.. currentmodule:: django.forms

.. _form-and-field-validation:

Form and field validation
=========================

Form validation happens when the data is cleaned. If you want to customize
this process, there are various places to make changes, each one serving a
different purpose. Three types of cleaning methods are run during form
processing. These are normally executed when you call the ``is_valid()``
method on a form. There are other things that can also trigger cleaning and
validation (accessing the ``errors`` attribute or calling ``full_clean()``
directly), but normally they won't be needed.

In general, any cleaning method can raise ``ValidationError`` if there is a
problem with the data it is processing, passing the relevant information to
the ``ValidationError`` constructor. :ref:`See below <raising-validation-error>`
for the best practice in raising ``ValidationError``. If no ``ValidationError``
is raised, the method should return the cleaned (normalized) data as a Python
object.

Most validation can be done using `validators`_ - simple helpers that can be
reused easily. Validators are simple functions (or callables) that take a single
argument and raise ``ValidationError`` on invalid input. Validators are run
after the field's ``to_python`` and ``validate`` methods have been called.

Validation of a form is split into several steps, which can be customized or
overridden:

* The ``to_python()`` method on a ``Field`` is the first step in every
  validation. It coerces the value to a correct datatype and raises
  ``ValidationError`` if that is not possible. This method accepts the raw
  value from the widget and returns the converted value. For example, a
  ``FloatField`` will turn the data into a Python ``float`` or raise a
  ``ValidationError``.

* The ``validate()`` method on a ``Field`` handles field-specific validation
  that is not suitable for a validator. It takes a value that has been
  coerced to a correct datatype and raises ``ValidationError`` on any error.
  This method does not return anything and shouldn't alter the value. You
  should override it to handle validation logic that you can't or don't
  want to put in a validator.

* The ``run_validators()`` method on a ``Field`` runs all of the field's
  validators and aggregates all the errors into a single
  ``ValidationError``. You shouldn't need to override this method.

* The ``clean()`` method on a ``Field`` subclass is responsible for running
  ``to_python()``, ``validate()``, and ``run_validators()`` in the correct
  order and propagating their errors. If, at any time, any of the methods
  raise ``ValidationError``, the validation stops and that error is raised.
  This method returns the clean data, which is then inserted into the
  ``cleaned_data`` dictionary of the form.

* The ``clean_<fieldname>()`` method is called on a form subclass -- where
  ``<fieldname>`` is replaced with the name of the form field attribute.
  This method does any cleaning that is specific to that particular
  attribute, unrelated to the type of field that it is. This method is not
  passed any parameters. You will need to look up the value of the field
  in ``self.cleaned_data`` and remember that it will be a Python object
  at this point, not the original string submitted in the form (it will be
  in ``cleaned_data`` because the general field ``clean()`` method, above,
  has already cleaned the data once).

  For example, if you wanted to validate that the contents of a
  ``CharField`` called ``serialnumber`` was unique,
  ``clean_serialnumber()`` would be the right place to do this. You don't
  need a specific field (it's just a ``CharField``), but you want a
  formfield-specific piece of validation and, possibly,
  cleaning/normalizing the data.

  This method should return the cleaned value obtained from ``cleaned_data``,
  regardless of whether it changed anything or not.

* The form subclass's ``clean()`` method can perform validation that requires
  access to multiple form fields. This is where you might put in checks such as
  "if field ``A``is supplied, field ``B`` must contain a valid email address".
  This method can return a completely different dictionary if it wishes, which
  will be used as the ``cleaned_data``.

  Since the field validation methods have been run by the time ``clean()`` is
  called, you also have access to the form's ``errors`` attribute which
  contains all the errors raised by cleaning of individual fields.

  Note that any errors raised by your :meth:`Form.clean()` override will not
  be associated with any field in particular. They go into a special
  "field" (called ``__all__``), which you can access via the
  :meth:`~django.forms.Form.non_field_errors` method if you need to. If you
  want to attach errors to a specific field in the form, you need to call
  :meth:`~django.forms.Form.add_error()`.

  Also note that there are special considerations when overriding
  the ``clean()`` method of a ``ModelForm`` subclass. (see the
  :ref:`ModelForm documentation
  <overriding-modelform-clean-method>` for more information)

These methods are run in the order given above, one field at a time.  That is,
for each field in the form (in the order they are declared in the form
definition), the ``Field.clean()`` method (or its override) is run, then
``clean_<fieldname>()``. Finally, once those two methods are run for every
field, the :meth:`Form.clean()` method, or its override, is executed whether
or not the previous methods have raised errors.

