python - Add fields dynamically to WTForms form

Question

I want to define a form class with fields based on a dict of name: label. I tried the following, which nearly worked. However, rendering the fields in a template gave AttributeError: 'UnboundField' object has no attribute '__call__'. How can I dynamically add fields to a form?

def build_form(name, record):
    class ContactForm(FlaskForm):
        name = StringField(name)
        fieldlist = {}

        for key, value in record.items():
            fieldlist[key] = StringField(key)

    @app.route('/', methods=['GET', 'POST'])
    def showform():
        form = ContactForm(request.form)

        if request.method == 'POST':
            return 'form processed'

        return render_template('cardcompare.tpl', record=record, form=form)

<form method=post>
    {{ form.name() }}
    {% for key, value in record.items() %}
        {{ form.fieldlist[key]() }}
    {% endfor %}
    <input type=submit value=Register>
</form>

Answer 1

Use setattr to add new fields as attributes of the form class. This will cause WTForms to set up the field correctly instead of keeping the unbound field.

# form class with static fields
class MyForm(FlaskForm):
    name = StringField('static field')

record = {'field1': 'label1', 'field2': 'label2'}

# add dynamic fields
for key, value in record.items():
    setattr(MyForm, key, StringField(value))

In the template you can iterate over the fields using the attr filter.

{% for key, value in record.items() %}:
    {{ form|attr(key)() }}
{% endfor %}