Deform uses Colander for form validation.

See Schema Node Objects in Colander's documentation for more information.

Colander stock validators

You can use default Colander validators in Deform forms:


class NewsletterSender(CSRFSchema):
    """Send a news letter."""

    preview = colander.SchemaNode(colander.Boolean())

    # Validate emaik
    email = colander.SchemaNode(colander.String(), validator=colander.Email(), missing=colander.Drop)

Custom validator functions

Here is an example of a data-driven validator that validates the form submission against an SQLAlchemy query.

import colander

def validate_unique_user_email(node, value, **kwargs):
"""Prevent duplicate usernames."""

    request = node.bindings["request"]
    dbsession = request.dbsession
    User = get_user_class(request.registry)
    if dbsession.query(User).filter_by(email=value).first():
        raise colander.Invalid(node, "Email address already taken")

class MySchema(CSRFSchema):
    email = colander.SchemaNode(colander.String(), validator=validate_unique_user_email)

Form level validation

Sometimes you might need to do complex validation where a colander.deferred() pattern complicates things too much. You might just want to do the validation within your view. Here is an example of how to do it.

class NewsletterSend(CSRFSchema):
    """Send a newsletter."""

    preview = colander.SchemaNode(colander.Boolean(), description="Is this a preview send.", default=True)

    email = colander.SchemaNode(colander.String(), title="Preview email", description="Send preview email to this email address", validator=colander.Email(), missing=colander.null)

    def validator(self, node: "NewsletterSend", appstruct: dict):
        """Custom schema level validation code."""

        # appstruct is Colander appstruct after all other validations have passed
        # Note that this method might not ever be reached
        if appstruct["preview"] and appstruct["email"] == colander.null:
            # This error message appears at the top of the form
            raise colander.Invalid(node["email"], "Please fill in email field if you want to send a preview email.")

Setting errors after validation

Sometimes you want to react to an error that arises after the form validation happens, for example, when you call a third party API service. You can convert exceptions to form validation errors.

Use form.widget.set_error() to set a form level error.

def newsletter(context: Admin, request: Request):
    """Newsletter admin form."""
    schema = NewsletterSend().bind(request=request)

    # Create a styled button with some extra Bootstrap 3 CSS classes
    b = deform.Button(name='process', title="Send", css_class="btn-block btn-lg")
    form = deform.Form(schema, buttons=(b, ), resource_registry=ResourceRegistry(request))

    rendered_form = None

    # User submitted this form
    if request.method == "POST":
        if 'process' in request.POST:

                appstruct = form.validate(request.POST.items())

                if appstruct["preview"]:
                    send_newsletter(request, appstruct["subject"], preview_email=appstruct["email"])
                    messages.add(request, "Preview email sent.")
                    send_newsletter(request, appstruct["subject"])
                    messages.add(request, "Newsletter sent.")

                return httpexceptions.HTTPFound(request.url)

            except MailgunError as e:
                # API call failed
                # Do a form level error message
                exc = colander.Invalid(form.widget, "Could not sent newsletter:" + str(e))
                form.widget.handle_error(form, exc)

            except deform.ValidationFailure as e:
                # Render a form version where errors are visible next to the fields,
                # and the submitted values are posted back
                rendered_form = e.render()
            # We don't know which control caused form submission
            return httpexceptions.HTTPBadRequest("Unknown form button pressed")

    # Render initial form
    # Default values for read only fields
    if rendered_form is None:
        rendered_form = form.render({
            "api_key": api_key,
            "domain": domain,
            "mailing_list": mailing_list,

    # This loads widgets specific CSS/JavaScript in HTML code,
    # if form widgets specify any static assets.
    form.resource_registry.pull_in_resources(request, form)

    return locals()