Subplot is software to help capture and communicate acceptance criteria for software and systems, and how they are verified, in a way that’s understood by all project stakeholders.

Another way to express that is that Subplot is for automated testing of acceptance criteria.

A simplified explanation is that Subplot provides a way to codify an agreement of when a customer is required to pay for a project.

Acceptance testing differ from other integration testing by looking at the software only from the user’s point of view, not the developers’. Because of this, Subplot produces two things: a standalone document aimed at the user to describe the accepance tests, and a test suite for developers to use to validate that the software meets those acceptance tests.

The document is meant to be a vehicle of communication between the various stakeholders in a project, for specifying in detail what the acceptance criteria are. The document both explains the criteria, and how they’re verified, and makes it possible to do the verification automatically.

Subplot generates the document, and a test program to execute the tests. Running the generated test program produces a test report which can be used as evidence of passing the tests.



Here is a quick example of using Subplot. Let’s imagine you are in need of a new implementation of the echo(1) command line utility, and you want to commission the implementation. You need to specify acceptance tests for it. After negotations with the developers, and their project manager, and your own sales and test teams, you end up with the following requirements:

  • If echo is run without arguments, it should write a newline character, and exit with a zero exit code.
  • If echo is run with arguments, it should write each argument, separated by a space character, and then a newline character, and finally exit with a zero exit code.

These are specified for Subplot in a Markdown document. A minimal one for echo might look like this:

title: echo acceptance criteria

No arguments

Run `echo` without arguments.

when user runs echo without arguments
then exit code is 0
and standard output contains a newline
and standard error is empty

Hello, world

Run `echo` to produce the output "hello, world".

when user runs echo with arguments hello, world
then exit code is 0
and standard output contains "hello, world\n"
and standard error is empty

Acceptance tests in Subplot are expressed as scenarios, which roughly correspond to use cases and acceptance requirements. A scenario has an overall structure of given/when/then:

  • given some initial context (setup)
  • when some action is taken (the thing to test)
  • then the end result looks in a specific way (verify what happened)

These are embedded in markdown using fenced code blocks marked as containing scenario text. Subplot extracts these and generates a test program to execute them.

The scenario steps need to be formulated in a way that all stakeholders in the project understand. This is crucial. The scenario steps, and the Markdown document in general, is typically written by the software architect or analyst of the developing organization, in collaboration with the client, and approved by the client.

Subplot is not magic artificial intelligence. Each scenario step needs to be implemented by programmers so that computers also understand them. Each step corresponds to a function, written in a programming language such as Python, and a YAML file binds the step to the function, using a pattern to capture relevant parts of the step. The pattern can be simple or a full PCRE regular expression. A binding file might look like this:

- when: user runs echo without arguments
  function: run_echo_without_args

- when: user runs echo with arguments {args}
  function: run_echo_with_args

- then: exit code is {exit_code}
  function: exit_code_is_zero

- then: standard output contains a newline
  function: stdout_is_a_newline

- then: standard output contains "(?P<text>.*)"
  function: stdout_is_text

- then: standard error is empty
  function: stderr_is_empty

This means that for a step saying ‘then standard output contains “foo”’, the Python function stdout_is_text is called and given the string foo as an argument called text.

The developers on the project, in collaboration with the testing team, need to supply both the bindings and the implementation functions in whatever language the test suite is written in (Python in this example). Subplot produces the code that extracts the interesting parts of scenario steps, and calls your functions in the right order, plus any other scaffolding needed.

Installing Subplot

To install Debian for Debian unstable, add the following to your APT sources lists:

deb unstable-ci main

Then run the following commands as root:

apt update  
apt install subplot

Producing a document

To produce a PDF document of the echo requirements, for you to review, the following commands are needed (prefix the commands with “./” if running from a git checkout):

subplot docgen -o echo.html
subplot docgen -o echo.pdf is the markdown input file. Subplot will find echo.yaml (the bindings) and or (step implementations in Python or Bash) from files mentioned in the markdown file’s metadata section.

Running the tests

To generate the test program, and running it to produce the test report, the following commands are needed:

subplot codegen -o --run

The output of the last command is the test report:

srcdir /home/liw/pers/subplot/subplot
datadir /tmp/tmp.MeW2f5NS7n
scenario: No arguments
  step: when user runs echo without arguments
  step: then exit code is 0
  step: then standard output contains a newline
  step: then standard error is empty
scenario: Hello, world
  step: when user runs echo with arguments hello, world
  step: then exit code is 0
  step: then standard output contains "hello, world"
  step: then standard error is empty
OK, all scenarios finished successfully


It would great if you could give us, Lars and Daniel, feedback on Subplot and this tutorial.

Questions for before you go through the tutorial (if we are interviewing you):

  • What kinds of testing do you already do?
  • Who are your usual stakeholders?
  • What sorts of things would you like to test better?

Questions for after you go through the tutorial:

  • What do you think of Subplot as a concept?
  • What’s good about Subplot?
  • What’s less good about Subplot?
  • Did you have any problems following the tutorial?
  • Do you see yourself using Subplot in the future?
  • Do you see Subplot as filling a gap?