Writing Web Applications in Python with Bottle

Web Applications

A web application consists of code that responds to HTTP requests to return an HTTP response. A web application is not the same as a web server; the server is listening on the network socket for requests and decoding them and then sending back the response. The web application is the code that takes the request information and generates the response. To write a web application we need a way to send the request information from the server to the application code; there are many ways to do this for different combinations of web server and application environment.

In Python, the low-level interface to web applications is called WSGI (Web Server Gateway Interface). It defines a standard way of defining a procedure to act as as web application.
While it is possible to write an application using the low-level WSGI standard, it is usually much more convenient to use a higher level abstraction in the form of a framework. This means that we use a python module that does some of the work for us and takes away some of the complexity of dealing with the low level WSGI standard. There are many frameworks to choose from for Python (see this list) but we will use the Bottle framework because it is simple and doesn't do everything for us - the goal of this course is for you to learn how the web works, so we don't yet want to use a framework that hides all of the detail from you.

We'll explore Bottle in lots of detail but to show you a simple example, a simple Bottle web application would be:

from bottle import Bottle

app = Bottle()

def index():
    return "Hello World, how are you?"

def about():
    return "Tell me about yourself."

The first thing that this example does is create a Bottle application object. This is going to be used when we come to run the code. (Note that you will see examples of Bottle apps without this, we'll use it because it helps with testing your code later on).

This example defines two 'pages' in the application - that is it will respond to two different URLs. Each URL is called a route and the code associates this route with a procedure that returns the content to be sent back to the browser. So if this application gets a request for the home page (eg. http://localhost:8000/) then it will return the "Hello World..." message. If it gets a request for the 'about page' (eg. http://localhost:8000/about) it will return the "Tell me..." message. In this simple example these are plain text; we'll see how to make them return HTML pages shortly.

This structure for the Bottle application emphasises one important feature of web applications. The job of the application is to respond to requests forwarded by the server; each request is for a particular URL. The application must receive the request, process it, and generate a response. Bottle allows you to arrange your code so that one procedure deals with one request; this makes it easy to see what your code is doing and helps with debugging.

Bottle is built on top of the WSGI standard and so it is compatible with all kinds of other Python web infrastructure. For example there are modules that implement authentication and sessions and they can be used directly with Bottle applications. You don't need to worry about this just now but it is reassuring to know that what you are doing is aligned with core Python infrastructure.

This is a simple example of an application, we now need to look at how to run it.

The Web Server

The web server is the piece of software that handles HTTP requests from the client and generates a response to send back. In a production environment, you will use a server like Apache, Ngnix or Microsoft Internet Information Server. These are large software systems that are designed to handle many requests per second as fast as possible. While we are developing code, there is no need to run under such a server and in fact it's a bit of a pain to have to deal with it. When working with Python it is common to use a simple server written in Python and that's what we will do. It means you don't need to install anything other than Python and Bottle to develop this code but since Bottle uses the WSGI standard interface, anything that you develop could be deployed on a production quality server quite easily.

Bottle provides a development server that you can use by adding a single line to your application. Here's the entire code of our first example, including the server:

from bottle import Bottle

app = Bottle()

def index():
    return "Hello World, how are you?"

def about():
    return "Tell me about yourself."

if __name__ == '__main__':

What we've added is a bit of code that will only run if we run this Python module (file) directly (if __name__=='__main__')) but not if it is imported by another module. This is a common Python trick and is equivalent to the main class in a Java program. In this case we have one line of code which just calls the run method on the application. We've passed one named argument to turn on debug mode in the server.

When you run the above Python code, it will start a web server listening on port 8080, you should see the following output:

Bottle v0.12.8 server starting up (using WSGIRefServer())...
Listening on
Hit Ctrl-C to quit.

This indicates that the server is listening for requests on port 8080 on the local machine ( is the IP address of the 'loopback' network interface on every computer, meaning that it refers to the computer itself). If you copy and paste that link into your browser you should see a page with our "Hello World..." message; you should also see the following printed in the terminal window: - - [11/Feb/2015 16:06:37] "GET / HTTP/1.1" 200 25 - - [11/Feb/2015 16:06:38] "GET /favicon.ico HTTP/1.1" 404 742

This is a log from the web server showing what requests were received and the response code (200) and content length (25 bytes) that were returned. The second request for /favicon.ico is not one you made directly, your web browser generally sends this request to any site you visit to get the little icon that is shown in the address bar. In this case, Bottle doesn't know what to do with a request for this route and so returns a 404 (Not Found) response. If you enter that URL into your browser directly ( then you'll see the debugging error message that Bottle generates.

You can also request the other URL that we've included in the application: This should give you the "Tell me..." page and you should see another line in the logs. Your application is working! We now need to see how to make it do something more useful.

From now on I won't include the server part of the script in my examples, since I will concentrate on the definition of the application itself. To run all of the future examples you can use the same main part as in the example here.


