Question

I have a Flask application that uses WTForms for user input. It uses a SelectMultipleField in a form. I can’t seem to get the app to POST all items in the field when selected; it only sends the first item selected regardless of how many the user selects.

The Flask documentation says this about the data sent from this field type, but I don’t see this behavior:

The data on the SelectMultipleField is stored as a list of objects,
each of which is checked and coerced from the form input.

Here’s a complete, minimal Flask app that illustrates this:

#!/usr/bin/env python

from flask import Flask, render_template_string, request
from wtforms import Form, SelectMultipleField

application = app = Flask('wsgi')

class LanguageForm(Form):
    language = SelectMultipleField(u'Programming Language', choices=[('cpp', 'C++'), ('py', 'Python'), ('text', 'Plain Text')])

template_form = """
{% block content %}
<h1>Set Language</h1>

<form method="POST" action="/">
    <div>{{ form.language.label }} {{ form.language(rows=3, multiple=True) }}</div>
    <button type="submit" class="btn">Submit</button>    
</form>
{% endblock %}

"""

completed_template = """
{% block content %}
<h1>Language Selected</h1>

<div>{{ language }}</div>

{% endblock %}

"""

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

    if request.method == 'POST' and form.validate():
        print "POST request and form is valid"
        language =  request.form['language']
        print "languages in wsgi.py: %s" % request.form['language']
        return render_template_string(completed_template, language=language)

    else:

        return render_template_string(template_form, form=form)

if __name__ == '__main__':
    app.run(debug=True)

Answer 1

Flask returns request.form as a werkzeug MultiDict object. This is kind of like a dictionary, only with traps for the unwary.

http://flask.pocoo.org/docs/api/#flask.request
http://werkzeug.pocoo.org/docs/datastructures/#werkzeug.datastructures.MultiDict

MultiDict implements all standard dictionary methods. Internally, it saves all values for a key as a list, but the standard dict access methods will only return the first value for a key. If you want to gain access to the other values, too, you have to use the list methods.

However, I think there’s an easier way.
Can you do me a favor and try replacing:

language =  request.form['language']

with

language =  form.language.data

and see if that’s any different? WTForms should handle the MultiDict object and just return a list for you since you’ve bound form data to it.