Validators are components that validate submitted data. This is certainly not a new concept, but in the previous form frameworks validation was hidden in many places:

  • Field/Widget Validation

    The schema field consists of a validate() method. Validation is automatically invoked when converting a unicode string to a field value using fromUnicode(). This makes it very hard to customize the field validation. No hooks were provided to exert dditional restriction at the presentation level.

  • Schema/Form Validation

    This type of validation was not supported at all initially. zope.formlib fixed this problem by validating against schema invariants. While this was a first good step, it still made it hard to customize validators, since it required touching the base implementations of the forms.

  • Action Validation

    zope.formlib supports the notion of action validatos. Actions have a success and failure handler. If the validation succeeds, the success handler is called, otherwise the failure handler is chosen. We believe that this design was ill-conceived, especially the default, which required the data to completely validate in order for the action to successful. There are many actions that do not even care about the data in the form, such as “Help”, “Cancel” and “Reset” buttons. Thus validation should be part of the data retrieval process and not the action.

For me, the primary goals of the validator framework are as follows:

  • Assert additional restrictions on the data at the presentation level.

    There are several use cases for this. Sometimes clients desire additional restrictions on data for their particular version of the software. It is not always desireable to adjust the model for this client, since the framework knows how to handle the less restrictive case anyways. In another case, additional restrictions might be applied to a particular form due to limited restrictions.

  • Make validation pluggable.

    Like most other components of this package, it should be possible to control the validation adapters at a fine grained level.

    • Widgets: context, request, view, field[1], widget
    • Widget Managers: context, request, view, schema[2], manager

    [1].. This is optional, since widgets must not necessarily have fields. [2].. This is optional, since widget managers must not necessarily have manage field widgets and thus know about schemas.

  • Provide good defaults that behave sensibly.

    Good defaults are, like in anywhere in this pacakge, very important. We have chosen to implement the zope.formlib behavior as the default, since it worked very well – with exception of action validation, of course.

For this package, we have decided to support validators at the widget and widget manager level. By default the framework only supports field widgets, since the validation of field-absent widgets is generally not well-defined. Thus, we first need to create a schema.

>>> import zope.interface
>>> import zope.schema
>>> class IPerson(zope.interface.Interface):
...     login = zope.schema.TextLine(
...         title=u'Login',
...         min_length=1,
...         max_length=10)
...     email = zope.schema.TextLine(
...         title=u'E-mail')
...     @zope.interface.invariant
...     def isLoginPartOfEmail(person):
...         if not
...             raise zope.interface.Invalid("The login not part of email.")

Widget Validators

Widget validators only validate the data of one particular widget. The validated value is always assumed to be an internal value and not a widget value.

By default, the system uses the simple field validator, which simply uses the validate() method of the field. For instantiation, all validators have the following signature for its discriminators: context, request, view, field, and widget

>>> from z3c.form import validator
>>> simple = validator.SimpleFieldValidator(
...     None, None, None, IPerson['login'], None)

A validator has a single method validate(). When the validation is successful, None is returned:

>>> simple.validate(u'srichter')

A validation error is raised, when the validation fails:

>>> simple.validate(u'StephanCaveman3')
Traceback (most recent call last):
TooLong: (u'StephanCaveman3', 10)

Let’s now create a validator that also requires at least 1 numerical character in the login name:

>>> import re
>>> class LoginValidator(validator.SimpleFieldValidator):
...     def validate(self, value):
...         super(LoginValidator, self).validate(value)
...         if'[0-9]', value) is None:
...             raise zope.interface.Invalid('No numerical character found.')

Let’s now try our new validator:

>>> login = LoginValidator(None, None, None, IPerson['login'], None)
>>> login.validate(u'srichter1')
>>> login.validate(u'srichter')
Traceback (most recent call last):
Invalid: No numerical character found.

We can now register the validator with the component architecture, ...

