Handling errors in Go - Step-by-Step Execution

Concept Flow - Handling errors

Start function

↓

Try operation

↓

Error returned?

No→Use result

Yes↓

Handle error

↓

End function

The program tries an operation, checks if an error occurred, handles it if yes, otherwise uses the result.

Execution Sample

Go

package main
import (
 "errors"
 "fmt"
)

func divide(a, b int) (int, error) {
 if b == 0 {
  return 0, errors.New("cannot divide by zero")
 }
 return a / b, nil
}

func main() {
 result, err := divide(10, 0)
 if err != nil {
  fmt.Println("Error:", err)
 } else {
  fmt.Println("Result:", result)
 }
}

This code tries to divide 10 by 0, checks for error, and prints the error message.

Execution Table

Step	Action	Condition	Result	Output
1	Call divide(10, 0)	b == 0?	True	Return error "cannot divide by zero"
2	In main, check err != nil	err != nil?	True	Print "Error: cannot divide by zero"
3	End program	-	-	-

💡 Error detected in divide, handled in main, program ends after printing error.

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
a	10	10	10	10
b	0	0	0	0
result	undefined	0	0	0
err	undefined	error("cannot divide by zero")	error("cannot divide by zero")	error("cannot divide by zero")

Key Moments - 2 Insights

Why do we check if err != nil after calling divide?

Because Go functions return errors as values, we must check if err is not nil to know if an error happened, as shown in execution_table step 2.

What happens if we try to use result without checking err?

If err is not nil, result may be invalid or zero. Checking err prevents using wrong data, as seen in step 2 where error is handled instead of using result.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of err after step 1?

A error("cannot divide by zero")

B nil

C 0

D undefined

💡 Hint

Check the 'Result' column in step 1 of the execution_table.

At which step does the program print the error message?

A Step 1

B Step 2

C Step 3

D No print occurs

💡 Hint

Look at the 'Output' column in the execution_table for step 2.

If divide was called with b=2 instead of 0, what would happen in step 1?

A Return result 0 and error nil

B Return error "cannot divide by zero"

C Return result 5 and error nil

D Panic and stop program

💡 Hint

Recall that divide returns error only if b == 0, otherwise returns a/b and nil error.

Concept Snapshot

Go error handling:
- Functions return (value, error)
- Check if error != nil after call
- Handle error before using value
- Use errors.New() to create errors
- Prevent program crashes by checking errors

Full Transcript

This example shows how Go handles errors by returning an error value from functions. The divide function returns an error if dividing by zero. The main function checks if the error is not nil before using the result. If there is an error, it prints the error message. This prevents using invalid data and keeps the program safe. The execution table traces each step: calling divide, checking error, printing error, and ending. Variables track the values of inputs, result, and error through the steps. Key moments explain why checking error is important and what happens if skipped. The quiz tests understanding of error values and program flow. This method is simple and clear for beginners to handle errors safely in Go.

Practice

1. In Go, how do you check if a function returned an error?

easy

A. By checking if the error variable is zero

B. By checking if the error variable is an empty string

C. By checking if the error variable equals false

D. By checking if the error variable is not nil

Solution

1. Step 1: Understand error type in Go

   Errors in Go are values, and when a function returns an error, it returns a value of type error or nil if no error occurred.

2. Step 2: Check error value

   To know if an error happened, you check if the error variable is not nil, meaning an error value exists.

3. Final Answer:

   By checking if the error variable is not nil -> Option D

4. Quick Check:

   error != nil = true means error occurred [OK]

Hint: Always check if error != nil to detect errors [OK]

Common Mistakes:

• Checking error == 0 instead of error != nil
• Assuming error is a boolean
• Comparing error to empty string

2. Which of the following is the correct way to declare and check an error returned by a function doTask() in Go?

easy

A. error := doTask()
if error != false {
// handle error
}

B. err := doTask()
if err != nil {
// handle error
}

C. err := doTask()
if err == 0 {
// handle error
}

D. var err doTask()
if err == nil {
// handle error
}

Solution

1. Step 1: Declare error variable correctly

   Use short variable declaration with := to assign the error returned by doTask() to err.

2. Step 2: Check if error is not nil

   Use if err != nil to check if an error occurred and handle it inside the block.

