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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. See below
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 aField
is the first step in every validation. It coerces the value to a correct datatype and raisesValidationError
if that is not possible. This method accepts the raw value from the widget and returns the converted value. For example, aFloatField
will turn the data into a Pythonfloat
or raise aValidationError
.The
validate()
method on aField
handles field-specific validation that is not suitable for a validator. It takes a value that has been coerced to a correct datatype and raisesValidationError
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 aField
runs all of the field’s validators and aggregates all the errors into a singleValidationError
. You shouldn’t need to override this method.The
clean()
method on aField
subclass is responsible for runningto_python()
,validate()
, andrun_validators()
in the correct order and propagating their errors. If, at any time, any of the methods raiseValidationError
, the validation stops and that error is raised. This method returns the clean data, which is then inserted into thecleaned_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 inself.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 incleaned_data
because the general fieldclean()
method, above, has already cleaned the data once).For example, if you wanted to validate that the contents of a
CharField
calledserialnumber
was unique,clean_serialnumber()
would be the right place to do this. You don’t need a specific field (it’s just aCharField
), 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 fieldA
is supplied, fieldB
must contain a valid email address”. This method can return a completely different dictionary if it wishes, which will be used as thecleaned_data
.Since the field validation methods have been run by the time
clean()
is called, you also have access to the form’serrors
attribute which contains all the errors raised by cleaning of individual fields.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 thenon_field_errors()
method if you need to. If you want to attach errors to a specific field in the form, you need to calladd_error()
.Also note that there are special considerations when overriding the
clean()
method of aModelForm
subclass. (see the ModelForm documentation 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 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 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 Form.errors.as_data()
and
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.
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 ValidationError
if not. 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])
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 RegexValidator
constructed with the first
argument being the pattern: ^[-a-zA-Z0-9_]+$
. See the section on
writing 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
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."
)
In previous versions of Django, form.clean()
was required to return
a dictionary of cleaned_data
. This method may still return a dictionary
of data to be used, but it’s no longer required.
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 call to super(ContactForm, self).clean()
in the example code ensures
that any validation logic in parent classes is maintained. If your form
inherits another that doesn’t return a cleaned_data
dictionary in its
clean()
method (doing so is optional), then don’t assign cleaned_data
to the result of the super()
call and use self.cleaned_data
instead:
def clean(self):
super(ContactForm, self).clean()
cc_myself = self.cleaned_data.get("cc_myself")
...
The second approach for reporting validation errors 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 Raising ValidationError for
more details. Note that add_error()
automatically removes the field
from cleaned_data
.