Examples of each of these methods are provided below.

As mentioned, any of these methods can raise a ``ValidationError``. For any
field, if the ``Field.clean()`` method raises a ``ValidationError``, any
field-specific cleaning method is not called. However, the cleaning methods
for all remaining fields are still executed.

.. _raising-validation-error:

Raising ``ValidationError``
---------------------------

In order to make error messages flexible and easy to override, consider the
following guidelines:

* Provide a descriptive error ``code`` to the constructor::

      # Good
      ValidationError(_('Invalid value'), code='invalid')

      # Bad
      ValidationError(_('Invalid value'))

* Don't coerce variables into the message; use placeholders and the ``params``
  argument of the constructor::

      # Good
      ValidationError(
          _('Invalid value: %(value)s'),
          params={'value': '42'},
      )

      # Bad
      ValidationError(_('Invalid value: %s') % value)

* Use mapping keys instead of positional formatting. This enables putting
  the variables in any order or omitting them altogether when rewriting the
  message::

      # Good
      ValidationError(
          _('Invalid value: %(value)s'),
          params={'value': '42'},
      )

      # Bad
      ValidationError(
          _('Invalid value: %s'),
          params=('42',),
      )

* Wrap the message with ``gettext`` to enable translation::

      # Good
      ValidationError(_('Invalid value'))

      # Bad
      ValidationError('Invalid value')

Putting it all together::

    raise ValidationError(
        _('Invalid value: %(value)s'),
        code='invalid',
        params={'value': '42'},
    )

Following these guidelines is particularly necessary if you write reusable
forms, form fields, and model fields.

While not recommended, if you are at the end of the validation chain
(i.e. your form ``clean()`` method) and you know you will *never* need
to override your error message you can still opt for the less verbose::

    ValidationError(_('Invalid value: %s') % value)

The :meth:`Form.errors.as_data() <django.forms.Form.errors.as_data()>` and
:meth:`Form.errors.as_json() <django.forms.Form.errors.as_json()>` methods
greatly benefit from fully featured ``ValidationError``\s (with a ``code`` name
and a ``params`` dictionary).

Raising multiple errors
~~~~~~~~~~~~~~~~~~~~~~~

If you detect multiple errors during a cleaning method and wish to signal all
of them to the form submitter, it is possible to pass a list of errors to the
``ValidationError`` constructor.

As above, it is recommended to pass a list of ``ValidationError`` instances
with ``code``\s and ``params`` but a list of strings will also work::

    # Good
    raise ValidationError([
        ValidationError(_('Error 1'), code='error1'),
        ValidationError(_('Error 2'), code='error2'),
    ])

    # Bad
    raise ValidationError([
        _('Error 1'),
        _('Error 2'),
    ])

Using validation in practice
----------------------------

The previous sections explained how validation works in general for forms.
Since it can sometimes be easier to put things into place by seeing each
feature in use, here are a series of small examples that use each of the
previous features.

.. _validators:

Using validators
~~~~~~~~~~~~~~~~

Django's form (and model) fields support use of simple utility functions and
classes known as validators. A validator is merely a callable object or
function that takes a value and simply returns nothing if the value is valid or
raises a :exc:`~django.core.exceptions.ValidationError` if not. These can be
passed to a field's constructor, via the field's ``validators`` argument, or
defined on the :class:`~django.forms.Field` class itself with the
``default_validators`` attribute.

Simple validators can be used to validate values inside the field, let's have
a look at Django's ``SlugField``::

    from django.forms import CharField
    from django.core import validators

    class SlugField(CharField):
        default_validators = [validators.validate_slug]

As you can see, ``SlugField`` is just a ``CharField`` with a customized
validator that validates that submitted text obeys to some character rules.
This can also be done on field definition so::

    slug = forms.SlugField()

is equivalent to::

    slug = forms.CharField(validators=[validators.validate_slug])

Common cases such as validating against an email or a regular expression can be
handled using existing validator classes available in Django. For example,
``validators.validate_slug`` is an instance of
a :class:`~django.core.validators.RegexValidator` constructed with the first
argument being the pattern: ``^[-a-zA-Z0-9_]+$``. See the section on
:doc:`writing validators </ref/validators>` to see a list of what is already
available and for an example of how to write a validator.

Form field default cleaning
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Let's first create a custom form field that validates its input is a string
containing comma-separated email addresses. The full class looks like this::

    from django import forms
    from django.core.validators import validate_email

    class MultiEmailField(forms.Field):
        def to_python(self, value):
            "Normalize data to a list of strings."

