Tests that mock the code they test

Tests and verification: one of the ways AI coding agents bloat software, from lessloc's pattern catalog. lessloc catches it as empty-test.

What makes tests that mock the code they test a bad practice?

The standard that calls it a bad practice: Carr, TDD anti-patterns: The Mockery.

What does lessloc do about tests that mock the code they test?

lessloc catches it as empty-test.

• empty-test (tier 1): A test that asserts nothing or only a constant, or one that mocks the code it tests.

Examples

Caught as empty-test:

from unittest.mock import patch


@patch("shop.pay.charge")
def test_charge(mock_charge):
    mock_charge.return_value = 1
    assert charge(5) == 1

Written well:

from unittest.mock import patch


@patch("shop.pay.send_receipt")
def test_charge(mock_send):
    assert charge(5) == 5
    mock_send.assert_called_once()

Caught as empty-test:

import { test, vi } from "vitest";
import { total } from "./cart";
vi.mock("./cart");
test("total", () => {
  expect(total([1])).toBe(1);
});

Written well:

import { test, vi } from "vitest";
import { total } from "./cart";
vi.mock("./prices");
test("total", () => {
  expect(total([1])).toBe(1);
});

Related patterns

The other patterns in Tests and verification:

• Behavior added with no test changed in the same change: lessloc gives every session the rule: new behavior gets one focused test.
• Tests that assert nothing, or assert a constant: lessloc catches it as empty-test.
• Deleted, skipped or force-passed tests: lessloc catches it as weakened-tests.
• Test-only branches in production code: lessloc catches it as test-logic.
• Production code that branches on a value from the tests: lessloc catches it as test-logic.
• Stopping short: a requested function never written: lessloc catches it as unaddressed: names a request asks to exist ("add a --status flag", "implement export_csv()") are looked for in the final repository; one that appears nowhere holds the turn.
• Stopping short: part of the problem left failing: lessloc catches it as unfinished: in a turn that changed code, the agent's last reply is read sentence by sentence, with headings and lead-ins speaking for what is under them.
• Saying "done" without a passing run on the final state: lessloc catches it as tests-failed and unverified: the agent's own test runs, as its host reported them, are the evidence for the exact tree; a run that fails on the final state, or a change to how the tests run, holds the turn once, and a final state no run verified is written to the record; lessloc test, verify and ci record evidence too.
• A refactor that changes existing tests: lessloc catches it as refactor-tests.