The Flask Mega-Tutorial, Part III: Web Forms

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(Great news! There is a new version of this tutorial!)

This is the third article in the series in which I document my experience writing web applications in Python using the Flask microframework.

The goal of the tutorial series is to develop a decently featured microblogging application that demonstrating total lack of originality I have decided to call microblog.

NOTE: This article was revised in September 2014 to be in sync with current versions of Python and Flask.

Here is an index of all the articles in the series that have been published to date:


In the previous chapter of the series we defined a simple template for the home page and used fake objects as placeholders for things we don't have yet, like users or blog posts.

In this article we are going to fill one of those many holes we still have in our app, we will be looking at how to work with web forms.

Web forms are one of the most basic building blocks in any web application. We will be using forms to allow users to write blog posts, and also for logging in to the application.

To follow this chapter along you need to have the microblog app as we left it at the end of the previous chapter. Please make sure the app is installed and running.


To handle our web forms we are going to use the Flask-WTF extension, which in turn wraps the WTForms project in a way that integrates nicely with Flask apps.

Many Flask extensions require some amount of configuration, so we are going to setup a configuration file inside our root microblog folder so that it is easily accessible if it needs to be edited. Here is what we will start with (file

SECRET_KEY = 'you-will-never-guess'

Pretty simple, it's just two settings that our Flask-WTF extension needs. The WTF_CSRF_ENABLED setting activates the cross-site request forgery prevention (note that this setting is enabled by default in current versions of Flask-WTF). In most cases you want to have this option enabled as it makes your app more secure.

The SECRET_KEY setting is only needed when CSRF is enabled, and is used to create a cryptographic token that is used to validate a form. When you write your own apps make sure to set the secret key to something that is difficult to guess.

Now that we have our config file we need to tell Flask to read it and use it. We can do this right after the Flask app object is created, as follows (file app/

from flask import Flask

app = Flask(__name__)

from app import views

The user login form

Web forms are represented in Flask-WTF as classes, subclassed from base class Form. A form subclass simply defines the fields of the form as class variables.

Now we will create a login form that users will use to identify with the system. The login mechanism that we will support in our app is not the standard username/password type, we will have our users login using their OpenID. OpenIDs have the benefit that the authentication is done by the provider of the OpenID, so we don't have to validate passwords, which makes our site more secure to our users.

The OpenID login only requires one string, the so called OpenID. We will also throw a 'remember me' checkbox in the form, so that users can choose to have a cookie installed in their browsers that remembers their login when they come back.

Let's write our first form (file app/

from flask_wtf import Form
from wtforms import StringField, BooleanField
from wtforms.validators import DataRequired

class LoginForm(Form):
    openid = StringField('openid', validators=[DataRequired()])
    remember_me = BooleanField('remember_me', default=False)

I believe the class is pretty much self-explanatory. We imported the Form class, and the two form field classes that we need, StringField and BooleanField.

The DataRequired import is a validator, a function that can be attached to a field to perform validation on the data submitted by the user. The DataRequired validator simply checks that the field is not submitted empty. There are many more validators included with Flask-WTF, we will use some more in the future.

Form templates

We will also need a template that contains the HTML that produces the form. The good news is that the LoginForm class that we just created knows how to render form fields as HTML, so we just need to concentrate on the layout. Here is our login template (file app/templates/login.html):

<!-- extend from base layout -->
{% extends "base.html" %}

{% block content %}
  <h1>Sign In</h1>
  <form action="" method="post" name="login">
      {{ form.hidden_tag() }}
          Please enter your OpenID:<br>
          {{ form.openid(size=80) }}<br>
      <p>{{ form.remember_me }} Remember Me</p>
      <p><input type="submit" value="Sign In"></p>
{% endblock %}

Note that in this template we are reusing the base.html template through the extends template inheritance statement. We will actually do this with all our templates, to ensure a consistent layout across all pages.

There are a few interesting differences between a regular HTML form and our template. This template expects a form object instantiated from the form class we just defined stored in a template argument named form. We will take care of sending this template argument to the template next, when we write the view function that renders this template.

The form.hidden_tag() template argument will get replaced with a hidden field that implements the CSRF prevention that we enabled in the configuration. This field needs to be in all your forms if you have CSRF enabled. The good news is that Flask-WTF handles it for us, we just need to make sure it is included in the form.

The actual fields of our form are rendered by the field objects, we just need to refer to a {{form.field_name}} template argument in the place where each field should be inserted. Some fields can take arguments. In our case, we are asking the text field to generate our openid field with a width of 80 characters.

Since we have not defined the submit button in the form class we have to define it as a regular field. The submit field does not carry any data so it doesn't need to be defined in the form class.

Form views

The final step before we can see our form is to code a view function that renders the template.

