.. _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 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 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 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 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. This 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 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.

  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. 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 email 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. If you want to attach
  errors to a specific field in the form, you will need to access the
  ``_errors`` attribute on the form, which is
  :ref:`described later <modifying-field-errors>`.

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

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.

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.

.. _raising-validation-error:

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

.. versionchanged:: 1.6

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 ValidationErrror(
        _('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)

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'),
    ])

.. _modifying-field-errors:

Form subclasses and modifying field errors
------------------------------------------

Sometimes, in a form's ``clean()`` method, you will want to add an error
message to a particular field in the form. This won't always be appropriate
and the more typical situation is to raise a ``ValidationError`` from
``Form.clean()``, which is turned into a form-wide error that is available
through the ``Form.non_field_errors()`` method.

When you really do need to attach the error to a particular field, you should
store (or amend) a key in the ``Form._errors`` attribute. This attribute is an
instance of a ``django.forms.util.ErrorDict`` class. Essentially, though, it's
just a dictionary. There is a key in the dictionary for each field in the form
that has an error. Each value in the dictionary is a
``django.forms.util.ErrorList`` instance, which is a list that knows how to
display itself in different ways. So you can treat ``_errors`` as a dictionary
mapping field names to lists.

If you want to add a new error to a particular field, you should check whether
the key already exists in ``self._errors`` or not. If not, create a new entry
for the given key, holding an empty ``ErrorList`` instance. In either case,
you can then append your error message to the list for the field name in
question and it will be displayed when the form is displayed.

There is an example of modifying ``self._errors`` in the following section.

.. admonition:: What's in a name?

    You may be wondering why is this attribute called ``_errors`` and not
    ``errors``. Normal Python practice is to prefix a name with an underscore
    if it's not for external usage. In this case, you are subclassing the
    ``Form`` class, so you are essentially writing new internals. In effect,
    you are given permission to access some of the internals of ``Form``.

    Of course, any code outside your form should never access ``_errors``
    directly. The data is available to external code through the ``errors``
    property, which populates ``_errors`` before returning it).

    Another reason is purely historical: the attribute has been called
    ``_errors`` since the early days of the forms module and changing it now
    (particularly since ``errors`` is used for the read-only property name)
    would be inconvenient for a number of reasons. You can use whichever
    explanation makes you feel more comfortable. The result is the same.

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. These can be passed to a field's constructor, via
the field's ``validators`` argument, or defined on the 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])


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

Let's firstly 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

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
``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.")

            # Always return the full collection of cleaned data.
            return cleaned_data

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:
                # We know these are not in self._errors now (see discussion
                # below).
                msg = u"Must put 'help' in subject when cc'ing yourself."
                self._errors["cc_myself"] = self.error_class([msg])
                self._errors["subject"] = self.error_class([msg])

                # These fields are no longer valid. Remove them from the
                # cleaned data.
                del cleaned_data["cc_myself"]
                del cleaned_data["subject"]

            # Always return the full collection of cleaned data.
            return cleaned_data

As you can see, this approach requires a bit more effort, not withstanding the
extra design effort to create a sensible form display. The details are worth
noting, however. Firstly, earlier we mentioned that you might need to check if
the field name keys already exist in the ``_errors`` dictionary. In this case,
since we know the fields exist in ``self.cleaned_data``, they must have been
valid when cleaned as individual fields, so there will be no corresponding
entries in ``_errors``.

Secondly, once we have decided that the combined data in the two fields we are
considering aren't valid, we must remember to remove them from the
``cleaned_data``.

.. versionchanged:: 1.5

    Django used to remove the ``cleaned_data`` attribute entirely if there were
    any errors in the form. Since version 1.5, ``cleaned_data`` is present even if
    the form doesn't validate, but it contains only field values that did
    validate.
