Variable shadowing

Canonical guidance

• keep scopes narrow and names distinct
• be cautious with := when an outer variable already exists
• prefer = when reusing an existing binding is intended

Use when

• error-heavy control flow
• nested if blocks
• loops with temporary variables

Avoid

• shadowing err and then reading the outer err later
• reusing short names across distant nested scopes
• large functions where scope boundaries are hard to see

Preferred pattern

f, err := os.Open(name)
if err != nil {
	return err
}
defer f.Close()

_, err = io.Copy(dst, f)
if err != nil {
	return err
}

Anti-pattern

• mixing outer and inner err variables so later checks inspect the wrong one

Explanation: This anti-pattern is tempting because short declarations are convenient, but one extra := can quietly change which variable later code sees.

Why

• shadowing creates bugs that still compile and often still look reasonable in review