>>> import zope.component
>>> zope.component.provideAdapter(LoginValidator)

and look up the adapter using the usual way:

>>> from z3c.form import interfaces
>>> zope.component.queryMultiAdapter(
...     (None, None, None, IPerson['login'], None),
...     interfaces.IValidator)
<LoginValidator for IPerson['login']>

Unfortunately, the adapter is now registered for all fields, so that the E-mail field also has this restriction (which is okay in this case, but not generally):

>>> zope.component.queryMultiAdapter(
...     (None, None, None, IPerson['email'], None),
...     interfaces.IValidator)
<LoginValidator for IPerson['email']>

The validator module provides a helper function to set the discriminators for a validator, which can include instances:

>>> validator.WidgetValidatorDiscriminators(
...     LoginValidator, field=IPerson['login'])

Let’s now clean up the component architecture and register the login validator again:

>>> from zope.testing import cleanup
>>> cleanup.cleanUp()
>>> zope.component.provideAdapter(LoginValidator)
>>> zope.component.queryMultiAdapter(
...     (None, None, None, IPerson['login'], None),
...     interfaces.IValidator)
<LoginValidator for IPerson['login']>
>>> zope.component.queryMultiAdapter(
...     (None, None, None, IPerson['email'], None),
...     interfaces.IValidator)

Widget Validators and File-Uploads

File-Uploads behave a bit different than the other form elements. Whether the user did not choose a file to upload interfaces.NOT_CHANGED is set as value. But the validator knows how to handle this.

The example has two bytes fields where File-Uploads are possible, one field is required the other one not:

>>> class IPhoto(zope.interface.Interface):
...     data = zope.schema.Bytes(
...         title=u'Photo')
...     thumb = zope.schema.Bytes(
...         title=u'Thumbnail',
...         required=False)

There are several possible cases to differentiate between:

No widget

If there is no widget or the widget does not provide interfaces.IContextAware, no value is looked up from the context. So the not required field validates successfully but the required one has an required missing error, as the default value of the field is looked up on the field:

>>> simple_thumb = validator.SimpleFieldValidator(
...     None, None, None, IPhoto['thumb'], None)
>>> simple_thumb.validate(interfaces.NOT_CHANGED)
>>> simple_data = validator.SimpleFieldValidator(
...     None, None, None, IPhoto['data'], None)
>>> simple_data.validate(interfaces.NOT_CHANGED)
Traceback (most recent call last):
RequiredMissing: data

Widget which ignores context

If the context is ignored in the widget - as in the add form - the behavior is the same as if there was no widget:

>>> import z3c.form.widget
>>> widget = z3c.form.widget.Widget(None)
>>> zope.interface.alsoProvides(widget, interfaces.IContextAware)
>>> widget.ignoreContext = True
>>> simple_thumb = validator.SimpleFieldValidator(
...     None, None, None, IPhoto['thumb'], widget)
>>> simple_thumb.validate(interfaces.NOT_CHANGED)
>>> simple_data = validator.SimpleFieldValidator(
...     None, None, None, IPhoto['data'], widget)
>>> simple_data.validate(interfaces.NOT_CHANGED)
Traceback (most recent call last):
RequiredMissing: data

Look up value from default adapter

When the value is interfaces.NOT_CHANGED the validator tries to look up the default value using a interfaces.IValue adapter. Whether the adapter is found, its value is used as default, so the validation of the required field is successful here:

>>> data_default = z3c.form.widget.StaticWidgetAttribute(
...     'data', context=None, request=None, view=None,
...     field=IPhoto['data'], widget=widget)
>>> zope.component.provideAdapter(data_default, name='default')
>>> simple_data.validate(interfaces.NOT_CHANGED)

Look up value from context

If there is a context aware widget which does not ignore its context, the value is looked up on the context using a data manager:

>>> class Photo(object):
...     zope.interface.implements(IPhoto)
...     data = None
...     thumb = None
>>> photo = Photo()
>>> widget.ignoreContext = False
>>> zope.component.provideAdapter(z3c.form.datamanager.AttributeField)
>>> simple_thumb = validator.SimpleFieldValidator(
...     photo, None, None, IPhoto['thumb'], widget)
>>> simple_thumb.validate(interfaces.NOT_CHANGED)

If the value is not set on the context it is a required missing as neither context nor input have a valid value:

>>> simple_data = validator.SimpleFieldValidator(
...     photo, None, None, IPhoto['data'], widget)
>>> simple_data.validate(interfaces.NOT_CHANGED)
Traceback (most recent call last):
RequiredMissing: data

After setting the value validation is successful:

>>> = 'data'
>>> simple_data.validate(interfaces.NOT_CHANGED)


>>> gsm = zope.component.getGlobalSiteManager()
>>> gsm.unregisterAdapter(z3c.form.datamanager.AttributeField)
>>> gsm.unregisterAdapter(data_default, name='default')

Widget Manager Validators

The widget manager validator, while similar in spirit, works somewhat different. The discriminators of the widget manager validator are: context, request, view, schema, and manager.

