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Make your own Tutoron

In this walkthrough, we will create a Tutoron that detects and documents Java classes. After following this tutorial, you will be able to build Tutorons that detect and explain arbitrary programming languages. You will also be able to integrate the explanations in arbitrary web pages.

A Tutoron can't run without a server to host it. So before we start, download and set up the Tutorons server by following the instructions in the getting started guide. Follow the guide to its conclusion, where you launch a site on localhost.

Create a server module for the Tutoron

Make a Git repository for the Tutoron

All the code for your Tutoron should be stored in a separate git repository from the server. This lets future developers add the Tutoron to their servers if they want it, while not requiring them to have it and its dependencies installed if they don't want them.

Make a new Git repository on your favorite hosting service. For many people, this may mean creating a new free public repository on GitHub.

Add the Tutoron to the server as a Git submodule

Say you created your Git repository at Now we'll mount your Tutoron as a server module at tutorons/modules/java_classes:

# Assuming you're in the `tutorons-base` directory (the home directory of
# the Tutorons server) that you created in the getting started guide...
git submodule add \ \

Initialize the Tutoron's source code

Create a Tutoron with boilerplate detection and explanation code by running this command: \
  python starttutoron java_classes

This generates about a dozen files with everything a Tutoron needs: a code detector, code explainer, templates for formatting the explanation as a tooltip, and even unit tests. All of this is written to files in the tutorons/modules/java_classes directory you created when you added the Git submodule for the Tutoron. You'll touch most of these files as we tailor this Tutoron to document Java classes.

Integrate the Tutoron as a server "apps"

For the Tutorons server to find the Tutoron, we need to do two things. First, add the Tutoron as an "app" to the Django server by adding the following line to the list of INSTALLED_APPS in the tutorons/settings/ file:


Second, point the Tutorons server to the URLs for the new Tutoron. You can do this by adding these two URL patterns to the list of URL patterns in tutorons/

    url(r'^java_classes$', 'tutorons.modules.java_classes.views.scan', name='java_classes'),
    url(r'^java_classes/', include('tutorons.modules.java_classes.urls', namespace='java_classes')),

Check that the Tutoron was integrated successfully

Make sure that the new Tutoron is working by running the unit tests:


...and then by running the Tutoron on a web page! Start the server:


Then, open http://localhost:8002/java_classes/example in your browser. You should be able to see that the Tutoron currently detects and explains the variable foo in a made-up language. Neat!!

Customize the Tutoron to document Java classes

Let's say we want to show snippets of documentation about Java API classes whenever those classes show up on a web page. Let's tailor this Tutoron so that it finds Java API classes and provides such documentation snippets.

For documentation, we'll use some example "insights" from Christoph Treude and Martin Robillard's paper. They automatically mined insightful sentences about Java API members from posts on Stack Overflow—maybe they'll be useful for people who look at Java tutorials on the web!

Let's create a Python list of dictionaries with these insights. Each "insight" has the name of a class from the Java API and an insightful sentence about that API. Make a new file named tutorons/modules/java_classes/ Paste this list into that file:

    'class': 'ArrayList',
    'insight': "The list returned from asList has fixed size.",
    'class': 'LinkedList',
    'insight': "There is one common use case in which LinkedList " +
               "outperforms ArrayList: that of a queue.",

In the next two steps, we'll detect all references to Java API classes for which we have insights, and then create explanations that a web page visitor can see in a tooltip, attached to each reference.

Write code to detect Java class mentions

Before we update the Tutoron's detection code to detect mentions of Java classes, let's modify the detection test so that we have some way to check if we implemented detection correctly. Open up the current detection test at tutorons/modules/java_classes/tests/ Replace the existing test_detect_foo method with this method:

    def test_detect_java_builtin(self):

        html_doc = HtmlDocument('\n'.join([
            "  <body>",
            "    <div>",
            "      <code>import java.util.ArrayList;</code>",
            "    </div>",
            "  </body>",
        code_regions = detect_code(html_doc)

        self.assertEqual(1, len(code_regions))
        code_region = code_regions[0]
        self.assertEqual("ArrayList", code_region.string)
        self.assertEqual(17, code_region.start_offset)
        self.assertEqual(25, code_region.end_offset)

The test case creates a fake HTML page with a code element with a small line of Java code—import java.util.ArrayList;. The test checks the result of detection (detect_code) on the HTML page: detection should have yielded a single code region for the API member ArrayList, at characters 17 to 25 within a code element.

