Breaking Change between 2.3 and 3.0 on SelectField(validate_choice=False)

[Nathanmalnoury]

Actual Behavior

>>> import wtforms
>>> class F(wtforms.Form):
...     foo = wtforms.SelectField(validate_choice=False)
>>> f = F()
>>> f.validate()
Traceback (most recent call last):
  File "scratch_27.py", line 7, in <module>
    f.validate()
  File "venv/lib/python3.8/site-packages/wtforms/form.py", line 329, in validate
    return super().validate(extra)
  File "venv/lib/python3.8/site-packages/wtforms/form.py", line 146, in validate
    if not field.validate(self, extra):
  File "venv/lib/python3.8/site-packages/wtforms/fields/core.py", line 231, in validate
    self.pre_validate(form)
  File "venv/lib/python3.8/site-packages/wtforms/fields/choices.py", line 136, in pre_validate
    raise TypeError(self.gettext("Choices cannot be None."))
TypeError: Choices cannot be None.

A quick fix consists in adding choices=[] to the SelectedField:

>>> class F(wtforms.Form):
...     foo = wtforms.SelectField(choices=[], validate_choice=False)

Expected Behavior

As mentioned in the title, I wasn't expecting a breaking change on that point, as it wasn't in the changelog. I wasn't even expecting a change at all:

>>> import wtforms
>>> class F(wtforms.Form):
...     foo = wtforms.SelectField(validate_choice=False)
>>> f = F()
>>> f.validate()  # no erros

Environment

• Python version: 3.8
• wtforms version: 3.0.1

I am unsure if you'd rather add this to the documentation somewhere, or fix the compatibility break (which seems quite easy).
I've found a similar issue: #606; except that I'm expecting to be able to not provide any choices at all here.

[israel-oye]

I have the same issue atm. I keep getting the same error even after setting validate_choice=False
Have you found any fix?

[Nathanmalnoury]

For us, adding choices=[] alongside validate_choice=False did the trick.