en getting started first test

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1. Your first PASS and FAIL

Learn how a test judges a result and where to look when it fails. This lesson does not introduce clocks or verification components.

A four-line test

Open examples/getting_started/test_accumulator.py and find test_addition near the beginning. The file already contains later lessons; focus on these lines:

def test_addition():
    expected = 8
    actual = 3 + 5
    assert actual == expected

def defines a function. pytest discovers functions whose names begin with test_. Use four spaces for indentation. = assigns a value; == compares values. assert requires a condition to hold and fails the test otherwise. Here 3 + 5 stands in for the computation under test; the next lesson reads the actual value from a DUT.

Run in the terminal:

python3 -m pytest -q examples/getting_started/test_accumulator.py::test_addition

Expect output similar to this, with a different duration:

.                                                                        [100%]
1 passed in ...

. means a passing test and -q requests concise output. ::test_addition selects just that function. Running the file with Python alone does not automatically call its test functions.

Make it fail deliberately

Change expected = 8 to expected = 9 and repeat the command:

E       assert 8 == 9
1 failed in ...

The actual result is 8, while the test expected 9. Check the specification and test data before blaming the DUT. Restore 8 and confirm that the test passes again.

Add a check

Append this function to the same file:

def test_eight_bit_wraparound():
    actual = (250 + 10) % 256
    assert actual == 4

% computes a remainder. An unsigned 8-bit value holds 0–255; retaining the low 8 bits of 260 gives 4. Run the new function:

python3 -m pytest -q examples/getting_started/test_accumulator.py::test_eight_bit_wraparound

Expect 1 passed. This still checks the specification without accessing a DUT. Keep working in the same file and continue with inputs and clocks.