SelectMultipleField default values are ignored when a request object exists

[azmeuk]

When a request object and a data dict are passed to a form, the data dict values for SelectMultipleField are ignored.

>>> import wtforms, werkzeug
>>> choices=[("foo", "foo"), ("bar", "bar"), ("baz", "baz")]
>>> requestform = werkzeug.datastructures.ImmutableMultiDict([])

>>> class F(wtforms.Form):
...     multiple = wtforms.SelectMultipleField(choices=choices)

>>> F(multiple=["foo", "bar"]).multiple()
Markup('<select id="multiple" multiple name="multiple"><option selected value="foo">foo</option><option selected value="bar">bar</option><option value="baz">baz</option></select>')

>>> F(requestform, multiple=["foo", "bar"]).multiple()
Markup('<select id="multiple" multiple name="multiple"><option value="foo">foo</option><option value="bar">bar</option><option value="baz">baz</option></select>')
>>> # Nothing is 'selected' here...

A simple workaround is F(requestform or None, multiple=["foo", "bar"]).multiple() but I think this should probably work without it.

The problem does not exist with SelectField:

>>> class G(wtforms.Form):
...     single = wtforms.SelectField(choices=choices)
>>> G(requestform, single="foo").single()
Markup('<select id="single" name="single"><option selected value="foo">foo</option><option value="bar">bar</option><option value="baz">baz</option></select>')

Environment

• Python version: 3.8
• wtforms version: 2.3.1 and master

[davidism]

I think this is a bug with SelectField, not SelectMultipleField. If form data was passed in, default data should be ignored. But with many web frameworks it's impossible to tell that a form wasn't submitted vs submitted but empty by only looking at the form data object. This is a specific case of #402.