2012-05-13T06:26:42Z

The Flask Mega-Tutorial, Part III: Web Forms

(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:

Recap

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.

Configuration

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 config.py):

WTF_CSRF_ENABLED = True
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/__init__.py):

from flask import Flask

app = Flask(__name__)
app.config.from_object('config')

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/forms.py):

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() }}
      <p>
          Please enter your OpenID:<br>
          {{ form.openid(size=80) }}<br>
      </p>
      <p>{{ form.remember_me }} Remember Me</p>
      <p><input type="submit" value="Sign In"></p>
  </form>
{% 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/views.py):

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',
                           form=form)

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/views.py):

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

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):

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

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() }}
      <p>
          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>
      <p>{{ form.remember_me }} Remember Me</p>
      <p><input type="submit" value="Sign In"></p>
  </form>
{% 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 config.py):

WTF_CSRF_ENABLED = True
SECRET_KEY = 'you-will-never-guess'

OPENID_PROVIDERS = [
    {'name': 'Google', 'url': 'https://www.google.com/accounts/o8/id'},
    {'name': 'Yahoo', 'url': 'https://me.yahoo.com'},
    {'name': 'AOL', 'url': 'http://openid.aol.com/<username>'},
    {'name': 'Flickr', 'url': 'http://www.flickr.com/<username>'},
    {'name': 'MyOpenID', 'url': 'https://www.myopenid.com'}]

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' %
              (form.openid.data, str(form.remember_me.data)))
        return redirect('/index')
    return render_template('login.html', 
                           title='Sign In',
                           form=form,
                           providers=app.config['OPENID_PROVIDERS'])

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 = openid.search('<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
}
</script>
<h1>Sign In</h1>
<form action="" method="post" name="login">
    {{ form.hidden_tag() }}
    <p>
        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 }}', '{{ pr.name }}');">{{ pr.name }}</a> |
        {% endfor %}
    </p>
    <p>{{ form.remember_me }} Remember Me</p>
    <p><input type="submit" value="Sign In"></p>
</form>
{% 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:

Download microblog-0.3.zip.

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.

Miguel

246 comments

  • #76 Miguel Grinberg said 2013-10-17T14:14:24Z

    @Elbert: I don't think so. I believe your problem is that your form does not validate when csrf is enabled. Are you rendering the hidden_tag field in the template?

  • #77 Siyuan said 2013-11-02T10:48:17Z

    Hi Miguel, I have some questions. 1. Why although you didn't set the "action" attribute of the login form, the form can still submit to the login function? That is quite different from PHP.

    Suppose the form submit to login function, in other words, enter the login function again. "form = LoginForm()" create a fresh new form instance, isn't it? But the "flash" statement shows that it is the previous one. I am very confused about this.

    Please help, thanks.

  • #78 Miguel Grinberg said 2013-11-03T01:28:23Z

    @Siyuan: (1) the default when "action" is missing is to submit to the same URL. (2) you are correct, a form object is created when the form is presented, and a completely different form object is created from scratch when the form is submitted. But during submission the form object will detect form data in the request and will initialize itsefl with those values.

  • #79 cecep said 2013-11-16T12:09:21Z

    hi miguel, i has follow this tutorial, but when i run.. debugger said 'app.forms.LoginForm object' has no attribute 'error'.. any idea how i can solve this?

  • #80 Miguel Grinberg said 2013-11-16T20:37:47Z

    @cecep: the field should be "errors", not "error", I think.

  • #81 Joshua Grigonis said 2013-12-10T03:18:34Z

    The zip file is out of date. In forms.py it has:

    from flask.ext.wtf import Form, TextField, BooleanField from flask.ext.wtf import Required

    But it should be: from flask.ext.wtf import Form from wtforms import TextField, BooleanField from wtforms.validators import Required

    http://stackoverflow.com/questions/18297041/im-not-able-to-import-flask-wtf-textfield-and-booleanfield

  • #82 David Branner said 2013-12-19T00:54:33Z

    Typo "some amount configuration" => "some amount of configuration".

