Django4scientists

Programming the Django application

The Django application is about filling the files views.py and models.py with content. The mathematical computations are performed in compute.py so we copy this file from the mvc directory to the hw1 directory for convenience (we could alternatively add ../mvc to sys.path such that import compute would work from the hw1 directory).

The user interaction

The web application offers a text field where the user can write the value of r, see Figure 1. After clicking on the equals button, the mathematics is performed and a new page as seen in Figure 2 appears.

The model

The models.py file contains the model, which consists of the data we need in the application, stored in Django's data types. Our data consists of one number, called r, and models.py then look like

from django.db import models
from django.forms import ModelForm

class Input(models.Model):
    r = models.FloatField()

class InputForm(ModelForm):
    class Meta:
        model = Input

The Input class lists variables representing data as static class attributes. The django.db.models module contains various classes for different types of data, here we use FloatField to represent a floating-point number. The InputForm class has a the shown generic form across applications if we by convention apply the name Input for the class holding the data.

The view

The views.py file contains a function index which defines the actions we want to perform when invoking the URL ( here http://127.0.0.1:8000/hw1/). In addition, views.py has the present_output function from the view.py file in the mvc directory.

from django.shortcuts import render_to_response
from django.template import RequestContext
from django.http import HttpResponse
from models import InputForm
from compute import compute

def index(request):
    if request.method == 'POST':
        form = InputForm(request.POST)
        if form.is_valid():
            form = form.save(commit=False)
            return present_output(form)
    else:
        form = InputForm()

    return render_to_response('hw1.html',
            {'form': form}, context_instance=RequestContext(request))

def present_output(form):
    r = form.r
    s = compute(r)
    return HttpResponse('Hello, World! sin(%s)=%s' % (r, s))

The index function deserves some explanation. It must take one argument, usually called request. There are two modes in the function. Either the user has provided input on the web page, which means that request.method equals 'POST', or we show a new web page with which the user is supposed to interact.

Making the input page

The input consists of a web form with one field where we can fill in our r variable. This page is realized by the two central statements

# Make info needed in the web form
form = InputForm()
# Make HTML code
render_to_response('hw1.html',
    {'form': form}, context_instance=RequestContext(request))

The hw1.html file resides in the templates subdirectory and contains a template for the HTML code:

<form method="post" action="">{% csrf_token %}
    Hello, World! The sine of {{ form.r }}
    <input type="submit" value="equals" />
</form>

This is a template file because it contains instructions like {% csrf_token %} and variables like {{ form.r }}. Django will replace the former by some appropriate HTML statements, while the latter simply extracts the numerical value of the variable r in our form (specified in the Input class in models.py). Typically, this hw1.html file results in the HTML code

<form method="post" action="">
<div style='display:none'>
<input type='hidden' name='csrfmiddlewaretoken'
value='oPWMuuy1gLlXm9GvUZINv49eVUYnux5Q' /></div>
    Hello, World! The sine of <input type="text" name="r" id="id_r" />
    <input type="submit" value="equals" />
</form>

Making the results page

When then user has filled in a value in the text field on the input page, the index function is called again and request.method equals 'POST'. A new form object is made, this time with user info (request.POST). We can check that the form is valid and if so, proceed with computations followed by presenting the results in a new web page (see Figure 2):

def index(request):
    if request.method == 'POST':
        form = InputForm(request.POST)
        if form.is_valid():
            form = form.save(commit=False)
            return present_output(form)

def present_output(form):
    r = form.r
    s = compute(r)
    return HttpResponse('Hello, World! sin(%s)=%s' % (r, s))

The numerical value of r as given by the user is available as form.r. Instead of using a template for the output page, which is natural to do in more advanced cases, we here illustrate the possibility to send raw HTML to the output page by returning an HttpResponse object initialized by a string containing the desired HTML code.

Launch this application by filling in the address http://127.0.0.1:8000/hw1/ in your web browser. Make sure the Django development server is running, and if not, restart it by

Terminal> python ../../../django_project/manage.py runserver

Fill in some number on the input page and view the output. To show how easy it is to change the application, invoke the views.py file in an editor and add some color to the output HTML code from the present_output function:

    return HttpResponse("""
<font color='blue'>Hello</font>, World!
sin(%s)=%s
"""% (r, s))

Go back to the input page, provide a new number, and observe how the "Hello" word now has a blue color.

Equipping the input page with output results

Instead of making a separate output page with the result, we can simply add the sine value to the input page. This makes the user feel that she interacts with the same page, as when operating a calculator. The output page should then look as shown in Figure 3.

We need to make a new Django application, now called hw2. Instead of running the standard manage.py startapp hw2 command, we can simply copy the hw1 directory to hw2. We need, of course, to add information about this new application in settings.py and urls.py. In the former file we must have

TEMPLATE_DIRS = (
    relative2absolute_path('../../apps/django_apps/hw1/templates'),
    relative2absolute_path('../../apps/django_apps/hw2/templates'),
)

INSTALLED_APPS = (
    'django.contrib.auth',
    'django.contrib.contenttypes',
    'django.contrib.sessions',
    'django.contrib.sites',
    'django.contrib.messages',
    'django.contrib.staticfiles',
    # Uncomment the next line to enable the admin:
    # 'django.contrib.admin',
    # Uncomment the next line to enable admin documentation:
    # 'django.contrib.admindocs',
    'hw1',
    'hw2',
)

In urls.py we add the URL hw2 which is to call our index function in the views.py file of the hw2 app:

urlpatterns = patterns('',
    url(r'^hw1/', 'django_apps.hw1.views.index'),
    url(r'^hw2/', 'django_apps.hw2.views.index'),

The views.py file changes a bit since we shall generate almost the same web page on input and output. This makes the present_output function unnatural, and everything is done within the index function:

def index(request):
    s = None  # initial value of result
    if request.method == 'POST':
        form = InputForm(request.POST)
        if form.is_valid():
            form = form.save(commit=False)
            r = form.r
            s = compute(r)
    else:
        form = InputForm()

    return render_to_response('hw2.html',
            {'form': form,
             's': '%.5f' % s if isinstance(s, float) else ''
             }, context_instance=RequestContext(request))

Note that the output variable s is computed within the index function and defaults to None. The template file hw2.html looks like

<form method="post" action="">{% csrf_token %}
    Hello, World! The sine of {{ form.r }}
    <input type="submit" value="equals" />
{% if s != '' %}
{{ s }}
{% endif %}
</form>

The difference from hw1.html is that we right after the equals button write out the value of s. However, we make a test that the value is only written if it is computed, here recognized by being a non-empty string. The s in the template file is substituted by the value of the object corresponding to the key 's' in the dictionary we pass to the render_to_response. As seen, we pass a string where s is formatted with five digits if s is a float, i.e., if s is computed. Otherwise, s has the default value None and we send an empty string to the template. The template language allows tests using Python syntax, but the if-block must be explicitly ended by {% endif %}.

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