site is served only
  over SSL, you may want to consider setting a value and enabling :ref:`HTTP
  Strict Transport Security <http-strict-transport-security>`. Be sure to read
  the documentation first; enabling HSTS carelessly can cause serious,
  irreversible problems.
* **security.W005**: You have not set the
  :setting:`SECURE_HSTS_INCLUDE_SUBDOMAINS` setting to ``True``. Without this,
  your site is potentially vulnerable to attack via an insecure connection to a
  subdomain. Only set this to ``True`` if you are certain that all subdomains of
  your domain should be served exclusively via SSL.
* **security.W006**: Your :setting:`SECURE_CONTENT_TYPE_NOSNIFF` setting is not
  set to ``True``, so your pages will not be served with an
  ``'X-Content-Type-Options: nosniff'`` header. You should consider enabling
  this header to prevent the browser from identifying content types incorrectly.
* **security.W007**: Your ``SECURE_BROWSER_XSS_FILTER`` setting is not
  set to ``True``, so your pages will not be served with an
  ``'X-XSS-Protection: 1; mode=block'`` header. You should consider enabling
  this header to activate the browser's XSS filtering and help prevent XSS
  attacks. *This check is removed in Django 3.0 as the* ``X-XSS-Protection``
  *header is no longer honored by modern browsers.*
* **security.W008**: Your :setting:`SECURE_SSL_REDIRECT` setting is not set to
  ``True``. Unless your site should be available over both SSL and non-SSL
  connections, you may want to either set this setting to ``True`` or configure
  a load balancer or reverse-proxy server  to redirect all connections to HTTPS.
* **security.W009**: Your :setting:`SECRET_KEY` has less than 50 characters,
  less than 5 unique characters, or it's prefixed with ``'django-insecure-'``
  indicating that it was generated automatically by Django. Please generate a
  long and random value, otherwise many of Django's security-critical features
  will be vulnerable to attack.
* **security.W010**: You have :mod:`django.contrib.sessions` in your
  :setting:`INSTALLED_APPS` but you have not set
  :setting:`SESSION_COOKIE_SECURE` to ``True``. Using a secure-only session
  cookie makes it more difficult for network traffic sniffers to hijack user
  sessions.
* **security.W011**: You have
  :class:`django.contrib.sessions.middleware.SessionMiddleware` in your
  :setting:`MIDDLEWARE`, but you have not set :setting:`SESSION_COOKIE_SECURE`
  to ``True``. Using a secure-only session cookie makes it more difficult for
  network traffic sniffers to hijack user sessions.
* **security.W012**: :setting:`SESSION_COOKIE_SECURE` is not set to ``True``.
  Using a secure-only session cookie makes it more difficult for network traffic
  sniffers to hijack user sessions.
* **security.W013**: You have :mod:`django.contrib.sessions` in your
  :setting:`INSTALLED_APPS`, but you have not set
  :setting:`SESSION_COOKIE_HTTPONLY` to ``True``. Using an ``HttpOnly`` session
  cookie makes it more difficult for cross-site scripting attacks to hijack user
  sessions.
* **security.W014**: You have
  :class:`django.contrib.sessions.middleware.SessionMiddleware` in your
  :setting:`MIDDLEWARE`, but you have not set :setting:`SESSION_COOKIE_HTTPONLY`
  to ``True``. Using an ``HttpOnly`` session cookie makes it more difficult for
  cross-site scripting attacks to hijack user sessions.
* **security.W015**: :setting:`SESSION_COOKIE_HTTPONLY` is not set to ``True``.
  Using an ``HttpOnly`` session cookie makes it more difficult for cross-site
  scripting attacks to hijack user sessions.
* **security.W016**: :setting:`CSRF_COOKIE_SECURE` is not set to ``True``.
  Using a secure-only CSRF cookie makes it more difficult for network traffic
  sniffers to steal the CSRF token.
* **security.W017**: :setting:`CSRF_COOKIE_HTTPONLY` is not set to ``True``.
  Using an ``HttpOnly`` CSRF cookie makes it more difficult for cross-site
  scripting attacks to steal the CSRF token. *This check is removed in Django
  1.11 as the* :setting:`CSRF_COOKIE_HTTPONLY` *setting offers no practical
  benefit.*
* **security.W018**: You should not have :setting:`DEBUG` set to ``True`` in
  deployment.
* **security.W019**: You have
  :class:`django.middleware.clickjacking.XFrameOptionsMiddleware` in your
  :setting:`MIDDLEWARE`, but :setting:`X_FRAME_OPTIONS` is not set to
  ``'DENY'``. Unless there is a good reason for your site to serve other parts
  of itself in a frame, you should change it to ``'DENY'``.
* **security.W020**: :setting:`ALLOWED_HOSTS` must not be empty in deployment.
* **security.W021**: You have not set the
  :setting:`SECURE_HSTS_PRELOAD` setting to ``True``. Without this, your site
  cannot be submitted to the browser preload list.
* **security.W022**: You have not set the :setting:`SECURE_REFERRER_POLICY`
  setting. Without this, your site will not send a Referrer-Policy header. You
  should consider enabling this header to protect user privacy.
* **security.E023**: You have set the :setting:`SECURE_REFERRER_POLICY` setting
  to an invalid value.
* **security.E024**: You have set the
  :setting:`SECURE_CROSS_ORIGIN_OPENER_POLICY` setting to an invalid value.
* **security.W025**: Your
  :setting:`SECRET_KEY_FALLBACKS[n] <SECRET_KEY_FALLBACKS>` has less than 50
  characters, less than 5 unique characters, or it's prefixed with
  ``'django-insecure-'`` indicating that it was generated automatically by
  Django. Please generate a long and random value, otherwise many of Django's
  security-critical features will be vulnerable to attack.

