Parameters Validation

When using TurboGears, your controller methods get their arguments built from the various GET, POST, and URL mechanisms provided by TurboGears. The only downside is that all the arguments will be strings and you’d like them converted to their normal Python datatype: numbers to int, dates to datetime, etc.

This conversion functionality is provided by the FormEncode package and is applied to your methods using the validate decorator. FormEncode provides both validation and conversion as a single step, reasoning that you frequently need to validate something before you can convert it or that you’ll need to convert something before you can really validate it.

The @validate() decorator can evaluate both widget-based forms and the standard form arguments so they are not dependent on widgets at all.

Furthermore, the @validate() decorator is not really required at all. It just provides a convenience so that you can assume that you have the right kind of data inside your controller methods. This helps separate validation logic from application logic about what to do with valid data.

If you don’t put a @validate() decorator on your method, you’ll simply have to do the string conversion in your controller.

Validating Parameters

When not using forms, the story gets a bit more complex. Basically, you need to specify which validator goes with which argument using in the validate decorator. Here’s a simple example:

from tg import request, validate, expose, TGController
from formencode import validators

class RootController(TGController):
    @validate({"a":validators.Int(not_empty=True), "b":validators.Email})
    def two_validators(self, a=None, b=None, *args):
        validation_status = tg.request.validation

        errors = [{key, value} in validation_status['errors'].iteritems()]
        values =  validation_status['values']
        return dict(a=a, b=b, errors=str(errors), values=str(values))

The dictionary passed to validators maps the incoming field names to the appropriate FormEncode validators, Int in this example.

In case of a validation error TurboGears will provide the errors and values inside tg.request.validation.

FormEncode provides a number of useful pre-made validators for you to use: they are available in the formencode.validators module.

For most validators, you can pass keyword arguments for more specific constraints.

Validation Process Information

TurboGears provides some information on the currently running validation process while it is handling the validation error.

Whenever an error handling is in process some properties are available in the tg.request.validation to provide overview of the validation error:

  • tg.request.validation['values'] The submitted values before validation
  • tg.request.validation['errors'] The errors that triggered the error handling
  • tg.request.validation['exception'] The validation exception that triggered the error handling
  • tg.request.validation['error_handler'] The error handler that is being executed

The Error Handler

In many cases you don’t need the granularity provided by tg.request.validation and probably in case of an error you just want to send the user somewhere else (maybe to reinsert the data he provided).

This can be achieved by using the error_handler argument of validate. The provided function or controller method will be called to generate a response for the user in case of an error instead of continuing with the current action:

from tg import request, validate, expose, TGController
from formencode import validators

class RootController(TGController):
    def onerror(self, **kwargs):
        return 'An error occurred: %s' % request.validation['errors']

    @validate({"a": validators.Int(not_empty=True), "b": validators.Email},
    def two_validators(self, a=None, b=None, *args):
        return 'Values: %s, %s, %s' % (a, b, args)

Heading to /two_validators without providing a value for a will lead to an "An error occurred" message as the onerror method is executed instead of continuing with two_validators.


The method in question will be called, with the unvalidated data as its parameters, so it’s usually best to accept **kwargs. And error validation messages will be stored in tg.request.validation.

Validating Forms

For manually written forms you can use @validate on the action that processes the submitted data and add the errors in your template from tg.request.validation.

TurboGears also provides a more convenient way to create forms, validate submitted data and display error messages, those can be managed through Widgets & Forms which work together with validation by Validating Fields

Any widget based form can then be passed to the @validate which will automatically validate the submitted data against that form.


TurboGears applications will usually rely on three kind of validators:

  • Convert which is builtin into TurboGears an can be used for simple conversions like integers, floats and so on...
  • tw2.core.validation which provide ToscaWidgets validators for Forms
  • formencode.validators validators which can be used Standalone or with a Form

While in many cases Convert will suffice, the FormEncode library provides a pretty complete set of validators:

  • Attribute
  • Bool
  • CIDR
  • ConfirmType
  • Constant
  • CreditCardExpires
  • CreditCardSecurityCode
  • CreditCardValidator
  • DateConverter
  • DateTime
  • DateValidator
  • DictConverter
  • Email
  • Empty
  • False
  • FancyValidator
  • FieldStorageUploadConverter
  • FieldsMatch
  • FileUploadKeeper
  • FormValidator
  • IDeclarative
  • IPhoneNumberValidator
  • ISchema
  • IValidator
  • Identity
  • IndexListConverter
  • Int
  • Interface
  • Invalid
  • MACAddress
  • MaxLength
  • MinLength
  • NoDefault
  • NotEmpty
  • Number
  • OneOf
  • PhoneNumber
  • PlainText
  • PostalCode
  • Regex
  • RequireIfMissing
  • RequireIfPresent
  • Set
  • SignedString
  • StateProvince
  • String
  • StringBool
  • StringBoolean
  • StripField
  • TimeConverter
  • True
  • URL
  • UnicodeString
  • Validator
  • Wrapper

For the absolute most up-to date list of available validators, check the FormEncode validators module. You can also create your own validators or build on existing validators by inheriting from one of the defaults.

See the FormEncode documentation for how this is done.

You can also compose compound validators with logical operations, the FormEncode compound module provides All (all must pass), Any (any one must pass) and Pipe (all must pass with the results of each validator passed to the next item in the Pipe). You can use these like so:

from formencode.compound import All

Writing Custom Validators

If you can’t or don’t want to rely on the FormEncode library you can write your own validators.

Validators are simply objects that provide a to_python method which returns the converted value or raise tg.validation.TGValidationError

For example a validator that converts a paramter to an integer would look like:

from tg.validation import TGValidationError

class IntValidator(object):
    def to_python(self, value, state=None):
            return int(value)
            raise TGValidationError('Integer expected')

Then it is possible to pass an instance of IntValidator to the TurboGears @validate decorator.

Schema Validators

Sometimes you need more power and flexibility than you can get from validating individual form fields. Fortunately FormEncode provides just the thing for us – Schema validators.

If you want to do multiple-field validation, reuse validators or just clean up your code, validation Schema``s are the way to go. You create a validation schema by inheriting from :class:`formencode.schema.Schema` and pass the newly created ``Schema as the validators argument instead of passing a dictionary.

Create a schema:

class PwdSchema(schema.Schema):
    pwd1 = validators.String(not_empty=True)
    pwd2 = validators.String(not_empty=True)
    chained_validators = [validators.FieldsMatch('pwd1', 'pwd2')]

Then you can use that schema in @validate rather than a dictionary of validators:

def password(self, pwd1, pwd2):
    if tg.request.validation['errors']:
        return "There was an error"
        return "Password ok!"

Besides noticing our brilliant security strategy, please notice the chained_validators part of the schema that guarantees a pair of matching fields.

Again, for information about Invalid exception objects, creating your own validators, schema and FormEncode in general, refer to the FormEncode Validator documentation and don’t be afraid to check the Formencode.validators source. It’s often clearer than the documentation.

Note that Schema validation is rigorous by default, in particular, you must declare every field you are going to pass into your controller or you will get validation errors. To avoid this, add:

class MySchema( schema.Schema ):

to your schema declaration.