TDD with GitHub Copilot

by Paul Sobocinski

Will the arrival of AI coding assistants similar to GitHub Copilot imply that we gained’t want exams? Will TDD change into out of date? To reply this, let’s study two methods TDD helps software program improvement: offering good suggestions, and a way to “divide and conquer” when fixing issues.

TDD for good suggestions

Good suggestions is quick and correct. In each regards, nothing beats beginning with a well-written unit take a look at. Not guide testing, not documentation, not code overview, and sure, not even Generative AI. Actually, LLMs present irrelevant data and even hallucinate. TDD is very wanted when utilizing AI coding assistants. For a similar causes we want quick and correct suggestions on the code we write, we want quick and correct suggestions on the code our AI coding assistant writes.

TDD to divide-and-conquer issues

Drawback-solving by way of divide-and-conquer signifies that smaller issues will be solved ahead of bigger ones. This permits Steady Integration, Trunk-Based mostly Growth, and in the end Steady Supply. However do we actually want all this if AI assistants do the coding for us?

Sure. LLMs not often present the precise performance we want after a single immediate. So iterative improvement will not be going away but. Additionally, LLMs seem to “elicit reasoning” (see linked examine) once they remedy issues incrementally by way of chain-of-thought prompting. LLM-based AI coding assistants carry out greatest once they divide-and-conquer issues, and TDD is how we try this for software program improvement.

TDD suggestions for GitHub Copilot

At Thoughtworks, now we have been utilizing GitHub Copilot with TDD because the begin of the 12 months. Our purpose has been to experiment with, consider, and evolve a collection of efficient practices round use of the software.

0. Getting began

TDD represented as a three-part wheel with 'Getting Started' highlighted in the center

Beginning with a clean take a look at file doesn’t imply beginning with a clean context. We regularly begin from a person story with some tough notes. We additionally speak via a place to begin with our pairing accomplice.

That is all context that Copilot doesn’t “see” till we put it in an open file (e.g. the highest of our take a look at file). Copilot can work with typos, point-form, poor grammar — you identify it. However it could actually’t work with a clean file.

Some examples of beginning context which have labored for us:

  • ASCII artwork mockup
  • Acceptance Standards
  • Guiding Assumptions similar to:
    • “No GUI wanted”
    • “Use Object Oriented Programming” (vs. Practical Programming)

Copilot makes use of open recordsdata for context, so conserving each the take a look at and the implementation file open (e.g. side-by-side) vastly improves Copilot’s code completion potential.

1. Pink

TDD represented as a three-part wheel with the 'Red' portion highlighted on the top left third

We start by writing a descriptive take a look at instance identify. The extra descriptive the identify, the higher the efficiency of Copilot’s code completion.

We discover {that a} Given-When-Then construction helps in 3 ways. First, it reminds us to offer enterprise context. Second, it permits for Copilot to offer wealthy and expressive naming suggestions for take a look at examples. Third, it reveals Copilot’s “understanding” of the issue from the top-of-file context (described within the prior part).

For instance, if we’re engaged on backend code, and Copilot is code-completing our take a look at instance identify to be, “given the person… clicks the purchase button, this tells us that we should always replace the top-of-file context to specify, “assume no GUI” or, “this take a look at suite interfaces with the API endpoints of a Python Flask app”.

Extra “gotchas” to be careful for:

  • Copilot could code-complete a number of exams at a time. These exams are sometimes ineffective (we delete them).
  • As we add extra exams, Copilot will code-complete a number of strains as a substitute of 1 line at-a-time. It’ll typically infer the proper “prepare” and “act” steps from the take a look at names.
    • Right here’s the gotcha: it infers the proper “assert” step much less typically, so we’re particularly cautious right here that the brand new take a look at is appropriately failing earlier than transferring onto the “inexperienced” step.

2. Inexperienced

TDD represented as a three-part wheel with the 'Green' portion highlighted on the top right third

Now we’re prepared for Copilot to assist with the implementation. An already present, expressive and readable take a look at suite maximizes Copilot’s potential at this step.

Having stated that, Copilot typically fails to take “child steps”. For instance, when including a brand new technique, the “child step” means returning a hard-coded worth that passes the take a look at. So far, we haven’t been in a position to coax Copilot to take this strategy.

Backfilling exams

As a substitute of taking “child steps”, Copilot jumps forward and gives performance that, whereas typically related, will not be but examined. As a workaround, we “backfill” the lacking exams. Whereas this diverges from the usual TDD circulate, now we have but to see any severe points with our workaround.

Delete and regenerate

For implementation code that wants updating, the best technique to contain Copilot is to delete the implementation and have it regenerate the code from scratch. If this fails, deleting the strategy contents and writing out the step-by-step strategy utilizing code feedback could assist. Failing that, the easiest way ahead could also be to easily flip off Copilot momentarily and code out the answer manually.

3. Refactor

TDD represented as a three-part wheel with the 'Refactor' portion highlighted on the bottom third

Refactoring in TDD means making incremental modifications that enhance the maintainability and extensibility of the codebase, all carried out whereas preserving conduct (and a working codebase).

For this, we’ve discovered Copilot’s potential restricted. Think about two situations:

  1. “I do know the refactor transfer I need to strive”: IDE refactor shortcuts and options similar to multi-cursor choose get us the place we need to go sooner than Copilot.
  2. “I don’t know which refactor transfer to take”: Copilot code completion can’t information us via a refactor. Nevertheless, Copilot Chat could make code enchancment recommendations proper within the IDE. We have now began exploring that characteristic, and see the promise for making helpful recommendations in a small, localized scope. However now we have not had a lot success but for larger-scale refactoring recommendations (i.e. past a single technique/perform).

Typically we all know the refactor transfer however we don’t know the syntax wanted to hold it out. For instance, making a take a look at mock that may permit us to inject a dependency. For these conditions, Copilot might help present an in-line reply when prompted by way of a code remark. This protects us from context-switching to documentation or net search.


The frequent saying, “rubbish in, rubbish out” applies to each Knowledge Engineering in addition to Generative AI and LLMs. Said otherwise: greater high quality inputs permit for the potential of LLMs to be higher leveraged. In our case, TDD maintains a excessive degree of code high quality. This top quality enter results in higher Copilot efficiency than is in any other case attainable.

We subsequently advocate utilizing Copilot with TDD, and we hope that you simply discover the above suggestions useful for doing so.

Due to the “Ensembling with Copilot” workforce began at Thoughtworks Canada; they’re the first supply of the findings lined on this memo: Om, Vivian, Nenad, Rishi, Zack, Eren, Janice, Yada, Geet, and Matthew.