Error snippets

Customising error messages

When creating custom validators, as shown earlier in this manual, it is easy to tailor an error message. However, zope.schema and z3c.form already perform a fair amount of validation for us, which results in generic error messages. For example, if a required field is not completed, a rather bland error message (“Required input is missing”) will be shown. Sometimes, we may want to change these messages.

z3c.form allows error messages to be customised at various levels of detail. For example, it is possible to register a custom z3c.form.interfaces.IErrorViewSnippet adapter, which behaves like a mini-view and can output arbitrary HTML. However, in most cases, we only want to update the output text string. For this, we use what’s known as a “value adapter”. This is simply an adapter which z3c.form’s default IErrorViewSnippet implementations will look up to determine which message to show.

The easiest way to create an error message value adapter is to use the @form.error_message() decorator from plone.directives.form. This decorator should be applied to a function that takes as its only argument the (invalid) value that was submitted, and return a unicode string or message indicating the error. To illustrate this, we will add a new function to, just after the interface definition:

from five import grok
from plone.supermodel import model
from plone.directives import form

from zope.interface import invariant, Invalid
from zope.component import queryUtility

from zope import schema

from zope.schema.interfaces import IContextSourceBinder
from zope.schema.interfaces import RequiredMissing
from zope.schema.vocabulary import SimpleVocabulary


from example.dexterityforms.interfaces import MessageFactory as _


class IPizzaOrder(model.Schema):


    telephone = schema.ASCIILine(
            title=_(u"Telephone number"),
            description=_(u"We prefer a mobile number"),


@form.error_message(field=IPizzaOrder['telephone'], error=RequiredMissing)
def telephoneOmittedErrorMessage(value):
    return u"Without your telephone number, we can't contact you in case of a problem."

As with the @form.validator() decorator, the @form.error_message() validator takes a number of keyword arguments, used to control where the error message is applied. The allowable arguments are:

The type of error, which is normally represented by an exception class. The most general type will usually be a zope.schema.interfaces.ValidationError. See below for a list of other common exception types.
The current request. This is normally used to supply a browser layer marker interface. This is a good way to ensure a general error message is only in force when our product is installed.
The widget which was used to render the field.
The field to which the error message applies. If this is omitted, the message would apply to all fields on the form (provided form is supplied) of the given error (provided error is applied).
The form class. We can use this either to apply a single message to a given error across multiple fields in one form (in which case field would be omitted), or to customise an error message for a particular form only if a schema is used in more than one form.
The content item (context) on which the form is being rendered.


In almost all cases, you will want to supply both field and error at a minimum, although if you have multiple fields that may raise a particular error, and you want to create a message for all instances of that error, you can omit field and use form instead. If you supply just error, the validator will apply to all instances of that error, on all forms, site-wide, which is probably not a good idea if you intend your code to be-usable. At the very least, you should use the request field to specify a browser layer in this case, and install that layer with browserlayer.xml in your product’s installation profile.

The exception types which may be used for the error discriminator are field-specific. The standard fields as defined in zope.schema use the following exceptions, all of which can be imported from zope.schema.interfaces:

  • RequiredMissing, used when a required field is submitted without a value
  • WrongType, used when a field is passed a value of an invalid type
  • TooBig and TooSmall, used when a value is outside the min and/or max range specified for ordered fields (e.g. numeric or date fields)
  • TooLong and TooShort, used when a value is outside the min_length and/or max_length range specified for length-aware fields (e.g. text or sequence fields)
  • InvalidValue, used when a value is invalid, e.g. a non-ASCII character passed to an ASCII field
  • ConstraintNotSatisfied, used when a constraint method returns False
  • WrongContainedType, used if an object of an invalid type is added to a sequence (i.e. the type does not conform to the field’s value_type)
  • NotUnique, used if a uniqueness constraint is violated
  • InvalidURI, used for URI fields if the value is not a valid URI
  • InvalidId, used for Id fields if the value is not a valid id
  • InvalidDottedName, used for DottedName fields if the value is not a valid dotted name