# 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.

Checks can be triggered explicitly via the `check` command. Checks are triggered implicitly before most commands, including `runserver` and `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 `check`.

Serious errors will prevent Django commands (such as `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 `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 System check reference.

## 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 `**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 `CheckMessage`. An instance of `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 message framework or the logging framework. 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.

- `Debug`
- `Info`
- `Warning`
- `Error`
- `Critical`

### 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 `AppConfig.ready()` method.

`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 `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, 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.

Database backend checks were added.

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 `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

### 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)