3. Final Answer:

   err := doTask() if err != nil { // handle error } -> Option B

4. Quick Check:

   Use := and err != nil for error handling [OK]

Hint: Use err := func() and check err != nil [OK]

Common Mistakes:

• Using wrong variable declaration syntax
• Checking error against 0 or false
• Using reserved word 'error' as variable name

3. What will be the output of this Go code?

package main
import (
    "errors"
    "fmt"
)
func divide(a, b int) (int, error) {
    if b == 0 {
        return 0, errors.New("division by zero")
    }
    return a / b, nil
}
func main() {
    result, err := divide(10, 0)
    if err != nil {
        fmt.Println(err)
    } else {
        fmt.Println(result)
    }
}

medium

A. division by zero

B. 0

C. 10

D. runtime error

Solution

1. Step 1: Analyze divide function behavior

   If b is zero, divide returns 0 and an error with message "division by zero".

2. Step 2: Check main function output

   Since divide(10, 0) returns an error, err != nil is true, so fmt.Println(err) prints the error message.

3. Final Answer:

   division by zero -> Option A

4. Quick Check:

   Error message prints when err != nil [OK]

Hint: If error returned, print error message, not result [OK]

Common Mistakes:

• Assuming result prints even if error exists
• Ignoring error check and printing zero
• Expecting runtime panic instead of error

4. Identify the error in this Go code snippet:

package main
import (
    "fmt"
    "os"
)
func readFile() error {
    file, err := os.Open("data.txt")
    if err != nil {
        return err
    }
    defer file.Close()
    // read file contents
    return nil
}

func main() {
    err := readFile()
    if err != nil {
        fmt.Println("File read successfully")
    }
}

medium

A. The error check in main should be err == nil, not err != nil

B. The defer statement is misplaced and should be before error check

C. The function readFile should return a string, not error

D. os.Open does not return an error

Solution

1. Step 1: Examine the error handling in main

   The main function calls readFile() and prints "File read successfully" if err != nil.

2. Step 2: Identify the logic error

   err != nil means an error occurred, so printing success here is incorrect. Success should be printed only if err == nil (no error).

3. Final Answer:

   The error check in main should be err == nil, not err != nil -> Option A

4. Quick Check:

   Print success only if err == nil [OK]

Hint: Print success only when err == nil, not err != nil [OK]

Common Mistakes:

• Confusing err == nil and err != nil conditions
• Moving defer before error check incorrectly
• Expecting os.Open to never return error

5. You want to write a Go function that tries to open a file and returns a custom error message if it fails. Which code snippet correctly wraps the original error with a custom message using Go's standard library?

hard

A. file, err := os.Open("config.json")
if err != nil {
panic(err)
}
return file.Close()

B. file, err := os.Open("config.json")
if err != nil {
return errors.New("failed to open config")
}
return file.Close()

C. file, err := os.Open("config.json")
if err != nil {
return fmt.Errorf("failed to open config: %w", err)
}
return file.Close()

D. file, err := os.Open("config.json")
if err != nil {
return err.Error()
}
return file.Close()

Solution

1. Step 1: Understand error wrapping in Go

   Go 1.13+ supports wrapping errors with fmt.Errorf and %w verb to keep original error context.

2. Step 2: Check each option for correct wrapping

   file, err := os.Open("config.json") if err != nil { return fmt.Errorf("failed to open config: %w", err) } return file.Close() uses fmt.Errorf with %w to wrap original error, preserving it. file, err := os.Open("config.json") if err != nil { return errors.New("failed to open config") } return file.Close() creates new error losing original error. file, err := os.Open("config.json") if err != nil { panic(err) } return file.Close() panics instead of returning error. file, err := os.Open("config.json") if err != nil { return err.Error() } return file.Close() returns string instead of error type.

3. Final Answer:

   file, err := os.Open("config.json") if err != nil { return fmt.Errorf("failed to open config: %w", err) } return file.Close() -> Option C

4. Quick Check:

   Use fmt.Errorf with %w to wrap errors [OK]

Hint: Use fmt.Errorf("%w") to wrap errors with context [OK]

Common Mistakes:

• Replacing original error instead of wrapping
• Using panic instead of returning error
• Returning error message string instead of error type