MultiCheckboxField fails validation

Question

I've added in a MultiCheckboxField to my Flask app (as per the docs), but this field fails to validate when I use the validate_on_submit method on it, seemingly because it has no choices attribute even though I've explicitly set it.

class MultiCheckboxField(SelectMultipleField):
    """
    A multiple-select, except displays a list of checkboxes.

    Iterating the field will produce subfields, allowing custom rendering of
    the enclosed checkbox fields.
    """
    widget = ListWidget(prefix_label=False)
    option_widget = CheckboxInput()

class DocumentProcessForm(Form):
    """
    Allows the user to select which documents should be processed.
    """

    PROCESS = "0"
    DELETE = "-1"

    files = MultiCheckboxField("Select", coerce=int)
    #Custom fields I made that render <button>s:
    process_button = ButtonField("Process", name="action", value=PROCESS)
    delete_button = ButtonField("Delete",  name="action", value=DELETE)

@app.route(app.config["PROJECT_ROUTE"] + "<project_id>",
    methods=["GET", "POST"])
def project_show(project_id):
    """
    Show the files contained in a specific project. It also allows the user
    to upload a new document, much like projects().

    :param int project_id: The ID of the desired project.
    """

    process_form = forms.DocumentProcessForm(prefix="process")

    if process_form.validate_on_submit(): #This fails
        files = request.form.getlist("process-files")
        if len(files) > 0:
            if request.form["action"] == process_form.DELETE:
                delete(files)
            elif request.form["action"] == process_form.PROCESS:
                #TODO: process these files.
                pass

    # The template needs access to the ID of each file and its filename.
    process_form.files.choices = []
    file_objects = GetFiles(project_id) # database query, this works fine
    for file_object in file_objects:
        process_form.files.choices.append((file_object.id,
            os.path.split(file_object.path)[1]))

    return render_template("document_list.html",
        process_form=process_form,
        allowed_extensions=app.config["ALLOWED_EXTENSIONS"])

And in my template:

<form method="POST" role="form">
    {{ process_form.hidden_tag() }}
    <table>
        <thead>
            <tr>
                <th>Filename</th>
                <th>Select</th>
            </tr>
        </thead>
        <tbody>
        {% for file in process_form.files %}
            <tr>
                <td>
                    <label for="{{ file.id }}">
                    <a href="{{ url_for('document_show',
                        document_id=file.data,
                        project_id=project.id) }}">{{ file.label.text }}</a>
                    </label>
                </td>
                <td>
                    {{ file }}
                </td>
            </tr>
        {% endfor %}
        </tbody>
    </table>
    <h3>Selected file actions:</h3>
    {{ process_form.process_button(class="btn btn-primary") }}
    {{ process_form.delete_button(class="btn btn-danger") }}
</form>

Flask returns the error:

Traceback (most recent call last):
  File "/home/plasmasheep/prog/project/venv/lib/python2.7/site-packages/flask/app.py", line 1836, in __call__
    return self.wsgi_app(environ, start_response)
  File "/home/plasmasheep/prog/project/venv/lib/python2.7/site-packages/flask/app.py", line 1820, in wsgi_app
    response = self.make_response(self.handle_exception(e))
  File "/home/plasmasheep/prog/project/venv/lib/python2.7/site-packages/flask/app.py", line 1403, in handle_exception
    reraise(exc_type, exc_value, tb)
  File "/home/plasmasheep/prog/project/venv/lib/python2.7/site-packages/flask/app.py", line 1817, in wsgi_app
    response = self.full_dispatch_request()
  File "/home/plasmasheep/prog/project/venv/lib/python2.7/site-packages/flask/app.py", line 1477, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/home/plasmasheep/prog/project/venv/lib/python2.7/site-packages/flask/app.py", line 1381, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/home/plasmasheep/prog/project/venv/lib/python2.7/site-packages/flask/app.py", line 1475, in full_dispatch_request
    rv = self.dispatch_request()
  File "/home/plasmasheep/prog/project/venv/lib/python2.7/site-packages/flask/app.py", line 1461, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/home/plasmasheep/prog/project/app/views.py", line 84, in project_show
    if really_submitted(process_form):
  File "/home/plasmasheep/prog/project/app/views.py", line 22, in really_submitted
    return form.validate_on_submit() and form.submitted.data
  File "/home/plasmasheep/prog/project/venv/lib/python2.7/site-packages/flask_wtf/form.py", line 156, in validate_on_submit
    return self.is_submitted() and self.validate()
  File "/home/plasmasheep/prog/project/venv/lib/python2.7/site-packages/wtforms/form.py", line 271, in validate
    return super(Form, self).validate(extra)
  File "/home/plasmasheep/prog/project/venv/lib/python2.7/site-packages/wtforms/form.py", line 130, in validate
    if not field.validate(self, extra):
  File "/home/plasmasheep/prog/project/venv/lib/python2.7/site-packages/wtforms/fields/core.py", line 164, in validate
    self.pre_validate(form)
  File "/home/plasmasheep/prog/project/venv/lib/python2.7/site-packages/wtforms/fields/core.py", line 465, in pre_validate
    values = list(c[0] for c in self.choices)
TypeError: 'NoneType' object is not iterable

self.choices is None in this case. However, self.data does contain the selected checkboxes. I'm not sure what to make of this.

Answer 1

The problem was that I was adding to choices after I had run validate_on_submit. If I rearrange the project_show like so:

@app.route(app.config["PROJECT_ROUTE"] + "<project_id>",
    methods=["GET", "POST"])
def project_show(project_id):
    """
    Show the files contained in a specific project. It also allows the user
    to upload a new document, much like projects().

    :param int project_id: The ID of the desired project.
    """

    process_form = forms.DocumentProcessForm(prefix="process")

    # The template needs access to the ID of each file and its filename.
    # Note where this has been moved!
    process_form.files.choices = []
    file_objects = GetFiles(project_id) # database query, this works fine
    for file_object in file_objects:
        process_form.files.choices.append((file_object.id,
            os.path.split(file_object.path)[1]))

    if process_form.validate_on_submit(): #This works fine
        files = request.form.getlist("process-files")
        if len(files) > 0:
            if request.form["action"] == process_form.DELETE:
                delete(files)
            elif request.form["action"] == process_form.PROCESS:
                #TODO: process these files.
                pass

    return render_template("document_list.html",
        process_form=process_form,
        allowed_extensions=app.config["ALLOWED_EXTENSIONS"])

Then everything works fine. Further reading here and here.

Answer 2

This line

 files = MultiCheckboxField("Select", coerce=int)

should be changed to something like:

 my_choices = [('0', 'PROCESS'), ('-1', 'DELETE')]
 files = MultiCheckboxField("Select", choices = my_choices, coerce=int)

You need to provide the "choices" attribute when you instantiate your files field.