This is actually quite simple since we just need to pass a form object to the template. Here is our new view function (file app/

from flask import render_template, flash, redirect
from app import app
from .forms import LoginForm

# index view function suppressed for brevity

@app.route('/login', methods=['GET', 'POST'])
def login():
    form = LoginForm()
    return render_template('login.html', 
                           title='Sign In',

So basically, we have imported our LoginForm class, instantiated an object from it, and sent it down to the template. This is all that is required to get form fields rendered.

Let's ignore for now the flash and redirect imports. We'll use them a bit later.

The only other thing that is new here is the methods argument in the route decorator. This tells Flask that this view function accepts GET and POST requests. Without this the view will only accept GET requests. We will want to receive the POST requests, these are the ones that will bring in the form data entered by the user.

At this point you can try the app and see the form in your web browser. After you start the application you will want to open http://localhost:5000/login in your web browser, as this is the route we have associated with the login view function.

We have not coded the part that accepts data yet, so pressing the submit button will not have any effect at this time.

Receiving form data

Another area where Flask-WTF makes our job really easy is in the handling of the submitted form data. Here is an updated version of our login view function that validates and stores the form data (file app/

@app.route('/login', methods=['GET', 'POST'])
def login():
    form = LoginForm()
    if form.validate_on_submit():
        flash('Login requested for OpenID="%s", remember_me=%s' %
              (, str(
        return redirect('/index')
    return render_template('login.html', 
                           title='Sign In',

The validate_on_submit method does all the form processing work. If you call it when the form is being presented to the user (i.e. before the user got a chance to enter data on it) then it will return False, so in that case you know that you have to render the template.

When validate_on_submit is called as part of a form submission request, it will gather all the data, run all the validators attached to fields, and if everything is all right it will return True, indicating that the data is valid and can be processed. This is your indication that this data is safe to incorporate into the application.

If at least one field fails validation then the function will return False and that will cause the form to be rendered back to the user, and this will give the user a chance to correct any mistakes. We will see later how to show an error message when validation fails.

When validate_on_submit returns True our login view function calls two new functions, imported from Flask. The flash function is a quick way to show a message on the next page presented to the user. In this case we will use it for debugging, since we don't have all the infrastructure necessary to log in users yet, we will instead just display a message that shows the submitted data. The flash function is also extremely useful on production servers to provide feedback to the user regarding an action.

The flashed messages will not appear automatically in our page, our templates need to display the messages in a way that works for the site layout. We will add these messages to the base template, so that all our templates inherit this functionality. This is the updated base template (file app/templates/base.html):

    {% if title %}
    <title>{{ title }} - microblog</title>
    {% else %}
    {% endif %}
    <div>Microblog: <a href="/index">Home</a></div>
    {% with messages = get_flashed_messages() %}
      {% if messages %}
        {% for message in messages %}
            <li>{{ message }} </li>
        {% endfor %}
      {% endif %}
    {% endwith %}
    {% block content %}{% endblock %}

The technique to display the flashed message is hopefully self-explanatory. One interesting property of flash messages is that once they are requested through the get_flashed_messages function they are removed from the message list, so these messages appear in the first page requested by the user after the flash function is called, and then they disappear.

The other new function we used in our login view is redirect. This function tells the client web browser to navigate to a different page instead of the one requested. In our view function we use it to redirect to the index page we developed in previous chapters. Note that flashed messages will display even if a view function ends in a redirect.

This is a great time to start the app and test how the form works. Make sure you try submitting the form with the openid field empty, to see how the DataRequired validator halts the submission process.

Improving field validation

With the app in its current state, forms that are submitted with invalid data will not be accepted. Instead, the form will be presented back to the user to correct. This is exactly what we want.

What we are missing is an indication to the user of what is wrong with the form. Luckily, Flask-WTF also makes this an easy task.

When a field fails validation Flask-WTF adds a descriptive error message to the form object. These messages are available to the template, so we just need to add a bit of logic that renders them.

Here is our login template with field validation messages (file app/templates/login.html):

<!-- extend base layout -->
{% extends "base.html" %}

{% block content %}
  <h1>Sign In</h1>
  <form action="" method="post" name="login">
      {{ form.hidden_tag() }}
          Please enter your OpenID:<br>
          {{ form.openid(size=80) }}<br>
          {% for error in form.openid.errors %}
            <span style="color: red;">[{{ error }}]</span>
          {% endfor %}<br>
      <p>{{ form.remember_me }} Remember Me</p>
      <p><input type="submit" value="Sign In"></p>
{% endblock %}

The only change we've made is to add a for loop that renders any messages added by the validators below the openid field. As a general rule, any fields that have validators attached will have errors added under form.field_name.errors. In our case we use form.openid.errors. We display these messages in a red style to call the user's attention.

