Flask - Return the Same Form that was Submitted

How do I get Flask to return the user to the same page, filled out the same way, after he/she submits the form on the page? 'render_template' only seems to work if I know all the variables ahead of time. My form is dynamic, and the variables change depending on selections the user has made. See the psuedo-code below for what I'm thinking of. I know render_template doesn't work like this, but is there a way to just say "use the same form values that came in when rendering the template?"

@app.route('./MyPage', methods=['GET', 'POST'])
def MyPage():

  if request.method == 'POST':
    # Do some stuff
    # return render_template('MyPage.html', context=request.form)
  else:
    # Do some other stuff

Solution

The simplest way to do a download in the way you are asking it to use target="_blank" on your form:

<form action="/MyPage" method="POST" target="_blank">
  <ul>
  {% for input in form %}
    <li>{{ input.label }} {{ input }}</li>
  {% endfor %}
  </ul>
</form>

Then your POST-handling method doesn't need to do anything else but return the CSV:

@app.route('/MyPage', methods=['GET', 'POST'])
def MyPage():
  if request.method == 'POST':
    # Turn `request.form` into a CSV
    # see https://stackoverflow.com/q/26997679/135978 for an example
    headers = {'Content-Disposition': 'attachment; filename=saved-form.csv', 'Content-Type': 'text/csv'}
    return form_as_csv, headers
  else:
    # Do some other stuff

If you need multiple buttons on the form, then instead of setting target on the form, you can just set formtarget="_blank" on the button that triggers the CSV:

<form action="/MyPage" method="POST">
  <ul><!-- ... snip ... --></ul>
  <button name="submit_action" value="SAVE_AS_CSV" formtarget="_blank">Save as CSV</button>
  <button name="submit_action" value="RUN_CALCULATION">Run Calculation</button>
</form>

Then you just need to add a check for request.form['submit_action'] in your if request.method == 'POST' block and you're off to the races:

if request.method == 'POST':
    if request.form['submit_action'] == 'SAVE_AS_CSV':
        # Return the CSV response
    else:
        # Return the calculation response