安装web framework: lpthw.web

Your First Website

These final three exercises will be very hard and you should take your time with them. In this first one you'll build a simple web version of one of your games. Before you attempt this exercise you must have completed Exercise 46 successfully and have a working pip installed such that you can install packages and know how to make a skeleton project directory. If you don't remember how to do this, go back to Exercise 46 and do it all over again.

Installing lpthw.web

Before creating your first web application, you'll first need to install the "web framework" called lpthw.web. The term "framework" generally means "some package that makes it easier for me to do something." In the world of web applications, people create "web frameworks" to compensate for the difficult problems they've encountered when making their own sites. They share these common solutions in the form of a package you can download to bootstrap your own projects.
In our case, we'll be using the lpthw.web framework, but there are many, many, many others you can choose from. For now, learn lpthw.web, then branch out to another one when you're ready (or just keep using lpthw.web since it's good enough).
Using pip, install lpthw.web:

$ sudo pip install lpthw.web
[sudo] password for zedshaw:
Downloading/unpacking lpthw.web
  Running setup.py egg_info for package lpthw.web

Installing collected packages: lpthw.web
  Running setup.py install for lpthw.web

Successfully installed lpthw.web
Cleaning up...

This will work on Linux and Mac OSX computers, but on Windows just drop the sudo part of the pip install command and it should work. If not, go back to Exercise 46 and make sure you can do it reliably.

Warning

Other Python programmers will warn you that lpthw.web is just a fork of another web framework called web.py and that web.py has too much "magic." If they say this, point out to them that Google App Engine originally used web.py and not a single Python programmer complained that it had too much magic, because they all worked at Google. If it's good enough for Google, then it's good enough for you to get started. Then, just get back to learning to code and ignore their goal of indoctrination over education.

Make a Simple "Hello World" Project

Now you're going to make an initial very simple "Hello World" web application and project directory using lpthw.web. First, make your project directory:

$ cd projects
$ mkdir gothonweb
$ cd gothonweb
$ mkdir bin gothonweb tests docs templates
$ touch gothonweb/__init__.py
$ touch tests/__init__.py

You'll be taking the game from Exercise 43 and making it into a web application, so that's why you're calling it gothonweb. Before you do that, we need to create the most basic lpthw.web application possible. Put the following code into bin/app.py:

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import web urls = ( '/', 'index' ) app = web.application(urls, globals()) class index: def GET(self): greeting = "Hello World" return greeting if __name__ == "__main__": app.run()

Then run the application like this:

$ python bin/app.py
http://0.0.0.0:8080/

However, if you did this:

$ cd bin/   # WRONG! WRONG! WRONG!
$ python app.py  # WRONG! WRONG! WRONG!

Then you are doing it wrong. In all Python projects you do not cd into a lower directory to run things. You stay at the top and run everything from there so that all of the system can access all the modules and files. Go reread Exercise 46 to understand a project layout and how to use it if you did this.
Finally, use your web browser and go to http://localhost:8080/ and you should see two things. First, in your browser you'll see Hello, world!. Second, you'll see your Terminal with new output like this:

$ python bin/app.py
http://0.0.0.0:8080/
127.0.0.1:59542 - - [13/Jun/2011 11:44:43] "HTTP/1.1 GET /" - 200 OK
127.0.0.1:59542 - - [13/Jun/2011 11:44:43] "HTTP/1.1 GET /favicon.ico" - 404 Not Found

Those are log messages that lpthw.web prints out so you can see that the server is working, and what the browser is doing behind the scenes. The log messages help you debug and figure out when you have problems. For example, it's saying that your browser tried to get /favicon.ico but that file didn't exist so it returned 404 Not Found status code.
I haven't explained the way any of this web stuff works yet, because I want to get you setup and ready to roll so that I can explain it better in the next two exercises. To accomplish this, I'll have you break your lpthw.web application in various ways and then restructure it so that you know how it's setup.

What's Going On?

Here's what's happening when your browser hits your application:

1. Your browser makes a network connection to your own computer, which is called localhost and is a standard way of saying "whatever my own computer is called on the network." It also uses port 8080.
2. Once it connects, it makes an HTTP request to the bin/app.py application and asks for the / URL, which is commonly the first URL on any website.
3. Inside bin/app.py you've got a list of URLs and what classes they match. The only one we have is the '/', 'index' mapping. This means that whenever someone goes to / with a browser, lpthw.web will find the class index and load it to handle the request.
4. Now that lpthw.web has found class index it calls the index.GET method on an instance of that class to actually handle the request. This function runs and simply returns a string for what lpthw.web should send to the browser.
5. Finally, lpthw.web has handled the request and sends this response to the browser, which is what you are seeing.

Make sure you really understand this. Draw up a diagram of how this information flows from your browser, to lpthw.web, then to index.GET and back to your browser.