Dealing with OpenIDs

In practice, we will find that a lot of people don't even know that they already have a few OpenIDs. It isn't that well known that a number of major service providers on the Internet support OpenID authentication for their members. For example, if you have an account with Google, you have an OpenID with them. Likewise with Yahoo, AOL, Flickr and many other providers. (Update: Google is shutting down their OpenID service on April 15 2015).

To make it easier for users to login to our site with one of these commonly available OpenIDs, we will add links to a short list of them, so that the user does not have to type the OpenID by hand.

We will start by defining the list of OpenID providers that we want to present. We can do this in our config file (file

SECRET_KEY = 'you-will-never-guess'

    {'name': 'Google', 'url': ''},
    {'name': 'Yahoo', 'url': ''},
    {'name': 'AOL', 'url': '<username>'},
    {'name': 'Flickr', 'url': '<username>'},
    {'name': 'MyOpenID', 'url': ''}]

Now let's see how we use this array in our login view function:

@app.route('/login', methods=['GET', 'POST'])
def login():
    form = LoginForm()
    if form.validate_on_submit():
        flash('Login requested for OpenID="%s", remember_me=%s' %
              (, str(
        return redirect('/index')
    return render_template('login.html', 
                           title='Sign In',

Here we grab the configuration by looking it up in app.config with its key. The array is then added to the render_template call as a template argument.

As I'm sure you guessed, we have one more step to be done with this. We now need to specify how we would like to render these provider links in our login template (file app/templates/login.html):

<!-- extend base layout -->
{% extends "base.html" %}

{% block content %}
<script type="text/javascript">
function set_openid(openid, pr)
    u ='<username>')
    if (u != -1) {
        // openid requires username
        user = prompt('Enter your ' + pr + ' username:')
        openid = openid.substr(0, u) + user
    form = document.forms['login'];
    form.elements['openid'].value = openid
<h1>Sign In</h1>
<form action="" method="post" name="login">
    {{ form.hidden_tag() }}
        Please enter your OpenID, or select one of the providers below:<br>
        {{ form.openid(size=80) }}
        {% for error in form.openid.errors %}
          <span style="color: red;">[{{error}}]</span>
        {% endfor %}<br>
        |{% for pr in providers %}
          <a href="javascript:set_openid('{{ pr.url }}', '{{ }}');">{{ }}</a> |
        {% endfor %}
    <p>{{ form.remember_me }} Remember Me</p>
    <p><input type="submit" value="Sign In"></p>
{% endblock %}

The template got somewhat long with this change. Some OpenIDs include the user's username, so for those we have to have a bit of javascript magic that prompts the user for the username and then composes the OpenID with it. When the user clicks on an OpenID provider link and (optionally) enters the username, the OpenID for that provider is inserted in the text field.

Below is a screenshot of our login screen after clicking the Google OpenID link:

Sign In screenshot

Final Words

While we have made a lot of progress with our login form, we haven't actually done anything to login users into our system, all we've done so far had to do with the GUI aspects of the login process. This is because before we can do real logins we need to have a database where we can record our users.

In the next chapter we will get our database up and running, and shortly after we will complete our login system, so stay tuned for the follow up articles.

The microblog application in its current state is available for download here:


Remember that the Flask virtual environment is not included in the zip file. For instructions on how to set it up see the first chapter of the series.

Feel free to leave comments or questions below. I hope to see you in the next chapter.


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  • #226 Miguel Grinberg said

    @Corey: Do you have the OPENID_PROVIDERS data structure in

  • #227 Bruce said

    Hi, Great tutorial! Starting to work my way through it. My only problem is getting localhost:5000 to work with IE9 (I'm on a Vista OS). It works fine with Google Chrome and Firefox.

  • #228 Ed said

    Hey Miguel,

    We have finished our system using Flask and Foundation. A powerful combination.
    The main data entry screens are from a complicated 24 page document which has 70+ Radio buttons.

    Typically, we approach this with the sample code below:
    M2410 = fields.RadioField(
    '(M2410) To which Inpatient Facility has the patient been admitted?',
    ('M2410_hospital', '1 - Hospital [Go to M2430]'),
    ('M2410_rehab', '2 - Rehabilitation facility [Go to M0903]'),
    ('M2410_nursing', '3 - Nursing home [Go to M2440]'),
    ('M2410_hospice', '4 - Hospice [Go to M0903]'),
    ('M2410_na', 'NA - No inpatient facility admission [Omit NA option on TRN]')])

    <bold>{{ Dform.M2410.label }}</bold>
    <div class="large-12 columns">
    {{ Dform.M2410 }}

    However, the option boxes are only aligned vertically.
    Do you know if it's possible to align them horizontally, or individually place the option boxes in different locations to approximate the paper form layout?


