Why does WTForms have unbound fields the first time the page is loaded? - flask-wtforms

Question

I have the following form classes:
class FieldsRequiredForm(FlaskForm):
class Meta:
def render_field(self, field, render_kw):
render_kw.setdefault('required', True)
return super().render_field(field, render_kw)
class SingleStringFieldForm(FieldsRequiredForm):
def __init__(self, field_label=None, question_id=None,
submit_label='Submit'):
super().__init__()
SingleStringFieldForm.answer = StringField(field_label)
SingleStringFieldForm.question_id = HiddenField(default=question_id)
SingleStringFieldForm.submit = SubmitField(submit_label)
class SingleRadioFieldForm(FieldsRequiredForm):
def __init__(self, field_label=None, question_id=None,
submit_label='Submit', choices=None):
super().__init__()
SingleRadioFieldForm.answer = RadioField(field_label, choices=choices)
SingleRadioFieldForm.question_id = HiddenField(default=question_id)
SingleRadioFieldForm.submit = SubmitField(submit_label)
The function that's using these forms looks like this:
#bp.route('/survey/<string:slug>', methods=['GET', 'POST'])
def question(slug):
survey = Survey.query.filter_by(slug=slug).first_or_404()
questions = survey.questions
question_ids = [question.id for question in questions]
if 'answers' not in session:
session['answers'] = json.dumps({id: None for id in question_ids})
answers = json.loads(session['answers'])
if request.method == 'POST':
record_submitted_answer()
answers = json.loads(session['answers'])
if None in answers.values():
question = get_next_question()
if question.category == 'word':
form = SingleStringFieldForm(field_label=question.question,
question_id=question.id)
elif question.category == 'likert':
form = SingleRadioFieldForm(field_label=question.question,
question_id=question.id,
choices=tuple(likert().items()))
else:
form = SingleStringFieldForm()
if form.validate_on_submit():
if None not in answers.values():
write_answers_to_database(survey=survey)
with open('app/static/ty.txt', 'r') as f:
ty = [x.strip() for x in f.readlines()]
return render_template('ty.html', ty=ty)
return redirect(url_for('survey.question', slug=slug))
return render_template('survey.html', form=form, answers=answers)
The first time I load the page after clearing the session, the form doesn't show up, and when I'm stepping through with a debugger when that happens, the debugger reports that form.answer has a value of:
<UnboundField(RadioField, ('Question 1',), {'choices': (('1', 'Strongly Agree'),
('2', 'Agree'), ('3', 'Neutral'), ('4', 'Disagree'), ('5', 'Strongly Disagree'))})>
If I reload the page, it has this value:
<app.survey.forms.SingleRadioFieldForm object at 0x110788d30>
I don't notice anything else different in the state of the program between the two page loads.
What is causing this the first time and how can I fix it?

Answer 1

While I still don't know why the original function wasn't working on first page load, but did on a reload, I went about this in another way and solved the problem. Instead of setting the fields within __init__, I subclassed within the function and edited the class directly.
#bp.route('/survey/<string:slug>', methods=['GET', 'POST'])
def survey(slug):
class F(FieldsRequiredForm):
pass
...
if None in answers.values():
question = get_next_question()
F.question_id = HiddenField(default=question.id)
if question.category == 'word':
F.answer = StringField(question.question)
elif question.category == 'likert':
F.answer = RadioField(question.question, choices=tuple(likert().items()))
F.submit = SubmitField('Submit')
else:
F.answer = StringField()
form = F()
...