Why Does Go\'s `fmt` Package Prefer `Error()` Over `String()` When Both Are Implemented?

Error() Priority Over String() in Go

In Go, the fmt package handles printing values based on their types. When an object implements both the Error() and String() methods, the Error() method takes precedence. This behavior stems from the implementation of fmt, which prioritizes error handling.

According to the fmt package documentation, if an operand implements the error interface, the Error() method is invoked to convert the object to a string before applying any formatting. This logic overrides the String() method, which is called by default for operands that implement it.

The following code demonstrates this behavior:

package main

import "fmt"

type Person struct {
    Name string
    Age  int
}

func (p *Person) String() string {
    return fmt.Sprintf("%v (%v years)", p.Name, p.Age)
}

func (p *Person) Error() string {
    return fmt.Sprintf("Failed")
}

func main() {
    a := &Person{"Arthur Dent", 42}
    z := &Person{"Zaphod Beeblebrox", 9001}
    fmt.Println(a, z)
}

Output:

Failed Failed

In this example, even though the Person type implements both String() and Error(), the fmt.Println function prints "Failed" for both persons because the Error() method takes precedence. This is because errors are considered more critical and require immediate attention.

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