This document describes a future that will be, not current status quo. Subplot is its initial, active development period, and we don’t want to update this document every time we make a change. Thus, be aware that parts of this document is speculative and may describe a version of Subplot that does not yet exist.


Subplot is a tool for acceptance testing of software. It helps describe acceptance criteria for all stakeholders in a software project, and also for computers. Such criteria may come from paying clients, users, the developers, their employers, or elsewhere.

Subplot is specifically meant for describing automated acceptance tests. It takes a two-pronged approach, where it lets developers write automated tests for all the acceptance criteria they have, and runs the tests. On the other hand, Subplot also produces a standalone document, which describes the automated tests all stakeholders, including non-technical ones, such as project managers and clients.

More concretely, Subplot helps developers specify the automated acceptance tests so that the tests can be executed, but also understood without requiring programming knowledge to understand.

Subplot is meant to be used to produce a document to facilitate communication between various shareholders of the software being developed.

Subplot’s overall working principle is that the tests are implemented and documented in a number of source files, which are processed by two Subplot tools. One tool produces a PDF or HTML document, for humans to read. The other produces a test program, which executes the acceptance tests.

A fairy tale of acceptance testing

The king was upset. This naturally meant the whole court was in a tizzy and chattering excitedly at each other, while trying to avoid the royal wrath.

“Who will rid me of this troublesome chore?” shouted the king, and quaffed a flagon of wine. “And no killing of priests, this time!”

The grand hall’s doors were thrown open. The grand wizard stood in the doorway, robe, hat, and staff everything, but quite still. After the court became silent, the wizard strode confidently to stand before the king.

“What ails you, my lord?”

The king looked upon the wizard, and took a deep breath. It does not do to shout at wizards, for they control dragons, and even kings are tasty morsels to the great beasts.

“I am tired of choosing what to wear every day. Can’t you do something?”

The wizard stoke his long, grey beard. He turned around, looked at the magnificent outfits worn by members of the court. He turned back, and looked at the king.

“I believe I can fix this. Just to be clear, your beef is with having to choose clothing, yes?”

“Yes”, said the king, “that’s what I said. When will you be done?”

The wizard raised his staff and brought it back down again, with a loud bang.

“Done” said the wizard, smugly.

The king was amazed and started smiling, until he noticed that everyone, including himself, was wearing identical burlap sacks and nothing on their feet. His voice was high, whiny, like that of a little child.

“Oh no, that’s not at all what I wanted! Change it back! Change it back now!”

The morale of this story is to be clear and precise in your acceptance criteria, and make sure everyone involved understands them the same way, or you might get something else than what you really, really wanted.

Motivation for Subplot

Keeping track of requirements and acceptance criteria is necessary for all but the simplest of software projects. Having all stakeholders in a project agree to them is crucial, as is that all agree how it is verified that the software meets the acceptance criteria. Subplot aims to provide a way for documenting the shared understanding of what the acceptance criteria are and how they can be checked automatically.

Stakeholders in any given project may include:

  • those who pay for the work to be done; this may be the employer of the developers for in-house projects (“customer”)
  • those who use the resulting systems, whether they pay for it or not (“user”)
  • those who install and configure the systems and keep them functional (“sysadmin”)
  • those who support the users (“helpdesk”)
  • those who develop the system in the first place (“developer”)

All stakeholders need to understand the acceptance criteria, and how the system is evaluated against them. In the simplest case, the customer and the developer need to both understand and agree so that the developer knows when the job is done, and the customer knows when they need to pay their bill.

However, even when the various stakeholder roles all fall upon the same person, or only on people who act as developers, the Subplot approach can be useful. A developer would understand acceptance criteria expressed only in code, but doing so may take time and energy that are not always available. The Subplot approach aims to encourage hiding unnecessary detail and documenting things in a way that is easy to understand with little effort.

Unfortunately, this does mean that for a Subplot output document to be good and helpful, writing it will require effort and skill. No tool can replace that.


This chapter lists requirements for Subplot itself. These requirements are not meant to be testable as such. For more specific, testable acceptance criteria, see the later chapter with acceptance tests for Subplot.

Each requirement here is given a unique mnemnoic id for easier reference in discussions.


Acceptance tests should be possible to express in a way that’s easily understood by non-programmers.


The markup language for writing documentation should be easy to write.


The formatted human-readable documentation should use good layout and typography to enhance comprension.


The code to implement the acceptance tests should not be embedded in the documentation source, but be in separate files. This makes it easier to edit without specialised tooling.