The following checks verify that your security-related settings are correctly
configured:

* **security.E100**: ``DEFAULT_HASHING_ALGORITHM`` must be ``'sha1'`` or
  ``'sha256'``. *This check appeared in Django 3.1 and 3.2*.
* **security.E101**: The CSRF failure view ``'path.to.view'`` does not take the
  correct number of arguments.
* **security.E102**: The CSRF failure view ``'path.to.view'`` could not be
  imported.

Signals
-------

* **signals.E001**: ``<handler>`` was connected to the ``<signal>`` signal with
  a lazy reference to the sender ``<app label>.<model>``, but app ``<app label>``
  isn't installed or doesn't provide model ``<model>``.

Templates
---------

The following checks verify that your :setting:`TEMPLATES` setting is correctly
configured:

* **templates.E001**: You have ``'APP_DIRS': True`` in your
  :setting:`TEMPLATES` but also specify ``'loaders'`` in ``OPTIONS``. Either
  remove ``APP_DIRS`` or remove the ``'loaders'`` option.
* **templates.E002**: ``string_if_invalid`` in :setting:`TEMPLATES`
  :setting:`OPTIONS <TEMPLATES-OPTIONS>` must be a string but got: ``{value}``
  (``{type}``).
* **templates.E003**:``<name>`` is used for multiple template tag modules:
  ``<module list>``.

Translation
-----------

The following checks are performed on your translation configuration:

* **translation.E001**: You have provided an invalid value for the
  :setting:`LANGUAGE_CODE` setting: ``<value>``.
* **translation.E002**: You have provided an invalid language code in the
  :setting:`LANGUAGES` setting: ``<value>``.
* **translation.E003**: You have provided an invalid language code in the
  :setting:`LANGUAGES_BIDI` setting: ``<value>``.
* **translation.E004**: You have provided a value for the
  :setting:`LANGUAGE_CODE` setting that is not in the :setting:`LANGUAGES`
  setting.

