# Τεκμηρίωση του Django - 1.10.7

# System check framework

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 System check topic guide.

## 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 `__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 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)

## Builtin checks

### Builtin tags

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

- `models`: Checks governing model, field and manager definitions.
- `signals`: Checks on signal declarations and handler registrations.
- `admin`: Checks of any admin site declarations.
- `compatibility`: Flagging potential problems with version upgrades.
- `security`: Checks security related configuration.
- `templates`: Checks template related configuration.
- `caches`: Checks cache related configuration.
- `urls`: Checks URL configuration.
- `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 `migrate` command or if you specify the `database` tag when calling the `check` command.

The `database` tag was added.

Some checks may be registered with multiple tags.

### Core system checks

#### 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 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 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: `index_together/unique_together` refers to the non-existent field `<field name>`.
- models.E013: `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 non-existent field `<field name>`.
- models.E016: `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>`.

#### 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 returning `(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.E100: `AutoField`s must set primary_key=True.
- fields.E110: `BooleanField`s do not accept null values.
- 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 `IntegerField`.
- 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 non-negative 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 accept blank values if null values are not allowed, 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.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.

#### File Fields

- fields.E200: `unique` is not a valid argument for a `FileField`.
- fields.E201: `primary_key` is not a valid argument for a `FileField`.
- 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 `<model>` which has been swapped out.
- fields.E302: Accessor for field `<field name>` clashes with field `<field name>`.
- fields.E303: Reverse query name for field `<field name>` clashes with field `<field name>`.
- fields.E304: Field name `<field name>` clashes with accessor for `<field name>`.
- fields.E305: Field name `<field name>` clashes with reverse query name for `<field name>`.
- fields.E306: Related name must be a valid Python identifier or end with a `'+'`.
- 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.E310: No subset of the fields `<field1>`, `<field2>`, ... on model `<model>` is unique. Add `unique=True` on any of those fields or add at least a subset of them to a unique_together constraint.
- fields.E311: `<model>` must set `unique=True` because it is referenced by a `ForeignKey`.
- 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.
- 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.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`.

#### 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>`.

#### Backwards Compatibility

The following checks are performed to warn the user of any potential problems that might occur as a result of a version upgrade.

- 1_6.W001: Some project unit tests may not execute as expected. This check was removed in Django 1.8 due to false positives.
- 1_6.W002: `BooleanField` does not have a default value. This check was removed in Django 1.8 due to false positives.
- 1_7.W001: Django 1.7 changed the global defaults for the `MIDDLEWARE_CLASSES.` `django.contrib.sessions.middleware.SessionMiddleware`, `django.contrib.auth.middleware.AuthenticationMiddleware`, and `django.contrib.messages.middleware.MessageMiddleware` were removed from the defaults. If your project needs these middleware then you should configure this setting. This check was removed in Django 1.9.
- 1_8.W001: The standalone `TEMPLATE_*` settings were deprecated in Django 1.8 and the `TEMPLATES` dictionary takes precedence. You must put the values of the following settings into your defaults `TEMPLATES` dict: `TEMPLATE_DIRS`, `TEMPLATE_CONTEXT_PROCESSORS`, `TEMPLATE_DEBUG`, `TEMPLATE_LOADERS`, `TEMPLATE_STRING_IF_INVALID`.
- 1_10.W001: The `MIDDLEWARE_CLASSES` setting is deprecated in Django 1.10 and the `MIDDLEWARE` setting takes precedence. Since you’ve set `MIDDLEWARE`, the value of `MIDDLEWARE_CLASSES` is ignored.

### Admin

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

The following checks are performed on any `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 an attribute 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_vertical[n]` refers to `<field name>`, which is not an attribute of `<model>`.
- admin.E020: The value of `filter_vertical[n]/filter_vertical[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 an attribute of `<model>`.
- admin.E023: The value of `radio_fields` refers to `<field name>`, which is not a `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 an attribute of `<model>`.
- admin.E028: The value of `prepopulated_fields` refers to `<field name>`, which must not be a `DateTimeField`, a `ForeignKey`, 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 an attribute 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 an attribute 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>`.

#### `ModelAdmin`

The following checks are performed on any `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 `BaseModelAdmin`.
- 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 an attribute 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 is not an attribute of `<model>`.
- admin.E128: The value of `date_hierarchy` must be a `DateField` or `DateTimeField`.