Fixing Errors

First, delete line 11 where you assign the greeting variable, then hit refresh in your browser. You should see an error page now that gives you lots of information on how your application just exploded. You know that the variable greeting is now missing, but lpthw.web gives you this nice error page to track down exactly where. Do each of the following with this page:

1. Look at each of the Local vars outputs (click on them) and see if you can follow what variables it's talking about and where they are.
2. Look at the Request Information section and see if it matches anything you're already familiar with. This is information that your web browser is sending to your gothonweb application. You normally don't even know that it's sending this stuff, so now you get to see what it does.
3. Try breaking this simple application in other ways and explore what happens. Don't forget to also look at the logs being printed into your Terminal as lpthw.web will put other stack traces and information there too.

Create Basic Templates

You can break your lpthw.web application, but did you notice that "Hello World" isn't a very good HTML page? This is a web application, and as such it needs a proper HTML response. To do that you will create a simple template that says "Hello World" in a big green font.
The first step is to create a templates/index.html file that looks like this:

$def with (greeting)

<html>
    <head>
        <title>Gothons Of Planet Percal #25</title>
    </head>
<body>

$if greeting:
    I just wanted to say <em style="color: green; font-size: 2em;">$greeting</em>.
$else:
    <em>Hello</em>, world!

</body>
</html>

If you know what HTML is, then this should look fairly familiar. If not, research HTML and try writing a few web pages by hand so you know how it works. This HTML file, however, is a template, which means that lpthw.web will fill in "holes" in the text depending on variables you pass in to the template. Every place you see $greeting will be a variable you'll pass to the template that alters its contents.
To make your bin/app.py do this, you need to add some code to tell lpthw.web where to load the template and to render it. Take that file and change it like this:

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import web urls = ( '/', 'Index' ) app = web.application(urls, globals()) render = web.template.render('templates/') class Index(object): def GET(self): greeting = "Hello World" return render.index(greeting = greeting) if __name__ == "__main__": app.run()

Pay close attention to the new render variable and how I changed the last line of index.GET so it returns render.index() passing in your greeting variable.
Once you have that in place, reload the web page in your browser and you should see a different message in green. You should also be able to do a View Source on the page in your browser to see that it is valid HTML.
This may have flown by you very fast, so let me explain how a template works:

1. In your bin/app.py you've added a new variable, render, which is a web.template.render object.
2. This render object knows how to load .html files out of the templates/ directory because you passed that to it as a parameter.
3. Later in your code, when the browser hits the index.GET like before, instead of just returning the string greeting, you call render.index and pass the greeting to it as a variable.
4. This render.index method is kind of a magic function where the render object sees that you're asking for index, goes into the templates/ directory, looks for a page named index.html, and then "renders" it, or converts it.
5. In the templates/index.html file you see the beginning definition that says this template takes a greeting parameter, just like a function. Also, just like Python this template is indentation sensitive, so make sure you get them right.
6. Finally, you have the HTML in templates/index.html that looks at the greeting variable and, if it's there, prints one message using the $greeting, or a default message.

To get deeper into this, change the greeting variable and the HTML to see what effect it has. Also create another template named templates/foo.html and render that using render.foo() instead of render.index() like before. This will show you how the name of the function you call on render is just matched to an .html file in templates/.

Study Drills

1. Read the documentation at http://webpy.org/ which is the same as the lpthw.web project.
2. Experiment with everything you can find there, including their example code.
3. Read about HTML5 and CSS3 and make some other .html and .css files for practice.
4. If you have a friend who knows Django and is willing to help you, then consider doing Exercises 50, 51, and 52 in Django instead to see what that's like.

Common Student Questions

I can't seem to connect to http://localhost:8080/.

Try going to http://127.0.0.1:8080/ instead.

What is the difference betwee lpthw.web and web.py?

No difference. I simply "locked" web.py at a particular version so that it would be consistent for students, then named it lpthw.web. Later versions of web.py might be different from this version.

I can't find index.html (or just about anything).

You probably are doing cd bin/ first and then trying to work with the project. Do not do this. All of the commands and instructions assume you are one directory above bin/, so if you can't type python bin/app.py then you are in the wrong directory.

Why do we assign greeting=greeting when we call the template?

You are not assigning to greeting. You are setting a named parameter to give to the template. It's sort of an assignment, but it only affects the call to the template function.

I can't use port 8080 on my computer.

You probably have an anti-virus program installed that is using that port. Try a different port.

After installing lpthw.web I get ImportError "No module named web".

You most likely have multiple versions of Python installed and are using the wrong one, or you didn't do the install correctly because of an old version of pip. Try uninstalling lpthw.web and reinstalling it. If that doesn't work make triple sure you're using the right version of Python.

 

from http://learnpythonthehardway.org/book/ex50.html