Converting Flask form data to JSON only gets first value

Question

I want to take input from an HTML form and give the output in JSON format. When multiple values are selected they are not converted into JSON arrays, only the first value is used.

@app.route('/form')
def show_form():
    return render_template('form.html')

@app.route("/result", methods=['POST'])
def show_result():
    result = request.form
    return render_template('result.html', result=result)

form.html:

<form method=POST>
   <input name=server>
   <select name=owners multiple>
       <option value="thor">thor</option>
       <option value="loki">loki</option>
       <option value="flash">flash</option>
       <option value="batman">batman</option>
   </select>
   <input type=submit>
</form>

result.html:

{{ result|tojson }}

When multiple values for owner are selected, "thor" and "flash", the output shows only one value:

{"server": "app-srv", "owners": "thor"}

I expect owners to be a list:

{"server": "app-srv", "owners": ["thor", "flash"]}

How do I display the form as JSON without losing list values?

Solution

request.form is a MultiDict. Iterating over a multidict only returns the first value for each key. To get a dictionary with lists of values, use to_dict(flat=False).

result = request.form.to_dict(flat=False)

All values will be lists, even if there's only one item, for consistency. If you want to flatten single-value items, you need to process the data manually. Use iterlists with a dict comprehension.

result = {
    key: value[0] if len(value) == 1 else value
    for key, value in request.form.iterlists()
}