Not possible to have two forms of same class at the same time

[abendebury]

If you have a form like so:

class MultiCheckboxField(SelectMultipleField):
    widget = ListWidget(prefix_label=False)
    option_widget = CheckboxInput()

class ActivityListForm(Form):
    activities = MultiCheckboxField(validators=[DataRequired()], choices=[])
    submit = SubmitField("Submit")

    def populate_activities(self, activities_set):
        activities_mapping = {}
        for activity in activities_set:
            activities_mapping[str(activity.id)] = activity
            choice_tuple = (str(activity.id), activity.question)
            self.activities.choices.append(choice_tuple)
        return activities_mapping

And a flask view like so:

def settings_experiment(exp_id):
    experiment = Experiment.query.get(exp_id)

    remove_activities_form = ActivityListForm(prefix="remove")
    add_activities_form = ActivityListForm(prefix="add")

    remove_activities_mapping = remove_activities_form.populate_activities(
        experiment.activities)

    add_activities_mapping = add_activities_form.populate_activities(
        Activity.query.\
            filter(not_(Activity.experiments.any(id=experiment.id))).all())

    return render_template("experiments/settings_experiment.html",
                           experiment=experiment,
                           remove_activities_form=remove_activities_form,
                           add_activities_form=add_activities_form,
                           add_activities_mapping=add_activities_mapping,
                           remove_activities_mapping=remove_activities_mapping)

Then by the end of the view function, remove_activities_mapping.activities.choices == add_activities_mapping.activities.choices.

Essentially, it looks like two forms of the same class necessarily have the same choices for a field, which makes it hard to reuse a field on the same page.

[davidism]

You've set the default choices to a mutable object, a list. WTForms stores the arguments passed to fields in order to construct bound fields later. This is the same situation as a mutable default argument in a normal Python function. Don't set a default, and instead initialize choices during __init__ (or populate_activities if it will always be called).

class F(Form):
    items = SelectField()

    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.items.choices = []

[davidism]

Patching SelectField.__init__ to do self.choices = copy(choices) seems reasonable, I'll do that.