How do you set a default value for a WTForms SelectField?

Question

When attempting to set the default value of a SelectField with WTForms, I pass in value to the \'default\' parameter like so.

class TestForm(Form):
  test_fi         


        

Answer 1

In case you're using flask_wtf and would like to set a default value for different SelectFields nested in a FieldList like this

from flask_wtf import FlaskForm


class FormWithSelect(FlaskForm):
    my_select_field = SelectField(validators=[DataRequired()], coerce=int)


class FormWithMultipleSelects(FlaskForm):
    select_fields = FieldList(FormField(FormWithSelect), min_entries=1)
    submit = SubmitField(label="save all the select field values")

the solution is to overwrite data, not default as you would expect.

def generate_my_form(my_data, overwrite_data=True):
    form = FormWithMultipleSelects(select_fields=[{
        "my_select_field": {
             "name": d["name"],
             # ...
        }
    } for d in my_data]))
    for n range(len(form.select_fields)):
        form.select_fields[n].my_select_field.choices = [(1,'foo'), (2,'bar')]
        # I wan't 'bar' to be the option selected by default
        # form.select_fields[n].my_select_field.default = 2  # has no effect!
        if overwrite_data:
            form.select_fields[n].my_select_field.data = 2 #  works

Basically this solution is similar to Elliots answer above; I just wanted to provide a solution for the more complex case of a FieldList. nikitz mentioned in a comment, that there is a side effect: form.validate_on_submit() does not work if you're overwriting data! You can build a simple workaround where to disable the overwriting of data when calling validate_on_submit().

 form_for_validation = generate_my_form(my_data, False)
 if form_for_validation.validate_on_submit():
      # do something with form_for_validation.data
 else:
     form_with_default_data = generate_my_form(my_data, True)
     render_template(..., form=form_with_default_data)

It's not super elegant, but it works.

PS: My reputation is too low to directly comment on the correct answer by Elliot.