How to use the wtforms.ValidationError function in WTForms

def validate_email(self, field):
        if User.query.filter_by(email=field.data).first():
            raise ValidationError('Email already registered.')

def validate_password(self, field):
        u = User.query.filter_by(login_name=self.username.data).first()
        if u is None or not u.verify_password(field.data):
            raise ValidationError('Username or password is invalid.')

def validate_email(self, field):
        if User.query.filter_by(email=field.data).first():
            raise ValidationError('Email already registered.')

def __call__(self, form, field):
        try:
            other = form[self.field_name]
        except KeyError:
            raise ValidationError(
                field.gettext("Invalid field name '%s'.") % self.field_name)

        # make sure both values are of the same type
        if not isinstance(field.data, type(other.data)):
            raise ValidationError(
                'Cannot compare the two fields: they are of different types.')

        if field.data > other.data:
            raise ValidationError(
                'Please make sure %s is not greater than %s.' %
                (field.label.text, other.label.text))

"""
    get_logger(__name__).debug("Importing new plugin report in server...")

    # Authorization code copy-pasted from server/api/base.py
    ws = Workspace.query.filter_by(name=workspace).one()
    if not (ws.active
            ):
        # Don't raise a 403 to prevent workspace name enumeration
        abort(404, "Workspace disabled: %s" % workspace)

    if 'file' not in request.files:
        abort(400)

    try:
        validate_csrf(request.form.get('csrf_token'))
    except ValidationError:
        abort(403)

    report_file = request.files['file']

    if report_file:

        chars = string.ascii_uppercase + string.digits
        random_prefix = ''.join(random.choice(chars) for x in range(12))
        raw_report_filename = '{0}{1}'.format(random_prefix, secure_filename(report_file.filename))

        try:
            file_path = os.path.join(
                CONF.getConfigPath(),
                'uploaded_reports/{0}'.format(raw_report_filename))
        except AttributeError:
            get_logger().warning(

def validate_email(self, field):
        if User.query.filter_by(email=field.data).first():
            raise ValidationError('邮箱已经存在')

raise ValidationError('The CSRF token is missing.')

    if field_name not in session:
        raise ValidationError('The CSRF session token is missing.')

    s = URLSafeTimedSerializer(secret_key, salt='wtf-csrf-token')

    try:
        token = s.loads(data, max_age=time_limit)
    except SignatureExpired:
        raise ValidationError('The CSRF token has expired.')
    except BadData:
        raise ValidationError('The CSRF token is invalid.')

    if not safe_str_cmp(session[field_name], token):
        raise ValidationError('The CSRF tokens do not match.')

def no_wiki_or_html(form, field):
    if '[/url]' in field.data:
        raise wtforms.ValidationError('Cannot search with wiki markup')
    if '' in field.data:
        raise wtforms.ValidationError('Cannot search with html markup')

def validate_email(self, field):
        if User.query.filter_by(email=field.data).first():
            raise ValidationError('Email already exists.')