  1. Make your own copy of the script above and run it to reproduce the results shown here.
  2. Modify the content of the 'page' returned for each URL by changing the string that each function returns - add some HTML markup, for example put a link to the About page in the home page and vice versa.
  3. See what happens when you request a URL that isn't dealt with by the application. Try a few different paths at the end of the URL and observe the log entry displayed and the results returned. Eg.,
  4. Modify the application so that it does respond to with a new message by adding a new route and a new procedure to the code.

More on Routes

The @app.route('/about') line before each procedure in a Bottle web application is a Python construct called a decorator. It is a way of changing the way a procedure works, in this case it is the way that Bottle associates your procedures with different URL paths. From the discussion of the structure of URLs we recall that the part of the URL after the network location is sent in the HTTP request to the server. So a request for http://example.org/home/index.html will result in an HTTP request being sent to the server at example.org that would look something like this:

GET /home/index.html HTTP/1.1
Host: example.org
Accept: text/*

The server accepts this request and looks at the path that has been requested - /home/index.html in this case. It decides how to deal with this, in this case it will send it along to our Bottle application. Bottle tries to match the path up with one of the routes that we have declared in our application, if it matches, it calls the procedure and returns the response that it generates.

So far we've seen a simple route that just names the URL path that it should apply to. However, Bottle provides a way to match patterns in the URL path and have a single procedure apply to more than one path. Let's illustrate this with a simple example that generates 'home pages' for different people based on their name. We want a URL like to return a custom page for user Steve. Our first implementation will just put the users name in the page. We use the URL pattern @app.route(/people/<who>) as where the <who> part is a pattern that will match anything in the URL. Since the text matched by the pattern can vary, the procedure now takes an argument which is passed the text from the URL. It can then use this text in generating the page. The name of the argument must be the same as the name between angle brackets in the path. Here's the code:

def homepage(who):
    """Generate the home page for a person"""

    return "<p>This is the home page for " + who + ".</p>"

If you add this to your application then you should be able to add any name to the URL to get a custom page; try the following:,,, Note that we can include special characters in a URL by encoding them via the % code numbers. %20 is a space character; what are the characters in the final example? Can you explain what appears in your browser?

The route pattern in angle brackets will only match one path component in a URL, so if you try to access, you will get a 404 error page saying that the page is not found - this is because there is no route to match the URL. We can use more than one pattern in a URL to match more path elements; for example to match first and last names:

def homepage(first, last):
    """Generate the home page for a person"""

    return "<p>This is the home page for " + first + " " + last + ".</p>"

Note that with two patterns in the URL, the procedure needs to accept two arguments for the two values matched. Again, the names of the arguments must match the names in the angle brackets. With this code in place we can access and get the generated page.

Rather than spelling out paths bit by bit, we might want to allow any number of path elements at the end of a URL. We can do this in Bottle by using the :path modifier after the pattern name. This then allows the pattern to match the / character. Let's modify the earlier example:

def homepage(who):
    """Generate the home page for a person"""

    return "<p>This is the home page for " + who + ".</p>"

Now when we access the URL the single pattern matches Steve/Cassidy and we get a page with the message "This is the home page for Steve/Cassidy.". Note that this pattern really just allows you to match names with the slash (/) character in them, they don't have to be paths as such. So what about the example: Can you explain what you see in your browser here? Why does this example only work with this latest code?

Routes in Bottle can match any URL pattern and pass information from the URL to the procedure that will generate the result. This means that your application can respond to a possibly infinite number of URLs. The kind of URL we used above for a person is actually quite a common one in web applications - look at the URL of your Facebook profile page for example. Obviously you would normally do much more work to generate the page - the user name is used as a key to look up data in a database to populate the page. We'll look at examples like this later on in the text.

Using the Request

All input to our web application comes via the HTTP request that is sent to the web server. The server receives this request and parses the header to determine how to deal with it - in our case the request is sent along to our python script using the WSGI standard and Bottle arranges for the right procedure to be called depending on the URL in the request. Many other things are included in an HTTP request that can be of use in generating the response to be sent back to the client. These can include form fields and uploaded files but also things like the IP address of the user, the page that linked to this page etc. Later we'll learn how to use some of this information in generating responses. For now we'll look at a simple example of generating a page from the request.

Bottle provides a global variable request who's value is an object that represents the current HTTP request. We need to import this from the bottle module in order to use it in our code. Let's look at a simple example that includes one property of the request in the page that is returned. We'll use the IP address of the browser (client) and the user-agent header that the browser sends as part of the request.

from bottle import Bottle, response, request

app = Bottle()

def about():

    result =  "<p>Your IP address is:" + request.remote_addr + "</p>"
    result += "<p>Your browser is: " + request.environ['HTTP_USER_AGENT'] + "</p>"

    return result

This is the first example of a web application that doesn't just return something we've typed in. We access the two bits of information we want in slightly different ways. The bottle request object has a property request.remote_addr for easy access to the remote IP address. However, to get at the user-agent string we need to look in the WSGI environment dictionary that is held in request.environ. This just illustrates two different ways to access information from the request in your code. Most of the time you'll just use the properties that Bottle exposes but sometimes it will be necessary to look into the environment.

