Form and Field validation

Form validation happens when the data is cleaned. If you want to customise 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 validate() method on a form or you interact with a field when the form is using event-based validation.

In general, any cleaning method can throw a ValidationError if there is a problem with the data it is processing, passing the relevant information to the ValidationError constructor.

Most validation can be done using validators – helpers that can be reused easily. Validators are functions that take a single argument and throw a ValidationError on invalid input. Validators are run after the field’s toJavaScript() and validate() methods have been called.

Validation steps and order

Validation of a Form is split into several steps, which can be customised or overridden:

  • The toJavaScript() method on a Field is the first step in every validation. It coerces the value to the correct datatype and throws a 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 JavaScript Number or throw 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 the correct datatype and throws a 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 runValidators() 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. This is responsible for running toJavaScript, validate and runValidators in the correct order and propagating their errors. If, at any time, any of the methods throws a ValidationError, the validation stops and that error is thrown. This method returns the clean data, which is then inserted into the cleanedData object of the form.

  • Field-specific cleaning/validation hooks on the Form. If your form includes a clean<FieldName>() (or clean_<fieldName>()) method in its definition, it will be called for the field its name matches. This method is not passed its field’s data as an argument. You will need to look up the value of the field in this.cleanedData (it will be in cleanedData because the general field clean() method, above, has already cleaned the data once).

    For example, if you wanted to validate that the content of a CharField called serialNumber was unique, implementing cleanSerialNumber() would provide the right place to do this.

  • The Form clean() method. This method can perform any validation that requires access to multiple fields from the form at once. This is where you would perform checks like password or email confirmation fields being equal to the original input.

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

    Note that any errors thrown 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 nonFieldErrors() method if you need to. If you want to attach errors to a specific field in the form, you need to call form.addError().

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>() (or clean_<fieldName>()) if defined. Finally, the form.clean() method, or its override, is executed whether or not the previous methods have thrown errors.

Examples of each of these methods are provided below.

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

Throwing ValidationError

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

  • Provide a descriptive error code to the constructor when possible:

    forms.ValidationError('Invalid value', {code: 'invalid'})
  • Don’t coerce variables into the message; use placeholders and the params argument of the constructor:

    forms.ValidationError('Invalid value: {value}', {params: {value: '42'}})

Putting it all together:

throw forms.ValidationError('Invalid value: {value)', {
  code: 'invalid',
  params: {value: '42'}

Following these guidelines is particularly useful to others if you write reusable forms and form fields.

If you’re at the end of the validation chain (i.e. your form’s clean()) and you know you will never need to override your error message (or even just... because) you can still opt for the less verbose:

forms.ValidationError('Invalid value: ' + value)

Throwing 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.

It’s recommended to pass a list of ValidationError instances with codes and params but a list of strings will also work:

throw forms.ValidationError([
  forms.ValidationError('Error 1', {code: 'error1'}),
  forms.ValidationError('Error 2', {code: 'error2'})

throw forms.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

Fields support use of utility functions known as validators. A validator is a function that takes a value and returns nothing if the value is valid, or thriws a ValidationError() if not. These can be passed to a field’s constructor, via the field’s validators argument, or defined on the field’s prototype as a defaultValidators property.

Let’s have a look at a basic implementation of newforms’ SlugField:

var MySlugField = forms.CharField.extend({
  defaultValidators: [forms.validators.validateSlug]

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

var field = new MySlugField()

is equivalent to:

var field = forms.CharField({validators: [forms.validators.validateSlug]})

Common cases such as validating against an email or a regular expression can be handled using existing validators available in newforms. For example, validateSlug() is a function created by passing a slug-matching RegExp to the RegexValidator() function factory.

Form field default cleaning

Let’s firstly create a custom form field that validates its input is a string containing comma-separated email addresses:

var MultiEmailField = forms.Field.extend({
   * Normalise data to a list of strings.
  toJavaScript: function(value) {
    // Return an empty list if no input was given
    if (this.isEmptyValue(value)) {
      return []
    return value.split(/, ?/g)

   * Check if value consists only of valid emails.
  validate: function(value) {
    // Use the parent's handling of required fields, etc., value)

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

var ContactForm = forms.Form.extend({
  subject: forms.CharField({maxLength: 100}),
  message: forms.CharField(),
  sender: forms.EmailField(),
  recipients: new MultiEmailField(),
  ccMyself: forms.BooleanField({required: false})

Cleaning a specific field

Suppose that in our ContactForm, we want to make sure that the recipients field always contains the address "". This is validation that is specific to our form, so we don’t want to put it into the general MultiEmailField. Instead, we write a cleaning function that operates on the recipients field, like so:

var ContactForm = forms.Form.extend({
  // Everything as before
  // ...

  cleanRecipients: function() {
    var recipients = this.cleanedData.recipients
    if (recipients.indexOf('') == -1) {
      throw forms.ValidationError('You forgot about Fred!')

Changed in version 0.10: You can no longer return a value from a custom field cleaning method to update the field’s cleanedData.