A simple default implementation is provided that checks the invariants of the schemas:

>>> invariants = validator.InvariantsValidator(
...     None, None, None, IPerson, None)

Widget manager validators have the option to validate a data dictionary,

>>> invariants.validate(
...     {'login': u'srichter', 'email': u''})

or an object implementing the schema:

>>> class Person(object):
...     zope.interface.implements(IPerson)
...     login = u'srichter'
...     email = u''
>>> stephan = Person()
>>> invariants.validateObject(stephan)

Since multiple errors can occur during the validation process, all errors are collected in a tuple, which is returned. If the tuple is empty, the validation was successful. Let’s now generate a failure:

>>> errors = invariants.validate(
...     {'login': u'srichter', 'email': u''})
>>> for e in errors:
...     print e.__class__.__name__ + ':', e
Invalid: The login not part of email.

Let’s now have a look at writing a custom validator. In this case, we want to ensure that the E-mail address is at most twice as long as the login:

>>> class CustomValidator(validator.InvariantsValidator):
...     def validateObject(self, obj):
...         errors = super(CustomValidator, self).validateObject(obj)
...         if len( > 2 * len(obj.login):
...             errors += (zope.interface.Invalid('Email too long.'),)
...         return errors

Since the validate() method of InvatiantsValidator simply uses validateObject() it is enough to only override validateObject(). Now we can use the validator:

>>> custom = CustomValidator(
...     None, None, None, IPerson, None)
>>> custom.validate(
...     {'login': u'srichter', 'email': u''})
>>> errors = custom.validate(
...     {'login': u'srichter', 'email': u''})
>>> for e in errors:
...     print e.__class__.__name__ + ':', e
Invalid: Email too long.

To register the custom validator only for this schema, we have to use the discriminator generator again.

>>> from z3c.form import util
>>> validator.WidgetsValidatorDiscriminators(
...     CustomValidator, schema=util.getSpecification(IPerson, force=True))
Note: Of course we could have used the zope.component.adapts() function
from within the class, but I think it is too tedious, since you have to specify all discriminators and not only the specific ones you are interested in.

After registering the validator,

>>> zope.component.provideAdapter(CustomValidator)

it becomes the validator for this schema:

>>> zope.component.queryMultiAdapter(
...     (None, None, None, IPerson, None), interfaces.IManagerValidator)
<CustomValidator for IPerson>
>>> class ICar(zope.interface.Interface):
...     pass
>>> zope.component.queryMultiAdapter(
...     (None, None, None, ICar, None), interfaces.IManagerValidator)

The Data Wrapper

The Data class provides a wrapper to present a dictionary as a class instance. This is used to check for invariants, which always expect an object. While the common use cases of the data wrapper are well tested in the code above, there are some corner cases that need to be addressed.

So let’s start by creating a data object:

>>> context = object()
>>> data = validator.Data(IPerson, {'login': 'srichter', 'other': 1}, context)

When we try to access a name that is not in the schema, we get an attribute error:

>>> data.address
Traceback (most recent call last):
AttributeError: address
>>> data.other
Traceback (most recent call last):
AttributeError: other

If the field found is a method, then a runtime error is raised:

>>> class IExtendedPerson(IPerson):
...     def compute():
...         """Compute something."""
>>> data = validator.Data(IExtendedPerson, {'compute': 1}, context)
>>> data.compute
Traceback (most recent call last):
RuntimeError: ('Data value is not a schema field', 'compute')

Finally, the context is available as attribute directly:

>>> data.__context__ is context

It is used by the validators (especially invariant validators) to provide a context of validation, for example to look up a vocabulary or access the parent of an object. Note that the context will be different between add and edit forms.

Validation of interface variants when not all fields are displayed in form

We need to register the data manager to access the data on the context object:

>>> from z3c.form import datamanager
>>> zope.component.provideAdapter(datamanager.AttributeField)

Sometimes you might leave out fields in the form which need to compute the invariant. An exception should be raised. The data wrapper is used to test the invariants and looks up values on the context object that are left out in the form.

>>> invariants = validator.InvariantsValidator(
...     stephan, None, None, IPerson, None)
>>> errors = invariants.validate({'email': ''})
>>> errors[0].__class__.__name__
>>> errors[0].args[0]
'The login not part of email.'