We can extend this example a bit further and look in more detail at what is in the WSGI environment. This example will also illustrate how we can write helper procedures to generate parts of the page for us.

As we saw in earlier chapters, a dictionary in Python is an associative array where names (keys) are associated with values. A dictionary is an object and has a number of useful methods. We're interested here in using an iterator over the keys in the dictionary - for key in d: will iterate over every key in turn. We then use the square bracket notation (d[key]) to retrieve the value associated with a given key. Given a dictionary, here's the Python code to print out all of the key-value pairs:

d = {'name': 'Steve',
     'age': 21,
     'favourite_colour': 'green',
     'likes_cheese': 'yes'

for key in d:
    print(key, d[key])

Building on this, we can write a procedure to take a dictionary as a parameter and return an HTML string listing the keys and values:

def dict_to_html(dd):
    """Generate an HTML list of the keys and
    values in the dictionary dd, return a
    string containing HTML"""

    html = "<ul>"
    for key in dd.keys():
        html += "<li><strong>%s: </strong>%s</li>" % (key, dd[key])
    html += "</ul>"
    return html

We can now make use of this procedure in a new WSGI application that returns an HTML page containing this information:

def about():

    result = dict_to_html(request.environ)

    return result

On my machine, a request for http://localhost:8080/ when running this application gives me the following page:

screenshot of web browser showing the environment

When you run the application you will see something different. Note that the environment that we are given in the request.environ parameter contains information about the request (e.g. HTTP_HOST, REQUEST_METHOD) but also general information about the execution environment (e.g. HOME, DISPLAY).


  1. The list of environment variables looks a little messy the way it was generated above. Make it a little better by sorting the list of keys before generating the page.
  2. When you change the URL you are requesting, part of the environment changes to communicate this to your script. Access the following URLs and note what has changed in the environment:
    • http://localhost:8080/about?register=yes
    • http://localhost:8080/about?name=Steve

Manipulating the Response

In the examples above, the response returned to the browser was interpreted as an HTML page because the default behaviour in Bottle is to return a response with the Content-Type header set to text/html. In some cases we might want to tell the browser that we're returning a different kind of data, for example, plain text or an image. To do this we need to manipulate the response before it goes back to the server and is sent on to the client. Bottle provides a global object called response to allow us to change any part of the response that will be sent back. Here's an example of changing the content type of the response:

from bottle import Bottle, response

app = Bottle()

def about():
    response.content_type = 'text/plain'
    return "Tell me about yourself."

If you make this change to your sample program and run it, then request in your browser you should see a slightly different result. The text should show up in a fixed width font and if there is any HTML markup you'll see it literally. If you peek at the HTTP request using your browser's developer tools, you'll see that the response type returned is text/plain.

The response object allows us to manipulate other aspects of the response as we'll see when we start developing more complex applications later. For example you can change the status code to generate an error page or a redirect response.

Static Files

In any web application there are some resources that will be static in that they are not generated by application code but are fixed for the whole application. Examples are CSS stylesheets, Javascript files and images. These will be referenced by the HTML generated by your application and so we need to know their URL so that we can reference them properly.

Since these files don't change, there is no real need to invoke an application python script to generate them. In a production setting, these files might be served directly by the main web server or even given to an offsite web server to handle. However, during development, we need them to be served by our python development server, hence we need some way of handling them in our code.

A common pattern is to use a fixed URL path when referring to static files; so, for example, I might use the prefix /static/ and arrange for all static resources to be served from there. In a standard server setup, this URL path would be mapped to a directory on the server. If we put our Javascript file in a subdirectory, eg. js/fancy.js then we'd use the URL /static/js/fancy.js to reference it in the HTML files. Similarly images/face.png would map to /static/images/face.png.

In production, the web server would be configured so that any URL starting with /static was served directly from the configured directory. This will be much faster than invoking our Python application to serve the file. Web servers are optimised for serving static files and do it very efficiently.

In development, we need to add some code to our application to handle this URL prefix. This code will look at the URL and extract the filename that corresponds to the resource, read that file and send it back. It's possible (and informative) to write this code but Bottle also gives us an implementation that is easy to use. Here is the standard route handler that we can include in our application to handle requests for static files:

from bottle import static_file

def server_static(filepath):
    return static_file(filepath, root='static')

The route uses the :path qualifier which matches any path after the URL, not just a single filename. We import the static_file handler from Bottle and pass it the path and the location of the root directory. In this example I've used a relative path which will be interpreted relative to the current working directory. This is convenient for development, we create a directory called static inside our project (next to main.py) and store static files in there.