Pass variable to Python Flask form

You can solve this problem by using the global context variable g. The trick is not to use the builtin validator but a custom validator. The reason is that the builtin validator is a factory function. This means the validationMessage function you pass to it will only be executed on class creation. If you build a custom validator and read the language from the global context variable it will work.

def custom_validator(form, field):    language = g.language    msg = Message.query.filter_by(title=message).first()    lang = Language.query.filter_by(abbr=language).first()    text = messageBody.query.filter_by(message_id=msg.id, language_id=lang.id).first().text    if not len(field.data):        raise validators.ValidationError(text)

Replace the validator in your form with the custom validator:

class messageForm(FlaskForm):    title = StringField('title', validators=[custom_validator])

In the view function just create the language property for the global context variable.

# Messages@admin.route('/<string:language>/message', methods=['GET'])def messageView(language='dk')    g.language = language    form=messageForm()    ...