A unit test runs a small piece of behaviour with known input and checks an expected result automatically.
A unit test runs a small piece of behaviour with known input and checks an expected result automatically. A useful test fails when the implementation is wrong and covers both normal cases and important boundaries rather than simply executing the code.
Learning goal: explain why Write a Small Unit Test behaves this way, apply it to a small example, and verify the result independently. Begin by being able to justify this first step: Choose one behaviour of a small function and define the expected result for a controlled input.
Treat this as a sequence of observable decisions rather than one opaque command. Stage 1: Choose one behaviour of a small function and define the expected result for a controlled input. Stage 2: Arrange the input/state, call the function, then assert the observable result. Stage 3: Add at least one boundary or invalid case that could fail for a different reason. Final checkpoint: Run the test after a deliberate small code change to confirm that it can actually detect a regression.
Choose one behaviour of a small function and define the expected result for a controlled input.
Arrange the input/state, call the function, then assert the observable result.
Add at least one boundary or invalid case that could fail for a different reason.
Choose one behaviour of a small function and define the expected result for a controlled input. This is an input-preparation stage for Write a Small Unit Test. Verify the relevant type, shape, units, keys, missingness or assumptions before later steps depend on them.
# Step 1 — Define the reusable `add_tax` function; its indented body describes what happens for each call.
def add_tax(amount, rate):
# Step 2 — Return the computed value to the caller so the result can be reused or tested.
return amount * (1 + rate)
# Step 3 — Define the reusable `test_add_tax` function; its indented body describes what happens for each call.
def test_add_tax():
# Step 4 — Assert an assumption that must hold here; failure exposes an invalid state early.
assert add_tax(100, 0.10) == 110
# Step 5 — Call the function with these arguments and use its returned value or side effect in the workflow.
test_add_tax()
# Step 6 — Display the current value explicitly so the result/state can be inspected during execution.
print("passed")The assertion is silent when correct; the final message appears only because the expected result matched.
For Write a Small Unit Test, trace the specific input through the mechanism above and independently verify one returned value, state change or side effect.
InputObjects/values supplied to the operation.StateNames or mutable objects that may change during execution.OutputReturned value, side effect, file, plot or exception to inspect.Use Write a Small Unit Test when it answers a defined question in Errors, Debugging & Testing and its inputs/assumptions match the current data or program state.
Reconsider Write a Small Unit Test when the required information is unavailable, the operation would violate a validation/data boundary, or a simpler operation answers the question more transparently.
Construct a tiny example of Write a Small Unit Test. First choose one behaviour of a small function and define the expected result for a controlled input. Then arrange the input/state, call the function, then assert the observable result. Predict the result before execution and explain one boundary or failure case.
Which approach best demonstrates understanding of Write a Small Unit Test?
Step 1Choose one behaviour of a small function and define the expected result for a controlled input.Step 2Arrange the input/state, call the function, then assert the observable result.Step 3Add at least one boundary or invalid case that could fail for a different reason.Step 4Keep the test deterministic and independent of unrelated files, clocks or network state when possible.