Pass variable to Python Flask form

Question

I'm currently stuck trying to build a variable message flashing function for my Flask forms

Since I'm building a tool that has to have multiple languages available, I wish to create custom messages for my flask form validation based on specific language input.

My form looks something like this:

class messageForm(FlaskForm):
    title = StringField(
        'title',
        validators=[
            validators.DataRequired(validationMessage('validateRequired', language=language))
        ]
    )

the function "validationMessage" looks like this:

def validationMessage(message, 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
    return text

As you notice, I do some lookup in a few database tables to produce my message text.

My trouble is now... How do I pass my language variable from my view to my form so that I can pass it on to the "validationMessage" function?

The language variable is based on a variable in my view endpoint

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

I have considered using my session for this, but as I understand it, I can only use this within my view and therefore not within either my form or the message function

You could always add a HiddenField to the form and then set that parameter after you construct it in your view. The value should be then available to you in your form instance — shiv
– shiv, Commented Dec 20, 2016 at 17:53
I could do this, if I needed to pass the variable through to my template, unfortunately I need to pass the variable to my Form class, and as such it won't work for me. — Henrik Poulsen
– Henrik Poulsen, Commented Dec 20, 2016 at 19:22
I'm assuming you're using flask-wtforms? If so, the form can be instantiated with kwargs for the fields. So if you had a HiddenField called lang, your form could be instantiated as form=messageForm(lang=language). Then set the validators language argument to be the value of your HiddenField — shiv
– shiv, Commented Dec 20, 2016 at 20:36
Trying this I unfourtunately get the following error: <UnboundField(HiddenField, (), {})> — Henrik Poulsen
– Henrik Poulsen, Commented Dec 21, 2016 at 6:36
The problem is actually that the validationMessage function only gets called when initializing your validator. And that happens right when you start your Flask app. It is not called later on form validation anymore. So this approach won't work at all. — MrLeeh
– MrLeeh, Commented Dec 21, 2016 at 7:12

MrLeeh · Accepted Answer · 2016-12-21 07:37:57Z

1

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()
    ...

answered Dec 21, 2016 at 7:37

MrLeeh

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1 Comment

gustavz Over a year ago

Does this also work for custom choices argument passed to a SelectField? If yes, how to create a custom choices function? I tried it with a function using global variables returning the choices where the flobal variable is set at the beginning of the main() function. But this does not work

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