URLs
----

The following checks are performed on your URL configuration:

* **urls.W001**: Your URL pattern ``<pattern>`` uses
  :func:`~django.urls.include` with a ``route`` ending with a ``$``. Remove the
  dollar from the ``route`` to avoid problems including URLs.
* **urls.W002**: Your URL pattern ``<pattern>`` has a ``route`` beginning with
  a ``/``. Remove this slash as it is unnecessary. If this pattern is targeted
  in an :func:`~django.urls.include`, ensure the :func:`~django.urls.include`
  pattern has a trailing ``/``.
* **urls.W003**: Your URL pattern ``<pattern>`` has a ``name``
  including a ``:``. Remove the colon, to avoid ambiguous namespace
  references.
* **urls.E004**: Your URL pattern ``<pattern>`` is invalid. Ensure that
  ``urlpatterns`` is a list of :func:`~django.urls.path` and/or
  :func:`~django.urls.re_path` instances.
* **urls.W005**: URL namespace ``<namespace>`` isn't unique. You may not be
  able to reverse all URLs in this namespace.
* **urls.E006**: The :setting:`MEDIA_URL`/ :setting:`STATIC_URL` setting must
  end with a slash.
* **urls.E007**: The custom ``handlerXXX`` view ``'path.to.view'`` does not
  take the correct number of arguments (…).
* **urls.E008**: The custom ``handlerXXX`` view ``'path.to.view'`` could not be
  imported.
* **urls.E009**: Your URL pattern ``<pattern>`` has an invalid view, pass
  ``<view>.as_view()`` instead of ``<view>``.

``contrib`` app checks
======================

``admin``
---------

Admin checks are all performed as part of the ``admin`` tag.

The following checks are performed on any
:class:`~django.contrib.admin.ModelAdmin` (or subclass) that is registered
with the admin site:

* **admin.E001**: The value of ``raw_id_fields`` must be a list or tuple.
* **admin.E002**: The value of ``raw_id_fields[n]`` refers to ``<field name>``,
  which is not a field of ``<model>``.
* **admin.E003**: The value of ``raw_id_fields[n]`` must be a foreign key or
  a many-to-many field.
* **admin.E004**: The value of ``fields`` must be a list or tuple.
* **admin.E005**: Both ``fieldsets`` and ``fields`` are specified.
* **admin.E006**: The value of ``fields`` contains duplicate field(s).
* **admin.E007**: The value of ``fieldsets`` must be a list or tuple.
* **admin.E008**: The value of ``fieldsets[n]`` must be a list or tuple.
* **admin.E009**: The value of ``fieldsets[n]`` must be of length 2.
* **admin.E010**: The value of ``fieldsets[n][1]`` must be a dictionary.
* **admin.E011**: The value of ``fieldsets[n][1]`` must contain the key
  ``fields``.
* **admin.E012**: There are duplicate field(s) in ``fieldsets[n][1]``.
* **admin.E013**: ``fields[n]/fieldsets[n][m]`` cannot include the
  ``ManyToManyField`` ``<field name>``, because that field manually specifies a
  relationship model.
* **admin.E014**: The value of ``exclude`` must be a list or tuple.
* **admin.E015**: The value of ``exclude`` contains duplicate field(s).
* **admin.E016**: The value of ``form`` must inherit from ``BaseModelForm``.
* **admin.E017**: The value of ``filter_vertical`` must be a list or tuple.
* **admin.E018**: The value of ``filter_horizontal`` must be a list or tuple.
* **admin.E019**: The value of ``filter_vertical[n]/filter_horizontal[n]``
  refers to ``<field name>``, which is not a field of ``<model>``.
* **admin.E020**: The value of ``filter_vertical[n]/filter_horizontal[n]``
  must be a many-to-many field.
* **admin.E021**: The value of ``radio_fields`` must be a dictionary.
* **admin.E022**: The value of ``radio_fields`` refers to ``<field name>``,
  which is not a field of ``<model>``.
* **admin.E023**: The value of ``radio_fields`` refers to ``<field name>``,
  which is not an instance of ``ForeignKey``, and does not have a ``choices``
  definition.
* **admin.E024**: The value of ``radio_fields[<field name>]`` must be either
  ``admin.HORIZONTAL`` or ``admin.VERTICAL``.
* **admin.E025**: The value of ``view_on_site`` must be either a callable or a
  boolean value.
* **admin.E026**: The value of ``prepopulated_fields`` must be a dictionary.
* **admin.E027**: The value of ``prepopulated_fields`` refers to
  ``<field name>``, which is not a field of ``<model>``.
* **admin.E028**: The value of ``prepopulated_fields`` refers to
  ``<field name>``, which must not be a ``DateTimeField``, a ``ForeignKey``,
  a ``OneToOneField``, or a ``ManyToManyField`` field.
* **admin.E029**: The value of ``prepopulated_fields[<field name>]`` must be a
  list or tuple.
* **admin.E030**: The value of ``prepopulated_fields`` refers to
  ``<field name>``, which is not a field of ``<model>``.
* **admin.E031**: The value of ``ordering`` must be a list or tuple.
* **admin.E032**: The value of ``ordering`` has the random ordering marker
  ``?``, but contains other fields as well.
* **admin.E033**: The value of ``ordering`` refers to ``<field name>``, which
  is not a field of ``<model>``.
* **admin.E034**: The value of ``readonly_fields`` must be a list or tuple.
* **admin.E035**: The value of ``readonly_fields[n]`` is not a callable, an
  attribute of ``<ModelAdmin class>``, or an attribute of ``<model>``.
* **admin.E036**: The value of ``autocomplete_fields`` must be a list or tuple.
* **admin.E037**: The value of ``autocomplete_fields[n]`` refers to
  ``<field name>``, which is not a field of ``<model>``.
* **admin.E038**: The value of ``autocomplete_fields[n]`` must be a foreign
  key or a many-to-many field.
* **admin.E039**: An admin for model ``<model>`` has to be registered to be
  referenced by ``<modeladmin>.autocomplete_fields``.
* **admin.E040**: ``<modeladmin>`` must define ``search_fields``, because
  it's referenced by ``<other_modeladmin>.autocomplete_fields``.

