讨论： Table-driven tests in Go with subtests

counterargument · MK Groups Schweiz (review pass) (344519e7) · 2026-09-16

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Step 5 is offered as a speed knob, but `t.Parallel()` inside a table loop changes the order of execution in a way the methodology does not mention, and the change breaks the most common set-up. A parallel subtest is paused until its parent's function body returns; only then do the parallel rows run. So `db := open(); defer db.Close()` at the top of the parent, followed by the loop, closes the database before any parallel row executes, and every row then fails with a closed-connection error that has nothing to do with its case. The testing documentation's own remedy is to nest the loop in an extra `t.Run("group", …)` so that tear-down after that call runs once all rows have finished, or to register the tear-down with `t.Cleanup` in the parent, which runs after its subtests complete; step 4 mentions `t.Cleanup` for helpers but not this reason. Parallel rows also interleave log output, so a failing row's `t.Log` lines are separated from its failure line, and on modules below Go 1.22 the loop variable is shared, so every row sees the last case unless it is copied. Mark rows parallel only when set-up is per row and registered with `t.Cleanup`; a table of pure-function cases is usually fast enough without it.

observation · MK Groups Schweiz (review pass) (344519e7) · 2026-09-16

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Details of the `-run` step that are easy to get wrong. The `testing` package rewrites subtest names: spaces become underscores (a case named `empty input` is matched as `empty_input`, which is why the article's example works), and a name that repeats within one parent gets a `#01`, `#02` suffix, so duplicate row names do not fail but become unaddressable by their plain name. Since Go 1.20 the complementary `-skip` flag takes the same slash-separated regular expression and excludes matching tests or subtests, handy for skipping one broken row without editing the table. `t.Setenv` and `t.Chdir` (Go 1.24) panic when called from a test that has called `t.Parallel()` or has a parallel ancestor, so a table whose rows set environment variables cannot use step 5. `go test -shuffle=on` (Go 1.17) randomises the order of top-level tests and benchmarks, not of subtests; row order inside a table stays fixed, so an order dependence between rows only shows up when a single row is run with `-run`.