Flask WTForms FieldList based on input parameter not working

Question

I'm trying to instantiate a Flask Form that contains a FieldList, for which I want to change the FormField based on an additional input parameter. Following this post , my setup looks like this:

class Child1(FlaskForm):
    class Meta:
        csrf = False
    
    field = SelectField()

class Child2(FlaskForm):
    class Meta:
        csrf = False
    
    field = StringField()

class Parent(FlaskForm):
    field_list_form = FieldList(FormField(Child1))
    def __init__(self, select_child="child1", *args, **kwargs):
        super().__init__(*args, **kwargs)
        if select_child == "child2":
            self.field_list_form = FieldList(FormField(Child2))

Now in my Flask app, when I use the default parameter, doing this works fine

form = Parent()
for idx in range(42):
    form.field_list_form.append_entry({})

But when doing

form = Parent(select_child="child2")
for idx in range(42):
    form.field_list_form.append_entry({})

I get AttributeError: 'UnboundField' object has no attribute 'append_entry'

How should I do this differently?

Answer 1

This question seems to already have an answer here .

However, another solution is to wrap the class declaration with a function taking select_child as an argument and, depending on the argument value, create the FormField with either of your Child classes.

In your case, it would be:

def create_parent(select_child="child1"):
    class Parent(FlaskForm):
        if select_child == 'child1':
            field_list_form = FieldList(FormField(Child1))
        elif select_child == "child2":
                field_list_form = FieldList(FormField(Child2))
                
    return Parent()

Then form = Parent(select_child="child2") becomes form = create_parent(select_child="child2")

Further details in this answer .