How to dynamically assign one of several values ​​to a WTForms descendant field when initializing a class instance?

Question

Flask

Good time.
# Project on Flask + WTForms

I have several custom SelectFields and a form in which one and the same field, depending on the conditions, must be assigned one or another SelectField.

# in view
@app.route('/some/<int:a>/', methods=['POST', 'GET'])
def f(a):
    form = MyForm(a=a)
    return render_template('t.html', form=form)

# in forms
class MyForm(Form):
    next = HiddenField()
    static_field1 = StringField()
    static_field2 = StringField()
    static_field3 = StringField()

    # здесь проблема
    # если a == 1 то необходимо сделать specific_field = CustomSelect1(),
    # но если a == 2, то specific_field = CustomSelect()2

    specific_field = ?

As far as I understand, it is necessary to overload __init__() MyForm in such a way that, depending on the value accepted as a parameter, one or another SelectField can be assigned to the field. Can

you please tell me how to implement this correctly?

Thank you.

Answer 1

This is usually done by metaclasses.

Answer 2

You can add the necessary fields to the form in the view:

if a == 1:
    MyForm.static_field1 = CustomSelect1()

Link to WTF documentation