How to Write Tests That Actually Prevent Bugs

You can write a lot of tests and still miss real defects. The key is writing tests that target risky behavior, prove key invariants, and catch regressions before they ship.

September 15, 2025

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This week we are focusing on test quality over test quantity. You will learn how to aim tests at real failure modes, keep them fast and focused, and build a suite that blocks regressions without blocking progress.

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Three Testing Habits That Stop Bugs

Target risky code paths
Identify the modules with the most change, complexity, or customer impact. Write tests that exercise edge cases, not just the happy path.

Assert behaviors, not implementation details
Test outcomes and contracts. Avoid coupling tests to private helpers or exact log messages that change often.

Make failures loud and useful
Each test should fail with a clear message that tells you what broke. Use names that describe behavior, for example

test_cancels_pending_order_on_timeout

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A Quick Example

Here’s a test with narrow coverage:

def test_apply_discount_simple(): assert apply_discount(100, 10) == 90

Here’s how to code better tests, behavior and boundaries:

import math
from hypothesis import given, strategies as st

# Contract: price >= 0, 0 <= pct <= 100, result never negative
@given(
    price=st.floats(min_value=0, allow_infinity=False, allow_nan=False),
    pct=st.floats(min_value=0, max_value=100)
)
def test_apply_discount_contract(price, pct):
    out = apply_discount(price, pct)
    assert out >= 0
    assert out <= price
    # Idempotence for zero and hundred percent
    if pct == 0:
        assert out == price
    if pct == 100:
        assert math.isclose(out, 0.0, rel_tol=0, abs_tol=1e-9)

That’s it for today.

Thanks for being part of the Hackr.io community. Keep learning, keep shipping, and keep your test suite fast and fearless.

The Hackr.io Team

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