How to use the formencode.validators.Invalid function in FormEncode

def validate_python(self, value, state):
        raise validators.Invalid('ERROR: Description', value, state)

except ValueError:
            wiki_in_url = -1
        if wiki_in_url >= len(url_parts) - 2:
            value = '/'.join(url_parts[:wiki_in_url])
        # normalize trailing slash
        value = value.rstrip('/') + '/'

        try:
            resp = requests.head(value, allow_redirects=True, timeout=10)
        except IOError:
            # fall through to 'raise' below
            pass
        else:
            if resp.status_code == 200:
                return value
        raise fev.Invalid(self.message('unavailable', state), value, state)

def to_python(self, value, state=None):
        value = super(ToolsValidator, self).to_python(value, state)
        valid = []
        invalid = []
        for name in value:
            importer = ToolImporter.by_name(name)
            if importer is not None and importer.source == self.source:
                valid.append(name)
            else:
                invalid.append(name)
        if invalid:
            pl = 's' if len(invalid) > 1 else ''
            raise fev.Invalid('Invalid tool%s selected: %s' %
                              (pl, ', '.join(invalid)), value, state)
        return valid

def _invalid(self, value, state):
        message = "please choose an item from the list"
        raise v.Invalid(message, value, state)

if value is None:
            return None
        if isinstance(value, float):
            value = str(value)
        if isinstance(value, string_type):
            try:
                connection = state.connection or state.soObject._connection
            except AttributeError:
                pass
            else:
                if hasattr(connection, "decimalSeparator"):
                    value = value.replace(connection.decimalSeparator, ".")
            try:
                return Decimal(value)
            except Exception:
                raise validators.Invalid(
                    "can not parse Decimal value '%s' "
                    "in the DecimalCol from '%s'" % (
                        value, getattr(state, 'soObject', '(unknown)')),
                    value, state)
        if isinstance(value, (int, long, Decimal, sqlbuilder.SQLExpression)):
            return value
        raise validators.Invalid(
            "expected a Decimal in the DecimalCol '%s', got %s %r instead" % (
                self.name, type(value), value), value, state)

@app.route('/save_template/', methods=['POST'])
def save_template(doc_id):
	doc = couch.PonyExpressTemplate.get(doc_id)
	try:
		form = TemplateForm.to_python(request.form)
	except formencode.validators.Invalid, error:
		return str(error)
	else:
		doc.contents = []
		for k,v in form.iteritems():
			if not k == 'contents':
				setattr(doc, k, v)
			elif k == 'contents':
				for content in v:
					if all([content.get(key, None) for key in ('lang','subject','body')]):
						doc.contents.append(couch.LocalTemplate(**content))
		doc.save()
		return redirect(url_for('view_template', doc_id=doc._id))

if value is None:
            return None
        if isinstance(value, float):
            value = str(value)
        if isinstance(value, basestring):
            try:
                connection = state.connection or state.soObject._connection
            except AttributeError:
                pass
            else:
                if hasattr(connection, "decimalSeparator"):
                    value = value.replace(connection.decimalSeparator, ".")
            try:
                return Decimal(value)
            except:
                raise validators.Invalid("can not parse Decimal value '%s' in the DecimalCol from '%s'" %
                    (value, getattr(state, 'soObject', '(unknown)')), value, state)
        if isinstance(value, (int, long, Decimal, sqlbuilder.SQLExpression)):
            return value
        raise validators.Invalid("expected a Decimal in the DecimalCol '%s', got %s %r instead" % \
            (self.name, type(value), value), value, state)

    @classmethod
    def _to_formencode_invalid(cls, ming_invalid):
        if ming_invalid.error_dict:
            return fev.Invalid(
                ming_invalid.msg, 
                ming_invalid.value, ming_invalid.state,
                error_dict=dict(
                    (k, cls._to_formencode_invalid(v))
                    for k,v in ming_invalid.error_dict.items()))
        elif ming_invalid.error_list:
            return fev.Invalid(
                ming_invalid.msg, 
                ming_invalid.value, ming_invalid.state,
                error_list=map(cls._to_formencode_invalid, ming_invalid.error_list))
        else:
            return fev.Invalid(
                ming_invalid.msg, 
                ming_invalid.value, ming_invalid.state)

flash('Cannot delete the admin tool, sorry....')
                    redirect('.')
                c.project.uninstall_app(self.app.config.options.mount_point)
                redirect('..')
            for opt in self.app.config_options:
                if opt in Application.config_options:
                    # skip base options (mount_point, mount_label, ordinal)
                    continue
                val = kw.get(opt.name, '')
                if opt.ming_type == bool:
                    val = asbool(val or False)
                elif opt.ming_type == int:
                    val = asint(val or 0)
                try:
                    val = opt.validate(val)
                except fev.Invalid as e:
                    flash(u'{}: {}'.format(opt.name, str(e)), 'error')
                    continue
                self.app.config.options[opt.name] = val
            if is_admin:
                # possibly moving admin mount point
                redirect('/'
                         + c.project._id
                         + self.app.config.options.mount_point
                         + '/'
                         + self.app.config.options.mount_point
                         + '/')
            else:
                redirect(request.referer or '/')

    @classmethod
    def _to_formencode_invalid(cls, ming_invalid):
        if ming_invalid.error_dict:
            return fev.Invalid(
                ming_invalid.msg, 
                ming_invalid.value, ming_invalid.state,
                error_dict=dict(
                    (k, cls._to_formencode_invalid(v))
                    for k,v in ming_invalid.error_dict.items()))
        elif ming_invalid.error_list:
            return fev.Invalid(
                ming_invalid.msg, 
                ming_invalid.value, ming_invalid.state,
                error_list=map(cls._to_formencode_invalid, ming_invalid.error_list))
        else:
            return fev.Invalid(
                ming_invalid.msg, 
                ming_invalid.value, ming_invalid.state)