Nicco Kunzmann


Documentation-Driven Development

Documentation-Driven Development

I stumbled upon this PyCon2016 talk. Whereas test-driven development is feeling the love of many people, I got reminded what it is really about: documenting behavior in code. So when I read the title I was inclined to listen to the talk. Only 26 views seem little for what it is: taking TDD to the next level.

There is some truth in it:

[…] understand information as the process of informing - as an activity. Understand documentation as a process of documenting […] [Minute 13 Second 13]

What can your Project do?

From the talk…

What I take from it

Documentation is often seen as something noone likes. But with the Django Project we can see that documentation actually drives development as it gets new people in and (in)forms the commuity.

When you write your code and you test it, you can do it to be the communication medium between the code and the test as you alter both while implementing the tests. I write my tests to think about how I would use the code instead in contrast to wanting my behavior implemented in an easy way.

Let’s think DDD further: What is missing when I write my tests is the intention behind the test and the overall picture. Also, tests are not right where the code lives. If I were new to the project, I would look for those intentions. The intentions should be found in the docs. Even if the tests were to be found right next to the function, their focus is not on the humans mental model and the bigger picture but on the specific behavior. Sometimes tests can and should be split up - it could be so much that the bigger picture is hard to find.

This is why I think that code, tests, documentation go hand in hand.

Fastforward -> “Documentation First” applied

I really like doing documentation first.

First, I created this class:

class InstructionToSVG(object):
    """This class maps instructions to SVGs."""
    def __init__(self):
        """Create a InstructionToSVG object."""

    def load(self):
        """Returns a loader object that allows loading SVG file from 
        various sources such as files and folders.
        `load.path(path)` loads an svg from a file named path
        `load.folder(path)` loads all SVG files for instructions in the folder
        If multiple files have the same name, the last occurrence is used."""
    def instruction_to_svg(self, instruction):
        """Returns an SVG representing the instruction.
        The SVG file is determined by the type of the instruction. 
        An instruction of type "knit" will be looked for in a file named
        The SVGs will be scaled to width and heigth of the instruction.
        The color of the instruction is taken into account.
        If nothing was loaded to display this instruction, a default image will
        be generated by `default_instruction_to_svg`."""

This was my mental model of how to render instructions to SVG. Then, I started writing some tests and they were much better structured than they usually are. It also came to be that I renamed a test method because it just did not represent what it should test. This was kind of mind-opening. Talking with the documentation about the tests had the following implications:

I come to think that documentation-first has the same implications on tests as test-first has on code.

Here is the test structure which would normally be just a lot of functions:

class TestHasSVGForInstruction(object):
    """This tests the `InstructionToSVG.has_instruction_to_svg` method."""
    # ...
class TestDefaultInstrucionToSVG(object):
    """This tests the `InstructionToSVG.default_instruction_to_svg` method.
    # ...

While writing the tests I started adding new documentation for the methods. This also works like a TODO-list of what needs to be done - which I would certainly loose track of while implementing tests. I created new methods:

class InstructionToSVG(object):
    # ...
    def has_svg_for_instruction(self, instruction):
        """Returns whether there is an image for the instruction. 
        This can be used before `instruction_to_svg` as it determines whether
        - the default value is used (`False`) 
        - or there is a dedicated svg representation (`True`). 
    def default_instruction_to_svg(self, instruction):
        """As `instruction_to_svg()` but it does not take the loaded files into
        In case no file is found for an instruction in `instruction_to_svg()`, 
        this method is used to determine the default svg for it.
        The content is created by replacing the text {instruction.type} a the 
        whole svg file named `default.svg`.
        If no file `default.svg` was loaded, an empty string is returned."""

These methods have a lot of things that I expect from them. An with the process

  1. document
  2. test
  3. implement

I am sure

Now, I have a lot of undocumented tests lying around which feels as bad as having untested code.


I am looking forward to using Sphinx. With its power I would like to add to each method

Documentation Coverage / Specification Coverage

We have test-coverage but we do not have documentation coverage, or do we? There is old research about it:

Research behind a pay wall. Certainly not what the author intended.

And, there is a test tool:

But there is no specification coverage tool like I wish to have it, it seems [Testing Tools for Python ].

What I would like to have is: