Form and field validation

=========================

.. versionchanged:: 1.2

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.

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.

      Just like the general field ``clean()`` method, above, this method

      should return the 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 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. 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 `described later`_.

      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.

.. _described later:

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

~~~~~~~~~~~~~~~~

.. versionadded:: 1.2

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 ``EmailField``::

    class EmailField(CharField):

        default_error_messages = {

            'invalid': _(u'Enter a valid e-mail address.'),

        }

        default_validators = [validators.validate_email]

As you can see, ``EmailField`` is just a ``CharField`` with customized error

message and a validator that validates e-mail addresses. This can also be done

on field definition so::

    email = forms.EmailField()

is equivalent to::

    email = forms.CharField(validators=[validators.validate_email],

            error_messages={'invalid': _(u'Enter a valid e-mail address.')})

Form field default cleaning

~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Let's firstly create a custom form field that validates its input is a string

containing comma-separated e-mail 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::

    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 way 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::

    class ContactForm(forms.Form):

        # Everything as before.

        ...

        def clean(self):

            cleaned_data = self.cleaned_data

            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.

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::

    class ContactForm(forms.Form):

        # Everything as before.

        ...

        def clean(self):

            cleaned_data = self.cleaned_data

            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``.

In fact, Django will currently completely wipe out the ``cleaned_data``

dictionary if there are any errors in the form. However, this behaviour may

change in the future, so it's not a bad idea to clean up after yourself in the

first place.