``ModelAdmin``
~~~~~~~~~~~~~~

The following checks are performed on any
:class:`~django.contrib.admin.ModelAdmin` that is registered
with the admin site:

* **admin.E101**: The value of ``save_as`` must be a boolean.
* **admin.E102**: The value of ``save_on_top`` must be a boolean.
* **admin.E103**: The value of ``inlines`` must be a list or tuple.
* **admin.E104**: ``<InlineModelAdmin class>`` must inherit from
  ``InlineModelAdmin``.
* **admin.E105**: ``<InlineModelAdmin class>`` must have a ``model`` attribute.
* **admin.E106**: The value of ``<InlineModelAdmin class>.model`` must be a
  ``Model``.
* **admin.E107**: The value of ``list_display`` must be a list or tuple.
* **admin.E108**: The value of ``list_display[n]`` refers to ``<label>``,
  which is not a callable, an attribute of ``<ModelAdmin class>``, or an
  attribute or method on ``<model>``.
* **admin.E109**: The value of ``list_display[n]`` must not be a
  ``ManyToManyField`` field.
* **admin.E110**: The value of ``list_display_links`` must be a list, a tuple,
  or ``None``.
* **admin.E111**: The value of ``list_display_links[n]`` refers to ``<label>``,
  which is not defined in ``list_display``.
* **admin.E112**: The value of ``list_filter`` must be a list or tuple.
* **admin.E113**: The value of ``list_filter[n]`` must inherit from
  ``ListFilter``.
* **admin.E114**: The value of ``list_filter[n]`` must not inherit from
  ``FieldListFilter``.
* **admin.E115**: The value of ``list_filter[n][1]`` must inherit from
  ``FieldListFilter``.
* **admin.E116**: The value of ``list_filter[n]`` refers to ``<label>``,
  which does not refer to a Field.
* **admin.E117**: The value of ``list_select_related`` must be a boolean,
  tuple or list.
* **admin.E118**: The value of ``list_per_page`` must be an integer.
* **admin.E119**: The value of ``list_max_show_all`` must be an integer.
* **admin.E120**: The value of ``list_editable`` must be a list or tuple.
* **admin.E121**: The value of ``list_editable[n]`` refers to ``<label>``,
  which is not a field of ``<model>``.
* **admin.E122**: The value of ``list_editable[n]`` refers to ``<label>``,
  which is not contained in ``list_display``.
* **admin.E123**: The value of ``list_editable[n]`` cannot be in both
  ``list_editable`` and ``list_display_links``.
* **admin.E124**: The value of ``list_editable[n]`` refers to the first field
  in ``list_display`` (``<label>``), which cannot be used unless
  ``list_display_links`` is set.
* **admin.E125**: The value of ``list_editable[n]`` refers to ``<field name>``,
  which is not editable through the admin.
* **admin.E126**: The value of ``search_fields`` must be a list or tuple.
* **admin.E127**: The value of ``date_hierarchy`` refers to ``<field name>``,
  which does not refer to a Field.
* **admin.E128**: The value of ``date_hierarchy`` must be a ``DateField`` or
  ``DateTimeField``.
* **admin.E129**: ``<modeladmin>`` must define a ``has_<foo>_permission()``
  method for the ``<action>`` action.
* **admin.E130**: ``__name__`` attributes of actions defined in
  ``<modeladmin>`` must be unique. Name ``<name>`` is not unique.

``InlineModelAdmin``
~~~~~~~~~~~~~~~~~~~~