We can see the new test fail when we run it:


Obviously, the test fails because we haven't yet implemented the new detection logic. To implement detection, open the tutorons/modules/java_classes/ file. Look at the current implementation: an "extractor" searches for the pattern of the string "foo" in the text of an HTML element. Every time it finds a substring of text that matches the pattern, it records a "region" that points to the position where the pattern was found.

We can reuse most of this code. The only thing that's different for us is that we want to look for several patterns—one for each class in the INSIGHTS list. The high-level strategy will be to loop over all entries in the INSIGHTS list, create a set of patterns from the class names in the list, and search for each pattern (class name) one-by-one in the text.

Okay, let's implement detection of Java classes. First, we need access to the INSIGHTS data. Beneath the current imports in tutoron/modules/java_classes/, add this import:

from insights import INSIGHTS

Then, add code to create a set of patterns from the classes in the list of insights. Add this code before the definition of the JavaClassesExtractor class:

java_class_patterns = []

for insight in INSIGHTS:
    if insight['class'] not in java_class_patterns:

Finally, do a substring search on the text for each pattern in java_class_patterns. Replace the code above the while loop (up through the pattern = "foo" line) with this code:

        # Initialize the list of regions outside of the for-loop: this list 
        # should contain regions found for *all* patterns.
        regions = []

        for pattern in java_class_patterns:
            # Reset pointer for substring match to the beginning of the text,
            # for each class.
            last_match_end = 0

            # The entirety of the `while True` loop will need to be indented
            # inside this for loop. The `return` statement should be outside of
            # the for loop.
            while True:

That's it! Now run the tests again and we can see that detection works:


We have successfully implemented a detector for Java classes! This is good progress, though note that currently the detector isn't that sophisticated and might yield false positives. See "Next Steps" for some pointers on building more sophisticated detectors.

Write code that provides documentation for Java classes

Now that we can detect Java classes, we need to write code that maps a class to an explanation that can be shown in a tooltip—in our case, an insight sentence.

Let's start with a test case that defines the expected explanation behavior. Open tutorons/modules/java_classes/tests/ and replace the test named test_explain_that_foos_do_bar with this test:

    def test_explain_ArrayList_with_insight(self):
        explanation = explain_code("ArrayList")
        self.assertIn("list returned from asList", explanation)

The test checks whether a string of code with the text "ArrayList" yields an explanation that contains a substring of the expected insight sentence ("list returned from asList"). This test should fail when we run it:


It's time to re-implement the code for making explanations in the explain_code method of tutorons/modules/java_classes/ Currently, the method generates a single static explanation for the word "foo". The method should instead give a custom explanation for each class, comprising of one of the insights from the list.

Import the INSIGHTS list into the file by adding this line below the rest of the imports:

from insights import INSIGHTS

Then, create a custom explanation for each Java class by replacing the code in the explain_code method with this:

    for insight in INSIGHTS:
        if code_string == insight['class']:
            explanation = insight['insight']

This code iterates over the list of insights, finds the first insight that was written for the class, and sets the explanation to that insight's text.

Run the tests again, and the Tutoron is now producing the insights as explanations.


Test the Tutoron on an example web page

We built a Tutoron that documents Java classes by writing custom detection and explanation code. But how do we know it works?

Every Tutoron created with starttutoron has a test page that can be used as an integration test. Visit http://localhost:8002/java_classes/example to see that page. (Reminder: the server needs to be running, or else you will see a "connection refused" error. Run the server with the ./ command.)

We need to update the test page, though: there are no Java classes to explain on the web page yet! So, we'll add some Java classes to the code on the example page. Open tutorons/modules/java_classes/templates/example.html. Then modify the content of the <pre><code> block. That line should look like this:

            <pre><code>import java.util.ArrayList;</code></pre>

To see detection working, refresh the test page. Did you see ArrayList get highlighted? And when you clicked on it, did you see the tooltip that appeared, containing the insight? That's the explanation we wrote!