            # Return an empty list if no input was given.
            if not value:
                return []
            return value.split(',')

        def validate(self, value):
            "Check if value consists only of valid emails."

            # Use the parent's handling of required fields, etc.
            super(MultiEmailField, self).validate(value)

            for email in value:
                validate_email(email)

Every form that uses this field will have these methods run before anything
else can be done with the field's data. This is cleaning that is specific to
this type of field, regardless of how it is subsequently used.

Let's create a simple ``ContactForm`` to demonstrate how you'd use this
field::

    class ContactForm(forms.Form):
        subject = forms.CharField(max_length=100)
        message = forms.CharField()
        sender = forms.EmailField()
        recipients = MultiEmailField()
        cc_myself = forms.BooleanField(required=False)

Simply use ``MultiEmailField`` like any other form field. When the
``is_valid()`` method is called on the form, the ``MultiEmailField.clean()``
method will be run as part of the cleaning process and it will, in turn, call
the custom ``to_python()`` and ``validate()`` methods.

Cleaning a specific field attribute
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Continuing on from the previous example, suppose that in our ``ContactForm``,
we want to make sure that the ``recipients`` field always contains the address
``"fred@example.com"``. This is validation that is specific to our form, so we
don't want to put it into the general ``MultiEmailField`` class. Instead, we
write a cleaning method that operates on the ``recipients`` field, like so::

    from django import forms

    class ContactForm(forms.Form):
        # Everything as before.
        ...

        def clean_recipients(self):
            data = self.cleaned_data['recipients']
            if "fred@example.com" not in data:
                raise forms.ValidationError("You have forgotten about Fred!")

            # Always return the cleaned data, whether you have changed it or
            # not.
            return data

.. _validating-fields-with-clean:

Cleaning and validating fields that depend on each other
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Suppose we add another requirement to our contact form: if the ``cc_myself``
field is ``True``, the ``subject`` must contain the word ``"help"``. We are
performing validation on more than one field at a time, so the form's
:meth:`~Form.clean()` method is a good spot to do this. Notice that we are
talking about the ``clean()`` method on the form here, whereas earlier we were
writing a ``clean()`` method on a field. It's important to keep the field and
form difference clear when working out where to validate things. Fields are
single data points, forms are a collection of fields.

By the time the form's ``clean()`` method is called, all the individual field
clean methods will have been run (the previous two sections), so
``self.cleaned_data`` will be populated with any data that has survived so
far. So you also need to remember to allow for the fact that the fields you
are wanting to validate might not have survived the initial individual field
checks.

There are two ways to report any errors from this step. Probably the most
common method is to display the error at the top of the form. To create such
an error, you can raise a ``ValidationError`` from the ``clean()`` method. For
example::

    from django import forms

    class ContactForm(forms.Form):
        # Everything as before.
        ...

        def clean(self):
            cleaned_data = super(ContactForm, self).clean()
            cc_myself = cleaned_data.get("cc_myself")
            subject = cleaned_data.get("subject")

            if cc_myself and subject:
                # Only do something if both fields are valid so far.
                if "help" not in subject:
                    raise forms.ValidationError(
                        "Did not send for 'help' in the subject despite "
                        "CC'ing yourself."
                    )

In this code, if the validation error is raised, the form will display an
error message at the top of the form (normally) describing the problem.

Note that the call to ``super(ContactForm, self).clean()`` in the example code
ensures that any validation logic in parent classes is maintained.

The second approach might involve assigning the error message to one of the
fields. In this case, let's assign an error message to both the "subject" and
"cc_myself" rows in the form display. Be careful when doing this in practice,
since it can lead to confusing form output. We're showing what is possible
here and leaving it up to you and your designers to work out what works
effectively in your particular situation. Our new code (replacing the previous
sample) looks like this::

    from django import forms

    class ContactForm(forms.Form):
        # Everything as before.
        ...

        def clean(self):
            cleaned_data = super(ContactForm, self).clean()
            cc_myself = cleaned_data.get("cc_myself")
            subject = cleaned_data.get("subject")

            if cc_myself and subject and "help" not in subject:
                msg = "Must put 'help' in subject when cc'ing yourself."
                self.add_error('cc_myself', msg)
                self.add_error('subject', msg)

The second argument of ``add_error()`` can be a simple string, or preferably
an instance of ``ValidationError``. See :ref:`raising-validation-error` for
more details. Note that ``add_error()`` automatically removes the field
from ``cleaned_data``.
