django/docs/ref/forms/validation.txt

.. _ref-forms-validation:

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

Form validation happens when the data is cleaned. If you want to customize
this process, there are various places you can change, 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 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 error message to
the ``ValidationError`` constructor. If no ``ValidationError`` is raised, the
method should return the cleaned (normalized) data as a Python object.

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.

The three types of cleaning methods are:

    * The ``clean()`` method on a Field subclass. This is responsible
      for cleaning the data in a way that is generic for that type of field.
      For example, a FloatField will turn the data into a Python ``float`` or
      raise a ``ValidationError``.

    * The ``clean_<fieldname>()`` method in 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.

    * The Form subclass's ``clean()`` method. This method can perform
      any validation that requires access to multiple fields from the form at
      once. This is where you might put in things to check that if field ``A``
      is supplied, field ``B`` must contain a valid e-mail address and the
      like. The data that this method returns is the final ``cleaned_data``
      attribute for the form, so don't forget to return the full list of
      cleaned data if you override this method (by default, ``Form.clean()``
      just returns ``self.cleaned_data``).

      Note that any errors raised by your ``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
      ``non_field_errors()`` method if you need to.

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 ``Form.clean()`` method, or its override, is executed.

As mentioned above, 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.

The ``clean()`` method for the ``Form`` class or subclass is always run. If
that method raises a ``ValidationError``, ``cleaned_data`` will be an empty
dictionary.

The previous paragraph means that if you are overriding ``Form.clean()``, you
should iterate through ``self.cleaned_data.items()``, possibly considering the
``_errors`` dictionary attribute on the form as well. In this way, you will
already know which fields have passed their individual validation requirements.

A simple example
~~~~~~~~~~~~~~~~

Here's a simple example of a custom field that validates its input is a string
containing comma-separated e-mail addresses, with at least one address. We'll
keep it simple and assume e-mail validation is contained in a function called
``is_valid_email()``. The full class::

    from django import forms

    class MultiEmailField(forms.Field):
        def clean(self, value):
            if not value:
                raise forms.ValidationError('Enter at least one e-mail address.')
            emails = value.split(',')
            for email in emails:
                if not is_valid_email(email):
                    raise forms.ValidationError('%s is not a valid e-mail address.' % email)
            return emails

Let's alter the ongoing ``ContactForm`` example to demonstrate how you'd use
this in a form. Simply use ``MultiEmailField`` instead of ``forms.EmailField``,
like so::

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