Update the tooltip's HTML

But wait, you say. Something's wrong. The explanation in the tooltip says, "You found the variable ArrayList, and ArrayList is a Java class, not a variable!"

You're right! Up until now, I've left out one of the important steps in explanation generation that we still need to fix for the Java classes Tutoron.

We've already looked at the first step of explanation creation. The explain_code function is that first step. It's written in Python because Python and APIs written in Python provide useful utilities for looking up explanations in databases and expressing complex logic for building up English sentences and examples.

We're missing the second step. In the second step, the HTML of the tooltip body is rendered using the tutorons/modules/java_classes/templates/explanation.html file. The explanation.html template takes as input data passed (such as a string or object representing an explanation of a piece of code) from the file, and formats it into the tooltip's HTML. To fix this gaffe with calling ArrayList a variable, we need to look at the part of the explanation that's created in the second step in this template file.

Look at tutorons/modules/java_classes/templates/explanation.html. When this template is rendered, {{ code_string }} and {{ explanation }} will resolve to the text of the detected code and the explanation generated by explain_code, respectively.

The problematic explanation is in the first p element, where the initial template assumed that any explained code would be for a variable. We'll update this for our Tutoron. Replace the first p element with this line:

<p>You found the Java class <code>{{ code_string }}</code>.</p>

Refresh the example page and see that the explanation is now correct.

You may wonder, what part of the explanation do I put in the explain_code method, and what part in the template? Try these rules of thumb: When part of an explanation will be shared across all explanations, put that part of the explanation in the template. Any HTML formatting should be done in the template. Dynamically generated parts of explanations should go in explain_code. For additional tips on generating HTML for explanations and code, see the Django docs about templates.

Integrating the Tutoron into web pages

Want to automatically detect and explain Java classes on another web page? Once you write your Tutoron, its explanations can be made available for any page on the web, with a couple of HTML additions.

First, you'll need to set up the Tutorons server (with your module installed) on a server with a domain name and with HTTPS communication enabled. You should absolutely enable HTTPS and disable all HTTP communication to this server. A user's visit to the page and, depending on the page, the way that page is rendered for them, can be sensitive information. You should respect this by making sure all communication with your Tutorons server is through HTTPS.

There are many options for setting up a Django server that communicates over HTTPS. DigitalOcean provides tutorials on serving Django apps with the Nginx reverse proxy and securing communication over Nginx with HTTPS. When in doubt, ask someone with experience to help you with setting up secure communication. Feel free to send me a message if you want pointers on how I did this for the central Tutorons server.

Once you have launched your Tutorons server with a publicly visible domain name, you can integrate your Tutoron into any web page by adding these <script> tags to the <head> of the page's HTML:

        <script src="//"></script>
          document.addEventListener("DOMContentLoaded", function (event) {
            var tutoronsConnection = new tutorons.TutoronsConnection(window, {
              endpoints: {
                  java_classes: "//",

Just replace with your domain name. Then, whenever someone opens the page, the tutorons-library.js script will be fetched from the central Tutorons server to load the Tutorons JavaScript API. Once the page fully loads, a call to tutoronsConnection.scanDom() uploads the page contents to your Tutorons server, requesting code detection and explanations. Your server will return a list of the detected code regions and their explanations, which the Tutorons JavaScript API will automatically attach to the web page.

Show me the source code

Didn't follow this tutorial, but want to play around with the result? See the finished source code for the Tutoron, and follow the instructions in the source code's README to test it out on a local server.

Next Steps

Improving detection

While this tutorial uses substring matching to detect explainable code, sometimes it's useful to have more accurate detection of code constructs. For example, if you wanted to explain ArrayList only when it was used in a constructor, you would first want to extract the code from a web page, run a parser over it, and traverse the parse tree, looking only at tree nodes that belonged to a constructor. Many common languages have parsers implemented in Python, or grammars that can be used with ANTLR to generate a parser. When you look for such a parser, make sure that it returns the character positions of all of the nodes in the abstract syntax tree so you can recover the exact character position of the matching code.

To get inspiration for more advanced parsing techniques, see this heavier version of the Tutorons server.

Updating the tooltip style

If you want to modify the CSS style of the Tutorons, you currently need to host your own version of the Tutorons library. Send us a message if you could use an easier way to style your Tutorons.