Creating Web Services

We will start by programming the server to return a responses to an http GET request.

We will always need to do two things:

• map a url to a view function
• define the view function

website/urls.py

This file matches urls to view functions.

When the django server receives a url. It searches in this file for one that matches. If it matches it executes the mapped function. If it doesn’t find anything you get a 404 - page not found error.

Saying hello

Django provides us with what it calls view functions.

These are orgindary pythong functions, but they take a request object and they response with a string or what is called an HTTPResponse object.

In your blog app, open the views.py file.

Add this to it:

from django.http import HttpResponse

def hello(request):
    return HttpResponse('hello')

Now we need to configure our website with which request will trigger this view function. We do this by adding a line to website/urls.py:

urlpatterns = patterns('',
    url(r'^hello$', 'blog.views.hello'),
    url(r'^admin/', include(admin.site.urls)),
)

In our browser, http://localhost:8000 responds with ‘hello’.

We have responded to a GET request.

We will often follow this pattern of creating a view function and hooking it up to a url.

GET parameters

http GET requests can pass parameters in the URL.

Here is an example:

http://localhost:8000/whoami/?name=greg&sex=male

The parameter section is defined by ? followed by & separated keys and values.

Here we have the parameters: - name, equal to greg - sex, equal to male

As usual we need to do two things create a view function and hook it up in website/urls.py

First the view function:

def whoami(request):

    sex = request.GET['sex']
    name = request.GET['name']

    response = 'You are ' + name + ' and of sex ' + sex

    return HttpResponse(response)

Note that we can extract anything passed in the url after the ? character using the request.GET dictionary.

Now website/urls.py:

urlpatterns = patterns('',
    url(r'^$', 'blog.views.hello'),
    url(r'^time$', 'blog.views.time'),
    url(r'^whoami/$', 'blog.views.whoami'),
    url(r'^admin/', include(admin.site.urls)),
)

You should now get as a response: You are greg and of sex male

Exercises

A clock service

You can get an exact time by doing the following:

>>> import datetime
>>> datetime.datetime.now()

Program your server to response the time when it recieves an http GET request to this url:

http://localhost:8000/time

You will need to create a view function in blog/views.py, and hook it up to a url in website/urls.py.

Body Mass Index Service

You have just been contracted by the NHS to provide a service that calculates the BMI. Both other websites and mobile apps will be using your service.

The endpoint (url) will respond successfully to the following type of url:

bmi/?mass=75&height=182

Look up the BMI equation on wikipedia, and write a bmi view function and hook it up to the website urls.

You may have to revisit the notion of type in Python. Remember there is a difference between ‘5’ and 5.

To transform a number as a string into a number you can cast it using either int() or float():

>>> float('5')
5.0
>>> int('5')
5

Your Serivce

By now you have discovered that you can trigger any type of programming sending ba GET request to your server. You simply hook up a url to a view function.

Come up with something that is useful to you!

Anything that involves simple maths is easily explored.

Solutions:

You can find some suggestions by adding _solutions to the above url.