#### `InlineModelAdmin`

The following checks are performed on any `InlineModelAdmin` that is registered as an inline on a `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>`.
- 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 `GenericInlineModelAdmin` that is registered as an inline on a `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 `AdminSite`:

- admin.E401: `django.contrib.contenttypes` must be in `INSTALLED_APPS` in order to use the admin application.
- admin.E402: `django.contrib.auth.context_processors.auth` must be in `TEMPLATES` in order to use the admin application.

### 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 `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!

### Content Types

The following checks are performed when a model contains a `GenericForeignKey` or `GenericRelation`:

- contenttypes.E001: The `GenericForeignKey` object ID references the non-existent field `<field>`.
- contenttypes.E002: The `GenericForeignKey` content type references the non-existent field `<field>`.
- contenttypes.E003: `<field>` is not a `ForeignKey`.
- contenttypes.E004: `<field>` is not a `ForeignKey` to `contenttypes.ContentType`.

### 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. They help you remember the simple things that 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 `SECURE_SSL_REDIRECT`. Use `SILENCED_SYSTEM_CHECKS` to silence unneeded checks.

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

- security.W001: You do not have `django.middleware.security.SecurityMiddleware` in your `MIDDLEWARE`/`MIDDLEWARE_CLASSES` so the `SECURE_HSTS_SECONDS`, `SECURE_CONTENT_TYPE_NOSNIFF`, `SECURE_BROWSER_XSS_FILTER`, and `SECURE_SSL_REDIRECT` settings will have no effect.
- security.W002: You do not have `django.middleware.clickjacking.XFrameOptionsMiddleware` in your `MIDDLEWARE`/`MIDDLEWARE_CLASSES`, 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 (`django.middleware.csrf.CsrfViewMiddleware` is not in your `MIDDLEWARE`/`MIDDLEWARE_CLASSES`). 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 `SECURE_HSTS_SECONDS` setting. If your entire site is served only over SSL, you may want to consider setting a value and enabling 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 `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 `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.
- security.W008: Your `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 `SECRET_KEY` has less than 50 characters or less than 5 unique characters. Please generate a long and random `SECRET_KEY`, otherwise many of Django’s security-critical features will be vulnerable to attack.
- security.W010: You have `django.contrib.sessions` in your `INSTALLED_APPS` but you have not set `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 `django.contrib.sessions.middleware.SessionMiddleware` in your `MIDDLEWARE`/`MIDDLEWARE_CLASSES`, but you have not set `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: `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 `django.contrib.sessions` in your `INSTALLED_APPS`, but you have not set `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 `django.contrib.sessions.middleware.SessionMiddleware` in your `MIDDLEWARE`/`MIDDLEWARE_CLASSES`, but you have not set `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: `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: `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: `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.
- security.W018: You should not have `DEBUG` set to `True` in deployment.
- security.W019: You have `django.middleware.clickjacking.XFrameOptionsMiddleware` in your `MIDDLEWARE`/`MIDDLEWARE_CLASSES`, but `X_FRAME_OPTIONS` is not set to `'DENY'`. The default is `'SAMEORIGIN'`, but 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: `ALLOWED_HOSTS` must not be empty in deployment.

### Sites

The following checks are performed on any model using a `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.

### Database

#### MySQL

If you’re using MySQL, the following checks will be performed:

- mysql.E001: MySQL does not allow unique `CharField`s to have a `max_length` > 255.
- mysql.W002: MySQL Strict Mode is not set for database connection ‘<alias>’. See also Setting sql_mode.

### Templates

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

- templates.E001: You have `'APP_DIRS': True` in your `TEMPLATES` but also specify `'loaders'` in `OPTIONS`. Either remove `APP_DIRS` or remove the `'loaders'` option.
- templates.E002: `string_if_invalid` in `TEMPLATES` `OPTIONS` must be a string but got: `{value}` (`{type}`).

### Caches

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

- caches.E001: You must define a `'default'` cache in your `CACHES` setting.

### URLs

The following checks are performed on your URL configuration:

- urls.W001: Your URL pattern `<pattern>` uses `include()` with a `regex` ending with a `$`. Remove the dollar from the `regex` to avoid problems including URLs.
- urls.W002: Your URL pattern `<pattern>` has a `regex` beginning with a `/`. Remove this slash as it is unnecessary. If this pattern is targeted in an `include()`, ensure the `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 `url()` instances.