Example Application: Simple Forms Processing

This guide will walk you through building a simple Pecan web application that will do some simple forms processing.

Project Setup

First, you’ll need to install Pecan:

$ pip install pecan

Use Pecan’s basic template support to start a new project:

$ pecan create mywebsite
$ cd mywebsite

Install the new project in development mode:

$ python setup.py develop

With the project ready, go into the templates folder and edit the index.html file. Modify it so that it resembles this:

<%inherit file="layout.html" />

<%def name="title()">
    Welcome to Pecan!
</%def>
    <header>
        <h1><img src="/images/logo.png" /></h1>
    </header>
    <div id="content">
        <form method="POST" action="/">
            <fieldset>
                <p>Enter a message: <input name="message" /></p>
                <p>Enter your first name: <input name="first_name" /></p>
                <input type="submit" value="Submit" />
            </fieldset>
        </form>
        % if not form_post_data is UNDEFINED:
            <p>${form_post_data['first_name']}, your message is: ${form_post_data['message']}</p>
        % endif
    </div>

What did we just do?

1. Modified the contents of the form tag to have two input tags. The first is named message and the second is named first_name

2. Added a check if form_post_data has not been defined so we don’t show the message or wording

3. Added code to display the message from the user’s POST action

Go into the controllers folder now and edit the root.py file. There will be two functions inside of the RootController class which will display the index.html file when your web browser hits the '/' endpoint. If the user puts some data into the textbox and hits the submit button then they will see the personalized message displayed back at them.

Modify the root.py to look like this:

from pecan import expose


class RootController(object):

    @expose(generic=True, template='index.html')
    def index(self):
        return dict()

    @index.when(method='POST', template='index.html')
    def index_post(self, **kwargs):
        return dict(form_post_data=kwargs)

What did we just do?

1. Modified the index function to render the initial index.html webpage

2. Modified the index_post function to return the posted data via keyword arguments

Run the application:

$ pecan serve config.py

Open a web browser: http://127.0.0.1:8080/

Adding Validation

Enter a message into the textbox along with a name in the second textbox and press the submit button. You should see a personalized message displayed below the form once the page posts back.

One problem you might have noticed is if you don’t enter a message or a first name then you simply see no value entered for that part of the message. Let’s add a little validation to make sure a message and a first name was actually entered. For this, we will use WTForms but you can substitute anything else for your projects.

Add support for the WTForms library:

$ pip install wtforms

Note

Keep in mind that Pecan is not opinionated when it comes to a particular library when working with form generation, validation, etc. Choose which libraries you prefer and integrate those with Pecan. This is one way of doing this, there are many more ways so feel free to handle this however you want in your own projects.

Go back to the root.py files and modify it like this:

from pecan import expose, request
from wtforms import Form, TextField, validators


class PersonalizedMessageForm(Form):
    message = TextField(u'Enter a message',
                        validators=[validators.required()])
    first_name = TextField(u'Enter your first name',
                           validators=[validators.required()])


class RootController(object):

    @expose(generic=True, template='index.html')
    def index(self):
        return dict(form=PersonalizedMessageForm())

    @index.when(method='POST', template='index.html')
    def index_post(self):
        form = PersonalizedMessageForm(request.POST)
        if form.validate():
            return dict(message=form.message.data,
                        first_name=form.first_name.data)
        else:
            return dict(form=form)

What did we just do?

1. Added the PersonalizedMessageForm with two textfields and a required field validator for each

2. Modified the index function to create a new instance of the PersonalizedMessageForm class and return it

3. In the index_post function modify it to gather the posted data and validate it. If its valid, then set the returned data to be displayed on the webpage. If not valid, send the form which will contain the data plus the error message(s)

Modify the index.html like this:

<%inherit file="layout.html" />

## provide definitions for blocks we want to redefine
<%def name="title()">
    Welcome to Pecan!
</%def>
    <header>
        <h1><img src="/images/logo.png" /></h1>
    </header>
    <div id="content">
        % if not form:
            <p>${first_name}, your message is: ${message}</p>
        % else:
            <form method="POST" action="/">
                <div>
                    ${form.message.label}:
                    ${form.message}
                    % if form.message.errors:
                        <strong>${form.message.errors[0]}</strong>
                    % endif
                </div>
               <div>
                    ${form.first_name.label}:
                    ${form.first_name}
                    % if form.first_name.errors:
                        <strong>${form.first_name.errors[0]}</strong>
                    % endif
                </div>
                <input type="submit" value="Submit">
            </form>
        % endif
    </div>

Note

Keep in mind when using the WTForms library you can customize the error messages and more. Also, you have multiple validation rules so make sure to catch all the errors which will mean you need a loop rather than the simple example above which grabs the first error item in the list. See the documentation for more information.

Run the application:

$ pecan serve config.py

Open a web browser: http://127.0.0.1:8080/

Try the form with valid data and with no data entered.