How to bind a field in __init__ function of a form

Question

class Example_Form(Form):
    field_1 = TextAreaField()
    field_2 = TextAreaField()

    def __init__(self, type, **kwargs):
        super(Example_Form, self).__init__(**kwargs)

        if type == 'type_1':
           self.field_3 = TextAreaField()

In some scenarios I need to dynamically add fields into the form. The field_3 added to example form turns out to be a UnboundField. I tried to bind field_3 to form in __init__ function, but it won't work.

field_3 = TextAreaField()
field_3.bind(self, 'field_3')

How to bind field_3 to example form?

Answer 1

Use self.meta.bind_field to create a bound field, and assign it to the instance and the _fields dict.

self.field_3 = self._fields['field_3'] = self.meta.bind_field(
    self, TextAreaField(),
    {'name': 'field_3', 'prefix': self._prefix}
)

---

In most cases, it's more clear to use a subclass and decide which class to use when creating the form instance.

class F1(Form):
    x = StringField()

class F2(F1):
    y = StringField()

form = F1() if type == 1 else F2()

If you need to be more dynamic, you can subclass the form and assign fields to it. Assigning fields to classes works directly, unlike with instances.

class F3(F1):
    pass

if type == 3:
    F3.z = StringField()

form = F3()

You can also define all fields, then choose to delete some before validating the form.

class F(Form):
    x = StringField()
    y = StringField()

form = F()

if type == 1:
    del form.y