FormEncode @validate, and TurboGears 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 arguments (without form widgets)

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

from formencode import validators

@validate(validators={"a":validators.Int(), "b":validators.Email})
def two_validators(self, a=None, b=None, *args):
    errors = [{key, value} in pylons.c.form_errors.iteritems()]
    values =  pylons.c.form_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.

If there’s a validation error, TurboGears calls the error_handler if it exists, but it always adds form_errors and form_values to the tmpl_context, so they will be available there for the rest of the request. In this case if there are validation errors, we grab both the error messages and the original unvalidated values and return them in the error message.

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.

Available 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

Validating widget-based forms

The simplest way to use @validate() is to pass in a reference to a widgets-based form:


The widgets system will take care of building a schema to handle the data conversions and you’ll wind up with the int or datetime objects you specified when building the form. When paired with the validate decorator, you can handle the common case of building a form, validating it, redisplaying the form if there are errors, and converting a valid form into the proper arguments in only a few lines of Python.

You can also pass the form using a keyword argument:


You might also want to tell TurboGears to pass off handling of invalid data to a different controller. To do that you just pass the method you want called to @validate via the error_handler param:

@validate(forms.myform, error_handler=process_form_errors)

The method in question will be called, with the unvalidated data as its parameters. And error validation messages will be stored in pylons.tmpl_context.

Here’s a quick example of how this all works:

def process_form_errors(self, **kwargs):
    #add error messages to the kwargs dictionary and return it
    kwargs['errors'] = pylons.tmpl_context.form_errors
    return dict(kwargs)

@validate(form=myform, error_handler=process_form_errors)
def send_to_error_handler(self, **kwargs):
    return dict(kwargs)

If there’s a validation error in myform, the send_to_error_handler method will never get called. Instead process_form_errors will get called, and the validation error messages can be picked up from the form_errors value of the template context object (pylons.tmpl_context).

Schema validation

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 ``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 pylons.c.form_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.

Converting URL strings to Python types manually

You can always use e.g. Python’s int() method to convert a string to an integer and use a try/except block to catch errors in the conversion process:

from tg import controllers, expose
class Root(controllers.RootController):

    # return the result of x+y
    def addnum(self, x, y)
           return str(int(x)+int(y))
           return 'value is not valid'

This isn’t that hard, but it quickly becomes unwieldy when you start converting large numbers of arguments. Moreover, you still have the problem of propagating the errors back to your users. In the end, it’s usually far simpler to use the validation framework.