  • #83 Dirk Swart said 2013-12-23T16:39:07Z

    Very nice tutorial

    I am getting a 405 "Method not allowed" on the form submission. This line: Should instead be:

    PS: Please consider numbering your code blocks, so they can be referenced more easily.

  • #84 Miguel Grinberg said 2013-12-23T17:46:46Z

    @Dirk: when the action is empty the form is submitted to the same address, so that should be fine. Method not allowed typically happens when you forget to add the "methods" argument to your app.route declaration. See the first code example in "Receiving Form Data".

  • #85 Dusan said 2013-12-24T11:04:13Z

    This is really amazing and awesome!!

  • #86 nick said 2013-12-25T07:22:23Z

    I get the output on the returned index page: Login requested for OpenID="nickolas", remember_me=False

    Is this correct?

    Thanks for the sweet tutorial. I like learning by doing.

  • #87 Miguel Grinberg said 2013-12-25T07:32:43Z

    @nick: Yes, that is expected at this stage of the tutorial .The test output will be replaced with the real thing after you learn a few more tricks.

  • #88 Dusan said 2013-12-25T10:17:09Z

    This is awesome!! Thanks for sharing this, dude!

  • #89 kyc said 2014-01-02T18:42:35Z

    I have a problem : builtins.AttributeError AttributeError: 'LoginForm' object has no attribute 'validate_on_submit'

  • #90 Miguel Grinberg said 2014-01-02T19:10:01Z

    @kyc: make sure your LoginForm class inherits from Flask-WTForm's Form base class.

  • #91 kyc said 2014-01-03T02:11:28Z

    @Miguel Grinberg: my forms.py code is :

    from wtforms import Form, BooleanField, TextField from wtforms import validators from wtforms.validators import Required

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

  • #92 Miguel Grinberg said 2014-01-03T18:13:37Z

    @kyc: You need to import class Form from flask.ext.wtf, not from wtforms. Flask-WTF adds functionality that the regular WTForms project does not have.

  • #93 Danny Goodall said 2014-01-06T17:18:41Z

    Great series of tutorials, many thanks for all the effort. I think I may have found a mis-ordering of the instructions. Just ABOVE the "Improving field validation" section above, you suggest that the reader should

    "Make sure you try submitting the form with the openid field empty, to see how the Required validator halts the submit process."

    Which I did, but I got no error in the form. So I've been trying to debug the validator process (which was very informative in and of itself BTW), when I realised that the validation messages are not in the login form until AFTER the "Improving field validation" section.

    Once I added in the template code that displays the form validation errors, it's working perfectly.

    Thanks again, and looking forward to the O'Reilly book on the subject.

  • #94 Miguel Grinberg said 2014-01-07T05:13:35Z

    @Danny: what I tried to convey with that statement is that the form is not be processed and returned to you. It's true that at that point there is no error message, but still, validation is running.

  • #95 Boris said 2014-01-07T19:20:34Z

    Thank you for doing this, Miguel.

    I have problems importing LoginForms:

    Traceback (most recent call last): File "./run.py", line 2, in from app import app File "/home/n3uplas/Stories/app/init.py", line 5, in from app import views File "/home/n3uplas/Stories/app/views.py", line 3, in from forms import LoginForm ImportError: cannot import name LoginForm

    What could be the problem?

  • #96 Miguel Grinberg said 2014-01-08T06:26:33Z

    @Boris: Do you have forms.py? Are you using Python 2 or 3?

  • #97 Boris said 2014-01-08T16:51:12Z

    Yes, I've got forms.py in the app folder. I'm using Python 2.7.3

  • #98 Miguel Grinberg said 2014-01-08T16:59:45Z

    @Boris: and do you have a LoginForm class in your forms.py? Please compare your version with mine on GitHub.

  • #99 Boris said 2014-01-09T22:34:32Z

    Apparently, there was something wrong with my version of the code. I copied yours from GitHub and it works!

    Thanks.

  • #100 James Nicholson said 2014-01-27T01:27:40Z

    In the "Improving field validation" doesn't the for loop add error messages below the open_id field, not to the right?