goScope errorMedium

Defer evaluation timing with variable assignment

Time: 0:00

BugFind the bug and fix the code

What it should do

Calculate and return execution status using deferred cleanup. Example: recordExecutionMetric(10) -> 20

What happens instead

Deferred call records old value 10 instead of updated 20

package main

import "fmt"

func recordExecutionMetric(initial int) int {
	res := initial
	// Arguments to deferred functions are evaluated immediately when defer is reached
	defer func(v int) {
		_ = v
	}(res)
	res += 10
	return res
}

func main() {
	fmt.Println(recordExecutionMetric(10))
}

Test a custom input to verify an edge case (e.g. empty lists, negative numbers, duplicates).

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This bug belongs to the scope and closure bugs pattern

Every callback reports the last value, a counter never increments, UnboundLocalError on a variable you can see.

Learn the scope and closure bugs pattern Step through your own code

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package main import "fmt" func recordExecutionMetric(initial int) int { res := initial // Arguments to deferred functions are evaluated immediately when defer is reached defer func(v int) { _ = v }(res) res += 10 return res } func main() { fmt.Println(recordExecutionMetric(10)) }

Defer evaluation timing with variable assignment

What it should do

What happens instead

More scope and closure bugs to practise

Defer evaluation timing with variable assignment

What it should do

What happens instead

More scope and closure bugs to practise