The developers implementing the acceptance tests should be free to use a language they’re familiar and comfortable with. Subplot should not require them to use a specific language.


Executing the acceptance tests should be fast.


The acceptance test tooling should assume the system under test is already deployed and available. Deploying is too big of a problem space to bring into the scope of acceptance testing, and there are already good tools for deployment.


The tests should produce a machine parseable result that can be archived, post-processed, and analyzed in ways that are of interest to the project using Subplot. For example, to see trends in how long tests take, how often tests fail, to find regressions, and to find tests that don’t provide value.

Subplot architecture

Subplot reads input files, and produces two outputs. On the one hand, it outputs a program that executes the tests specified in the input files. The person running the generated program gets a test report, with results of each of the test scenarios. On the other hand Subplot also outputs a human-readable document (as PDF or HTML), for communicating what is being tested.

Subplot is able to produce the test program in various languages, using a templating system to make it simple to add support for more languages. Subplot comes with support for Python and Rust. It’s the user’s choice which language they’re most comfortable with.

Acceptance tests are expressed to Subplot in the form of test scenarios, in which a sequence of actions are taken, and then the results are checked. If the checks fail, the scenario fails.

Subplot runs the scenarios concurrently (but see the USING keyword), within the constrains of hardware resources. If Subplot determines it doesn’t have all the resources to run all scenarios at once, it will run fewer, but randomly chosen scenarios concurrently, to more likely to detect unintentional dependencies between scenarios.

Note that Subplot or the scenarios it runs aren’t meant deploy the software, or start services to run in the background.

The Subplot input language

Subplot input consists of three types of files:

  • one or more markdown files which document the acceptance tests
  • a bindings file (in YAML) which binds scenario steps to their implementations
  • scenario step implementations, which are implemented in some programming language (e.g., Python or Rust); Subplot will combine this code with some scaffolding provided by Subplot itself

The input files for a simple acceptance test suite for Subplot would be divided into three files:, foo.yaml, and (assuming step implementation in Python).

Markdown files

The Subplot input language is markdown files using fenced code blocks with backticks. The code blocks must indicate that they contain Subplot language:

given a service
and I am Tomjon
when I access the service
then it's OK

Code blocks for the PlantUML, GraphViz dot markup, and a custom roadmap markup languages are also supported. These can be used for generating embedded images in the documented produced by Subplot.

Any unfenced code blocks (indented code blocks) are ignored by Subplot, as are fenced code blocks using unknown content types.

The Subplot code generator understands the full Markdown language, by ignoring everything except headings and its own code blocks. The Subplot document generator uses Pandoc to produce standalone files, and anything that Pandoc supports is OK to use.

Subplot treats multiple Markdown files as one, as if they had been concatenated with the cat(1) utility. Within the logical file, normal Markdown and Pandoc markup can be used to structure the document in any way that aids human understanding of the acceptance test suite, which the caveat that chapter or section headings are used by Subplot to group code blocks into scenarios.

All code blocks for the same scenario must be grouped under a single heading. Sub-headings are permitted within a scenario, but the next heading at the same or a higher level will end the scenario. This allows for scenarios to begin at any level, but not to leak into a wider scope within the acceptance document. For example, a scenario which starts after a level 2 heading may have subdivisions marked with level 3 or below headings, but will end at the next level 2 or level 1 heading.

Within the Subplot code blocks, Subplot understands a special language, inspired by Gherkin, as used by the Cucumber testing tool. The language understood by Subplot has the following general structure:

  • each logical line starts with a keyword at the beginning of the line
  • logical lines may be broken into physical lines, by starting the continuation lines with one or more space or TAB characters; the physical line break and white space characters are preserved
  • logical lines define steps in a test scenario
  • the meaning and implementation of the steps are defined by other Subplot input files
  • the keywords are: ASSUMING, USING, GIVEN, WHEN, THEN, and AND, with meanings defined below; keywords can be written in upper or lower case, or mixes, Subplot doesn’t care

The keywords have the following meanings:


A condition for the scenario. If an ASSUMING step fails, the scenario is skipped.

This is used for skipping test scenarios that require specific software to be installed in the test environment, or access to external services, but which can’t be required for all runs of the acceptance tests.


Indicate that the scenario uses a resource such as a database, that’s constrained and can’t be used by all scenarios if they run concurrently. When scenarios declare the resource, Subplot can limit which scenarios run concurrently.

For example, if several scenarios require uncontested use of the GPU, of which there is typically only one per machine, they can all declare “using the graphical processing unit”, and Subplot will run them one at a time.

