Test-only branches in production code; code shaped to pass a test

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

What makes test-only branches in production code a bad practice?

The standards that call it a bad practice: Meszaros: Test Logic in Production; Anthropic system cards: special-casing tests.

What does lessloc do about test-only branches in production code?

lessloc catches it as test-logic.

• test-logic (tier 1): Production code that checks whether tests are running, says it exists to make tests pass, or branches on a value that appears only in tests.

Examples

Caught as test-logic:

import os


def price(x):
    if "PYTEST_CURRENT_TEST" in os.environ:
        return 0
    return x

Written well:

import os


def price(x):
    return x * float(os.environ.get("RATE", "1"))

Caught as test-logic:

export function price(x) {
  // special case for the test fixture
  if (x === 42) return 0;
  return x * 2;
}

Written well:

export function price(x) {
  // Prices are stored in cents: 42 means $0.42
  return x * 2;
}

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.
• Tests that mock the code they test: lessloc catches it as empty-test.
• 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.