Passing on validators to select subfields seems wrong

[ThiefMaster]

This was added in #615 but it breaks custom select field subclasses that use e.g. a list of checkboxes as they now all get the required attribute.

Here's a failing test case (since both inputs get the required flag):

def test_required_flag_custom():
    class CustomWidget:
        def __call__(self, field, **kwargs):
            html = []
            for subfield in field:
                html.append(f'{subfield.label}: {subfield()}')
            return ''.join(html)

    class CustomSelectMultipleField(SelectMultipleField):
        widget = CustomWidget()
        option_widget = CheckboxInput()

    F = make_form(
        c=CustomSelectMultipleField(
            choices=[("a", "hello"), ("b", "bye")],
            validators=[validators.DataRequired()],
        )
    )
    form = F(DummyPostData(c=["a"]))
    assert form.c() == (
        '<label for="c-0">hello</label>: <input checked id="c-0" name="c" type="checkbox" value="a">'
        '<label for="c-1">bye</label>: <input id="c-1" name="c" type="checkbox" value="b">'
    )

I have the feeling that passing validators to subfields isn't the correct thing to do - only <input type="radio"> handles the required flag correctly, but of course for multi-select you'll never have radio buttons...

[homeworkprod]

Yes, please.

Is there a workaround?

[homeworkprod]

For the record, I had success fixing this issue for myself by using the patch done by @ThiefMaster in indico/indico#5185.

Would be great, though, to have this fixed in WTForms itself somehow.

[azmeuk]

Reproducible with:

>>> import wtforms
>>> import wtforms.widgets
>>> from werkzeug.datastructures import ImmutableMultiDict
...
>>> class CustomWidget:
...     def __call__(self, field, **kwargs):
...         html = []
...         for subfield in field:
...             html.append(f'{subfield.label}: {subfield()}')
...         return ''.join(html)
...
>>> class CustomSelectMultipleField(wtforms.SelectMultipleField):
...     widget = CustomWidget()
...     option_widget = wtforms.widgets.CheckboxInput()
...
>>> class F(wtforms.Form):
...     c=CustomSelectMultipleField(
...         choices=[("a", "hello"), ("b", "bye")],
...         validators=[wtforms.validators.DataRequired()],
...     )
...
>>> form = F(ImmutableMultiDict({"c":["a"]}))
>>> assert form.c() == (
...     '<label for="c-0">hello</label>: <input checked id="c-0" name="c" type="checkbox" value="a">'
...     '<label for="c-1">bye</label>: <input id="c-1" name="c" type="checkbox" value="b">'
... ), form.c()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AssertionError: <label for="c-0">hello</label>: <input checked id="c-0" name="c" required type="checkbox" value="a"><label for="c-1">bye</label>: <input id="c-1" name="c" required type="checkbox" value="b">

[azmeuk]

I am not sure how to do with this issue. render_kw being passed to sub-fields comes from #688
The idea was to the required flag could be passed to radio inputs:
https://developer.mozilla.org/en-US/docs/Web/HTML/Element/input/radio#required
However, indeed it seems awkward for select fields, and since #739 subfield render_kw can be set.

[azmeuk]

We should revert this behavior in v4.