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gofmt / go fmt — Interview Q&A

Labeled by level. Concise answers.

Junior

Q1. What does gofmt do? It rewrites Go source into the single canonical style — tab indentation, normalized spacing, grouped imports. Go has one official format and gofmt enforces it.

Q2. What's the difference between go fmt and gofmt? go fmt is a wrapper that runs gofmt -l -w on the packages you name (writes changes). gofmt is the underlying tool with flags like -l, -d, -w, -s.

Q3. How do you check formatting without changing files? gofmt -l . lists files that are not formatted; it prints nothing if all are clean. gofmt -d . shows a diff.

Q4. Can you configure gofmt's style? No — it has no style options by design. The lack of choices is the feature: no formatting debates.

Middle

Q5. What does gofmt -s add? Simplifications: removing redundant slice bounds (s[1:len(s)] → s[1:]), redundant composite-literal types ([]T{T{1}} → []T{{1}}), etc. Recommended baseline.

Q6. gofmt vs goimports? gofmt formats and groups imports but never adds or removes them. goimports does everything gofmt does plus adding missing and removing unused imports.

Q7. How should CI handle formatting? Check, don't rewrite: test -z "$(gofmt -l .)" (fail if anything is unformatted). Never run go fmt in CI — it writes and hides the violation.

Q8. What is gofumpt? A stricter superset of gofmt with extra opinionated rules; its output is still gofmt-valid. Teams adopt it for maximal consistency.

Senior

Q9. Why can a formatted file fail a formatting check in CI? gofmt's output is tied to the toolchain version. If CI runs a different Go version than the developer, the two gofmts can disagree. Fix: pin the toolchain (go directive + GOTOOLCHAIN).

Q10. How do you handle a formatter upgrade that reformats many files (e.g., Go 1.19 doc comments)? Do the gofmt -w ./... in a dedicated reformat commit isolated from logic, and add that commit to .git-blame-ignore-revs so blame stays clean.

Q11. Does gofmt type-check? No. It parses to an AST and re-prints, so it formats code that does not compile (as long as it parses). That is why format-on-save works mid-edit. Use gofmt -e to see all parse errors.

Q12. How do you keep generated code from failing the fmt check? Pass generator output through go/format.Source, or run gofmt -w on generated files as a post-step, so machine-written code is canonical.

Professional

Q13. What's the one formatting decision a team actually makes? Which formatter is the standard (gofmt, gofmt -s, or gofumpt) plus import handling — then enforce the identical tool and toolchain version across editor, pre-commit hook, and CI.

Q14. Why is gofmt valuable at the team level? It eliminates formatting bikeshedding, whitespace merge conflicts, and onboarding friction. The professional goal is to make inconsistent formatting impossible, not to format code by hand.

Common traps

  • Confusing gofmt (prints) with gofmt -w (writes).
  • Running go fmt in CI (rewrites instead of failing).
  • Expecting gofmt to add/remove imports (that's goimports).
  • Expecting gofmt to lint for bugs (that's vet/staticcheck).
  • Toolchain version skew causing spurious format failures.
  • Trying to configure gofmt's style.