Cleaning and validating fields that depend on each other


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 throw a ValidationError from the clean() method. For example:

var ContactForm = forms.Form.extend({
  // Everything as before
  // ...

  clean: function() {
    var ccMyself = this.cleanedData.ccMyself
    var subject = this.cleanedData.subject

    if (ccMyself && subject) {
      // Only do something if both fields are valid so far
      if (subject.indexOf('help') == -1) {
        throw forms.ValidationError(
          "Did not send for 'help' in the subject despite CC'ing yourself.")

Another 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 “ccMyself” rows in the form display:

var ContactForm = forms.Form.extend({
  // Everything as before
  // ...

  clean: function() {
    var cleanedData =
    var ccMyself = this.cleanedData.ccMyself
    var subject = this.cleanedData.subject

    if (ccMyself && subject && subject.indexOf('help') == -1) {
      var message = "Must put 'help' in subject when cc'ing yourself."
      this.addError('ccMyself', message)
      this.addError('subject', message)

The second argument oto addError() can be a simple string, or preferably an instance of ValidationError. See Throwing ValidationError for more details. Note that addError() automatically removes the field from cleanedData.

Specifying fields used in cross-field validation

New in version 0.9.

To let a form know which fields are used in cross-field validation, specify its clean() method as an array of field named followed by the cleaning function itself.

In scenarios where the form is being partially updated, such as when individual field input values are being updated and validated when an onChange event fires, if this information is available cross-field cleaning will only be performed if one of the fields it uses is affected.

var PersonForm = forms.Form.extend({
  firstName: forms.CharField({required: false, maxLength: 50}),
  lastName: forms.CharField({required: false, maxLength: 50}),
  jobTitle: forms.CharField({required: false, maxLength: 100}),
  organisation : forms.CharField({required: false}),

  clean: ['firstName', 'lastName', function() {
     if (!this.cleanedData.firstName && !this.cleanedData.lastName) {
       throw forms.ValidationError('A first name or last name is required.')

Asynchronous validation

New in version 0.10.

For some validation you may need to access an external data source, such as a web service, database or filesystem. In JavaScript, these tend to be asynchronous operations.

You can let newforms know that a custom field – or cross-field clean() – validation method will be async by defining it with a single parameter in its function signature. It doesn’t matter what this is called, but it’s conventionally called callback or cb:

cleanUsername: function(callback) {
  // ...

When your custom cleaning method is finished whatever async operation it needs to perform, it must call the callback function to let the form know it can finish the validation process.

The callback has the following signature:

function callback(error, validationError)
  • error – an Error indicating that something went wrong with the async operation. Any falsy value can be passed if there was no error, but it’s conventional to pass null in that case. This style of error reporting is known as an “errback”.
  • validationError – an error message if the field’s value was invalid, this can be a simple string or a ValidationError.

If async validation determines that the input is valid, you must still call the callback to let newforms know you’re done. It can be called without any arguments in this case.

The callback must only be called once, so take care with your custom validation logic, branching or returning early as necessary to avoid calling it multiple times.

Async field validation example

A common use case for async validation is checking if a username is available in a signup form:

cleanUsername: function(callback) {'/checkuser', {username: this.cleanedData.username}, function(err, res) {
    // There was an error during the HTTP request
    if (err) {
      return callback(err)

    // The username is already taken
    if (res.alreadyTaken) {
      return callback(null, forms.ValidationError('This username is already taken.'))

    // The username is available

In this live example, someone has registered every possible username containing a vowel from the English alphabet, which will randomly take between 1 and 2 seconds to validate:

Cancelling async validation

Asynchronous validation will be cancelled if the user is able to make a change which re-triggers validation while their last change is still being validated.

From the newforms side of things, this effectivly involves ignoring calls to the callback which was handed to async validation function.


In order to support cancelling asyncronous validation, you should always call the given callback with validation results, rather than modifying the form directly via this. This allows newforms to ignore your eventual callback if the data is was validating is stale.

If an async validation involves a potentially long-running or (in some way) expensive operation which you’d like to cancel should this happen, you can give newforms a callback to call by returning an object with an onCancel() function, after your async validation has started:

cleanExpensiveField: function(callback) {
  var request =
    // ...

  return {
    onCancel: function() {

Combining sync and async validation

If you have custom validation which can be performed locally, as well as async validation, you can combine the two in the same custom cleaning method.

If validation fails before it reaches the asynchronous part, throwing a ValidationError or explicitly returning false will let newforms know that asynchronous validation wes never initiated:

cleanUsername: function(callback) {
  var username = this.cleanedData.username

  // Throwing a ValidationError skips remaining validation for this field
  if (username != esrever.reverse(username)) {
    throw forms.ValidationError('Usernames must be palindromes.')

  // Returning false explicitly lets newforms know not to wait for a callback
  if (/[aeiou].test(username)) {
    this.addError('useranme', 'Usernames must not contain vowels.')
    return false
    // ...