The following checks are performed on any
:class:`~django.contrib.admin.InlineModelAdmin` that is registered as an
inline on a :class:`~django.contrib.admin.ModelAdmin`.

* **admin.E201**: Cannot exclude the field ``<field name>``, because it is the
  foreign key to the parent model ``<app_label>.<model>``.
* **admin.E202**: ``<model>`` has no ``ForeignKey`` to ``<parent model>``./
  ``<model>`` has more than one ``ForeignKey`` to ``<parent model>``. You must
  specify a ``fk_name`` attribute.
* **admin.E203**: The value of ``extra`` must be an integer.
* **admin.E204**: The value of ``max_num`` must be an integer.
* **admin.E205**: The value of ``min_num`` must be an integer.
* **admin.E206**: The value of ``formset`` must inherit from
  ``BaseModelFormSet``.

``GenericInlineModelAdmin``
~~~~~~~~~~~~~~~~~~~~~~~~~~~

The following checks are performed on any
:class:`~django.contrib.contenttypes.admin.GenericInlineModelAdmin` that is
registered as an inline on a :class:`~django.contrib.admin.ModelAdmin`.

* **admin.E301**: ``'ct_field'`` references ``<label>``, which is not a field
  on ``<model>``.
* **admin.E302**: ``'ct_fk_field'`` references ``<label>``, which is not a
  field on ``<model>``.
* **admin.E303**: ``<model>`` has no ``GenericForeignKey``.
* **admin.E304**: ``<model>`` has no ``GenericForeignKey`` using content type
  field ``<field name>`` and object ID field ``<field name>``.

``AdminSite``
~~~~~~~~~~~~~

The following checks are performed on the default
:class:`~django.contrib.admin.AdminSite`:

* **admin.E401**: :mod:`django.contrib.contenttypes` must be in
  :setting:`INSTALLED_APPS` in order to use the admin application.
* **admin.E402**: :mod:`django.contrib.auth.context_processors.auth`
  must be enabled in :class:`~django.template.backends.django.DjangoTemplates`
  (:setting:`TEMPLATES`) if using the default auth backend in order to use the
  admin application.
* **admin.E403**: A :class:`django.template.backends.django.DjangoTemplates`
  instance must be configured in :setting:`TEMPLATES` in order to use the
  admin application.
* **admin.E404**: ``django.contrib.messages.context_processors.messages``
  must be enabled in :class:`~django.template.backends.django.DjangoTemplates`
  (:setting:`TEMPLATES`) in order to use the admin application.
* **admin.E405**: :mod:`django.contrib.auth` must be in
  :setting:`INSTALLED_APPS` in order to use the admin application.
* **admin.E406**: :mod:`django.contrib.messages` must be in
  :setting:`INSTALLED_APPS` in order to use the admin application.
* **admin.E408**:
  :class:`django.contrib.auth.middleware.AuthenticationMiddleware` must be in
  :setting:`MIDDLEWARE` in order to use the admin application.
* **admin.E409**: :class:`django.contrib.messages.middleware.MessageMiddleware`
  must be in :setting:`MIDDLEWARE` in order to use the admin application.
* **admin.E410**: :class:`django.contrib.sessions.middleware.SessionMiddleware`
  must be in :setting:`MIDDLEWARE` in order to use the admin application.
* **admin.W411**: ``django.template.context_processors.request`` must be
  enabled in :class:`~django.template.backends.django.DjangoTemplates`
  (:setting:`TEMPLATES`) in order to use the admin navigation sidebar.

