validate_on_submit() expects a tuple list for SelectField even though a normal list is a valid argument

[ghost]

Actual Behavior

form.validate on submit() evaluates to False when choices in SelectField is a list of values only. The same form validation evaluates to True when using (value, label) tuples as used with version 2.2.1

class RegistrationForm(FlaskForm):

    language = SelectField('Language', choices=['python', 'ruby', 'rust'], validators=[DataRequired()])
    submit = SubmitField('Submit')


@app.route('/', methods=['GET', 'POST'])
def index():
    form = RegistrationForm()
    if form.validate_on_submit():
        return redirect(url_for('test'))
    return render_template('index.html', form=form)

The SelectField() is rendered properly in the HTML form and its value can be accessed with form.language.data but form.validate_on_submit() still evaluates to False.

Expected Behavior

form.validate on submit() to evaluate to True when choices in SelectField is a list of values only as well.

Environment

• Python version: 3.8.2
• Flask-wtf version: 0.14.3
• wtforms version: 2.3.1

[davidism]

Duplicate of pallets-eco/wtforms#598

@azmeuk Need to backport that to the 2.x branch and make a bugfix release.