Testing Flask-WTF forms that use `FieldList` and `FormField`

I wrote some forms using flask-wtf that use FieldList and FormField, and I want to test them using pytest. It's a bulk insertion of data from an uploaded CSV

These are my forms:

# myapp/admin/forms.py

from wtforms import Form as NoCsrfForm

class SimpleRegistrationForm(NoCsrfForm):

    email = StringField('Email')
    username = StringField('Username')
    password = StringField('Password')

from flask_wtf import FlaskForm    

class BulkUserCreationForm(FlaskForm):

    users = FieldList(FormField(SimpleRegistrationForm))
    submitted = HiddenField()
    submit = SubmitField('Register all')

    def is_submitted(self):
        from flask import request
        return super().is_submitted() and 'submitted' in request.form

Note that I'm skipping imports and other stuff. Also, I used submitted to stop my @app.route from passing validate_on_submit().

This is a part of my test:

# a part of a test
# (...) mumble mumble
        from myapp.admin.forms import (
            BulkUserCreationForm, SimpleRegistrationForm)

        usr_form_1 = SimpleRegistrationForm(username="user1",
                                            email="user1@mail.com",
                                            password="pwd1",)

        usr_form_2 = SimpleRegistrationForm(username="user2",
                                            email="user2@mail.com",
                                            password="pwd2",)

        usr_form_full = BulkUserCreationForm(
            users=[usr_form_1, usr_form_2])

        # user issues a POST request
        rv = client.post(
            url_for('bulk-import-users.edit_users')
            follow_redirects=True,
            data=usr_form_full.data)

        assert something_happened()

I'm struggling on how to craft the data part. So far I've read three approaches

• This solution uses data as tuples, but I don't understand the binding between tuples and forms for something like BulkUserCreationForm.users

• This solution relies on the data attribute of a form instance, which returns a dictionary. In the post it seems to work, but in my case (and for the code I show) I'm getting an error of the form:

  /src/venv/lib/python3.6/site-packages/werkzeug/test.py:349: DeprecationWarning: it's no longer possible to pass dicts as `data`.  Use tuples or FileStorage objects instead
  
  

• Googling this error returned this (and not much more). This solution uses hardcoded values, so instead I went for something more or less better:

  data_full = {field.label.field_id: field.data
               for form in usr_form_full.users
               for field in form}
  
  

and passed this to the data attribute. It didn't work. For some reason the rendered .data attribute does not behave like expected, returning a different repr (I expected to see the actual value).

    >>> print(data_full)
    {'users-0-email': <wtforms.fields.core.StringField object at 0x7f1704f68cc0>, 'users-0-username': <wtforms.fields.core.StringField object at 0x7f1704fa5e10>, 'users-0-password': <wtforms.fields.core.StringField object at 0x7f1704f539e8>, 'users-1-email': <wtforms.fields.core.StringField object at 0x7f1704f26a90>, 'users-1-username': <wtforms.fields.core.StringField object at 0x7f1704f26f98>, 'users-1-password': <wtforms.fields.core.StringField object at 0x7f1704f26828>}

In short, none of the above approaches worked in my test. What's the right approach? Do I need to pass submit and submitted values to my form instance as well?