(This is an intentionally simplistic way of controlling concurrency. The goal is to be simple and correct rather then achievee maximal concurrency.)

The actual management of resources belongs to the generated test program at runtime, not the Subplot compiler.


Set up the test environment for the action (WHEN). This might create files, start a background process, or something like that. This also sets up the reversal of the setup, so that any background processes are stopped automatically after the scenario has ended. The setup and cleanup must succeed, or the scenario will fail.

The cleanups are executed in the reverse order of the GIVENs, and they’re done always, whether the scenario succeeds or not.


Perform the action that is being tested. This must succeed. This might, for example, execute a command line program, and capture its output and exit code.


Test the results of the action. This would examine the output and exit code of the program run in a WHEN step, or examine current content of the database, or whatever is needed.


This keyword exists to make scenarios “read” better in English. The keyword indicates that this step should use the same keyword as the previous step, whatever that keyword is. For example, a step “THEN output is empty” might be followed by “AND the exit code is 0” rather than “THEN the exit code is 0”.

The order of steps is somewhat constrainted: first any ASSUMING steps, then any USING steps, at least one WHEN must come before a THEN.


FIXME: The binding specification needs thought. This is just a sketch.

Binding files match scenario steps to functions that implement them, using regular expressions. The bindings may also extract parts of the steps, and pass them onto the functions as parameters.

Binding files are YAML files, with lists of bindings, each binding being a dict. For example:

In the example above, the “I am” step extracts the name of the user from the step. It’s type is declared, and the value is saved for use by a later step.

The “I access” step expects the name to have been set by a previous step. Subplot will check that the name is set, and give an error if it isn’t, before any scenario runs. If name is set, it is given to the function to be called as a function argument.

The “I access” step further sets the variable “exit_code”, and the “it’s OK” step expects it to be set.

Step implementations

Continuing the example from the previous section, the following Python code might implement the functions:

With these bindings, Subplot produces a Python program, which calls these functions in order, and passes values between them via function arguments and return values. The generated program will handle running scenarios concurrently, and taking care of USING constraints, and other resource constraints.

Acceptance tests for Subplot

Shared files between scenarios

The scenarios will use the files under different names.

b.yaml—Bindings file—Python file with functions

The simplest possible scenario

This tests that Subplot can build a PDF and an HTML document, and execute a simple scenario successfully. The test is based on generating the test program from an input file, running the test program, and examining the log file.

given files, simple.yaml (from b.yaml), and (from

when I run docgen --pdf
then file simple.pdf exists

when I run docgen --html
then file simple.html exists

when I run codegen --run
then log file says scenario "Simple" was run
and log file says step "given precondition foo" was run
and log file says step "when I do bar" was run
and log file says step "then bar was done" was run
and program finished successfully—markdown file

A scenario step fails

This tests that Subplot can run handle a scenario step failing.

given files, fail.yaml (from b.yaml), and (from
when I run codegen --run

then log file says scenario "Fail" was run
and log file says step "given precondition foo" was run
and log file says step "then bar was done" failed
and program finished with an error—markdown file

Two scenarios in the same markdown file

This tests that Subplot can run two successful scenarios in the same Markdown file successfully.

FIXME: This and all other tests that run more than one scenario should handle scenarios being run in random order, and concurrently.

given files, two.yaml (from b.yaml), and (from
when I run codegen --run

then log file says scenario "First" was run
and log file says step "given precondition foo" was run
and log file says step "when I do bar" was run
and log file says step "then bar was done" was run

then log file says scenario "Second" was run
and log file says step "given precondition foo" was run
and log file says step "when I do bar" was run
and log file says step "then bar was done" was run

and test program finished successfully—markdown file

Two scenarios, one has a failing step

This tests that Subplot runs both scenarios, even if one has a failing step.

given files, onefails.yaml (from b.yaml), and (from
when I run codegen --run

then log file says scenario "First" was run
and log file says step "given precondition foo" was run
and log file says step "when I do bar" was run
and log file says step "then bar was done" was run

then log file says scenario "Second" was run
and log file says step "given precondition foo" was run
and log file says step "then bar was done" failed

and program finished with an error—markdown file

Two scenarios, both have a failing step

This tests that Subplot runs both scenarios, even when both have a failing step.

given files, twofail.yaml (from b.yaml), and (from
when I run codegen --run

then log file says scenario "First" was run
and log file says step "given precondition foo" was run
and log file says step "then bar was done" failed

then log file says scenario "Second" was run
and log file says step "given precondition foo" was run
and log file says step "then bar was done" failed

and program finished with an error—markdown file