``auth``
--------

* **auth.E001**: ``REQUIRED_FIELDS`` must be a list or tuple.
* **auth.E002**: The field named as the ``USERNAME_FIELD`` for a custom user
  model must not be included in ``REQUIRED_FIELDS``.
* **auth.E003**: ``<field>`` must be unique because it is named as the
  ``USERNAME_FIELD``.
* **auth.W004**: ``<field>`` is named as the ``USERNAME_FIELD``, but it is not
  unique.
* **auth.E005**: The permission codenamed ``<codename>`` clashes with a builtin
  permission for model ``<model>``.
* **auth.E006**: The permission codenamed ``<codename>`` is duplicated for model
  ``<model>``.
* **auth.E007**: The :attr:`verbose_name
  <django.db.models.Options.verbose_name>` of model ``<model>`` must be at most
  244 characters for its builtin permission names
  to be at most 255 characters.
* **auth.E008**: The permission named ``<name>`` of model ``<model>`` is longer
  than 255 characters.
* **auth.C009**: ``<User model>.is_anonymous`` must be an attribute or property
  rather than a method. Ignoring this is a security issue as anonymous users
  will be treated as authenticated!
* **auth.C010**: ``<User model>.is_authenticated`` must be an attribute or
  property rather than a method. Ignoring this is a security issue as anonymous
  users will be treated as authenticated!
* **auth.E011**: The name of model ``<model>`` must be at most 93 characters
  for its builtin permission names to be at most 100 characters.
* **auth.E012**: The permission codenamed ``<codename>`` of model ``<model>``
  is longer than 100 characters.

``contenttypes``
----------------

The following checks are performed when a model contains a
:class:`~django.contrib.contenttypes.fields.GenericForeignKey` or
:class:`~django.contrib.contenttypes.fields.GenericRelation`:

* **contenttypes.E001**: The ``GenericForeignKey`` object ID references the
  nonexistent field ``<field>``.
* **contenttypes.E002**: The ``GenericForeignKey`` content type references the
  nonexistent field ``<field>``.
* **contenttypes.E003**: ``<field>`` is not a ``ForeignKey``.
* **contenttypes.E004**: ``<field>`` is not a ``ForeignKey`` to
  ``contenttypes.ContentType``.
* **contenttypes.E005**: Model names must be at most 100 characters.

``postgres``
------------

The following checks are performed on :mod:`django.contrib.postgres` model
fields:

* **postgres.E001**: Base field for array has errors: ...
* **postgres.E002**: Base field for array cannot be a related field.
* **postgres.E003**: ``<field>`` default should be a callable instead of an
  instance so that it's not shared between all field instances. *This check was
  changed to* ``fields.E010`` *in Django 3.1*.

``sites``
---------

The following checks are performed on any model using a
:class:`~django.contrib.sites.managers.CurrentSiteManager`:

* **sites.E001**: ``CurrentSiteManager`` could not find a field named
  ``<field name>``.
* **sites.E002**: ``CurrentSiteManager`` cannot use ``<field>`` as it is not a
  foreign key or a many-to-many field.

The following checks verify that :mod:`django.contrib.sites` is correctly
configured:

* **sites.E101**: The :setting:`SITE_ID` setting must be an integer.

``staticfiles``
---------------

The following checks verify that :mod:`django.contrib.staticfiles` is correctly
configured:

* **staticfiles.E001**: The :setting:`STATICFILES_DIRS` setting is not a tuple
  or list.
* **staticfiles.E002**: The :setting:`STATICFILES_DIRS` setting should not
  contain the :setting:`STATIC_ROOT` setting.
* **staticfiles.E003**: The prefix ``<prefix>`` in the
  :setting:`STATICFILES_DIRS` setting must not end with a slash.
* **staticfiles.W004**: The directory ``<directory>`` in the
  :setting:`STATICFILES_DIRS` does not exist.