  • #229 Miguel Grinberg said

    @Ed: You are letting WTForms render your fields. See the documentation, you can render the individual radio buttons for greater control:

  • #230 Andromeda said

    The OpenID is now deprecated and no more support for my current project. Is there any suggestion which have a similar ability to OpenID?

    Thanks in advance

  • #231 Miguel Grinberg said

    I have written about OAuth authentication with Flask or also username/password. Neither is integrated with the mega-tutorial, but you can find articles in this blog and also on my O'Reilly book and videos.

  • #232 Terrance Moore said

    So I did not do a virtualenv installation and installed everything systemwide since I use a VPS and keep replacing it.

    My has #!/usr/bin/python, until .forms step all was good now I am getting

    Traceback (most recent call last):
    File "./", line 2, in <module>
    from app import app
    File "/var/www/app/app/", line 6, in <module>
    from app import views
    File "/var/www/app/app/", line 3, in <module>
    from .forms import app
    ImportError: cannot import name app

    If I change my to "#!flask/bin/python" I get:
    bash: ./ flask/bin/python: bad interpreter: No such file or directory

    Is there an alternative way for me to keep running it without virtualenv? or I have to go back to virtualenv?

    I have the extensions installed but it can't import .forms..
    Here is my tree output
    ├── app
    │   ├──
    │   ├── forms.pyc
    │   ├──
    │   ├── init.pyc
    │   ├── static
    │   ├── templates
    │   │   ├── base.html
    │   │   ├── index.html
    │   │   └── login.html
    │   ├──
    │   └── views.pyc
    ├── config.pyc
    └── tmp

  • #233 Miguel Grinberg said

    Terrence: you are importing "app" from forms, but the forms module only has the Flask-WTF forms. That should be importing LoginForm.

  • #234 dpg said

    Thanks a lot for this tutorial. Man, your work is a big help for many of us who want to give our first baby steps.

    Just few questions about the login procedure:
    Since Google OpenID will be deprecated soon, what would be the alternative in your opinion? Is OpenId still a good recommendation? What are the pros and cons of other alternatives? Do you have a post about this issue you could link in this tutorial? Could you write a paragraph about it? Maybe just three lines with your suggestions and the link to the corresponding webs with documentation...

    Thanks a lot again.

  • #235 Miguel Grinberg said

    @dpg: I have written about OAuth authentication on a recent article, that is probably my recommendation if you do not want to implement a standard username/password login. OpenID Connect replaces the old OpenID, but I feel it is still too new, we'll have to see if it takes off.

  • #236 Natanael Souza said

    Hi, I'm from Brazil and I'm wonder that you keep answer and supporting the readers from your blog, even of posts from 3 years ago. I would like to thank your for the blog, I have search in the web for a good Flash tutorial, and yours is the best.

  • #237 ale said

    thanks so much for yours tutorials!! :)

  • #238 Jorge E. said

    I just realized that Google shut down the OpenID service on April 15 2015. Just a suggestion that it would be great (and greatly appreciated) to update the tutorial to work with the new service. Cheers.

  • #239 Miguel Grinberg said

    @Jorge: see comment #235 above.

  • #240 Benjamin said

    Internal Server Error when {{ form.hidden_tag() }} is included, anyone else having this issue?

  • #241 Howard said

    Hi Miguel. First, I want to say thank you for publishing such informative tutorials. It is greatly appreciated.

    One question I had: Is it a common practice to have the variable and package module both named "app"? I found this to be very confusing as I'm just learning the materials.

  • #242 Miguel Grinberg said

    @howard: I'm not sure it is common. You are not the first that finds this confusing, so you are definitely not alone. A pattern that I see a lot of people follow is to name the app instance "app", but change the name of the application package to the app name, such as "microblog" in this case.

  • #243 Joe Jacob said

    What does this:
    {% with messages = get_flashed_messages() %}
    {% if messages %}

      {% for message in messages %}
    • {{ message }}

    • {% endfor %}

    {% endif %}
    {% endwith %}
    I don't understand the with/endwith syntax

  • #244 Miguel Grinberg said

    @Joe: the "with var = expression" sets the variable to the result of the expression inside the with/endwith block.

  • #245 sai gollapudi said

    Thanks a lot. I am thoroughly enjoying and learning Python+Flask and also web development through your tutorial. I appreciate your effort.

    Can you comment on use of Stormpath Vs OpenID? How does usage of Stormpath benefit us in this tutorial?

    again, thanks for your time

  • #246 Miguel Grinberg said

    @sal: It's really apples to oranges, Stormpath is a service, not a standard. If you prefer to pay and have user management done for you, then you may like Stormpath.