Be able to add a form to your Flask app and use POST HTTP requests correctly.


We’ll be continuing to use the silly dataset as an example, though you don’t have to if you don’t want to. If you want to follow along, I recommend you have your previous version of the lab code up or make a quick new Flask app to work with. If you need the previous two Flask labs for reference, they are here: Intro to Flask and Flask and HTML.

Table of Contents

If you want specific functionality, here are the sections:

Simple Row Display

We’re first going to use radio buttons to let the user choose which row of the silly dataset to view.

  1. Make a new route and function in your Flask app that just returns the specified row from the dataset, for example:
     def display_row():
         row = 1
         return str(data[row])
  2. To give the user a choice in which row to display, we need to add a form to the page. You can make a new HTML page, or just put this on index.html, whichever you prefer:
         <p>Which row would you like to see?</p>
         <form action="displayrow">
             <label for="1">Row 1</label>
             <input type="radio" name="rowchoice" value="1">
             <label for="2">Row 2</label>
             <input type="radio" name="rowchoice" value="2">
             <input type="submit" value="Submit">

    We’re only letting them choose between row 1 and row 2 currently and using radio buttons to force them to choose only one.

  3. If you run your app now, you’ll notice that Flask is ignoring what the user selected and still always showing row 1, so we need to update the Flask function so that it actually uses the information the user submitted. First, we need to import the request functionality in Flask:
     from flask import Flask, render_template, request

    Then we need to update the function to get information from the request object:

     def display_row():
         row = int(request.args['rowchoice'])
         return str(data[row])

    Now if you try your app again, you should see the data that the user requested!

POST Requests

You might have noticed in the URL that the user’s selection is shown there. This is fine for non-sensitive data, but might not always be what you want. If you don’t want the information included right in the URL, then you need to use a POST method.

  1. Add to your HTML another form, this time specifying that the method used should be post:
         <p>POST version: Which row would you like to see?</p>
         <form action="displayrow" method="post">
             <label for="1">Row 1</label>
             <input type="radio" name="rowchoice" value="1">
             <label for="2">Row 2</label>
             <input type="radio" name="rowchoice" value="2">
             <input type="submit" value="Submit">
  2. If you try to just run your app, you’ll get an error, because Flask functions default to only accepting GET requests. To also accept POST requests, we need to pass in a keyword parameter to the route:
     @app.route('/displayrow', methods=['GET', 'POST'])

    If you try to run it now, you’ll get another error, this time because POST requests access the variables a little bit differently. To be able to handle both POST and GET requests, we need to first detect which method is being used and then make the correct call to access the information:

     def display_row():
         if request.method == 'POST':
             row = int(request.form['rowchoice'])
         elif request.method == 'GET':
             row = int(request.args['rowchoice'])
             return 'Not a valid request protocol'
         return str(data[row])

    Try out your app now and verify that you are able to get the information using either request method. Awesome!

Dynamically Generated Dropdowns

You might have realized that it will be really annoying if you have to manually make radio buttons for every possible choice that the user might want to look up in your dataset. Fortunately, with a little bit of Jinja, we can automatically generate a dropdown menu.

  1. First we’ll need a couple of new functions. We need a function to give all the ‘titles’ of the rows (i.e. the first column):

     def getRowTitles():
         row_titles = []
         for row in data:
         return row_titles

    (This function obviously isn’t very efficient, but I’m not worried about that at the moment.)

    We also need a function to look up the row by a ‘title’:

     def getRowByTitle(title):
         for row in data:
             if row[0] == title:
                 return row
         return []

    (Also super not efficient.)

  2. With those functions in place, we need to pass the row titles to our homepage template:
     def home():
         return render_template('index.html', rows=getRowTitles())

    And adjust our HTML template to use that information by displaying it in a dropdown menu:

     <p>Dropdown version: Which row would you like to see?</p>
     <form action="rowbytitle">
         <label for="rows">Choose a row</label>
         <select name="rowchoice">
             {% for row in rows: %}
             <option value="{{row}}">{{row}}</option>
             {% endfor %}
         <input type="submit" value="Submit">
  3. Finally, we need to make the Flask route specified:

     def display_row_by_title():
         return str(getRowByTitle(request.args['rowchoice']))

Applying to your project

You’re now all set to implement a fancy way to navigate from the homepage to the data page for your individual front-end deliverable.

If you want, see if you can get a autocomplete search box working. Hint: text input always goes through POST, so make sure to have your Flask method set up for POST. You’ll also need to add the form tags and submit button and add a name argument to the input tag. You are welcome to use the ajax.googleapis.com scripts in your project, just be sure to cite the tutorial!