What are the Go equivalents to Python's `string.format` and how do they compare in flexibility and functionality?

Equivalent of Python string.format in Go?

In Python, you can use string.format to insert values into strings with placeholders like this:

"File {file} had error {error}".format(file=myfile, error=err)

Go has an equivalent function called Sprintf, which works in a similar way:

fmt.Sprintf("File %s had error %s", myfile, err)

However, Sprintf has a limitation: it requires the order of the parameters in the format string to match the order of the arguments. This can be a problem for internationalization (I18N), where the order of the placeholders may vary depending on the language.

Using text/template

Go provides a more flexible alternative to Sprintf called text/template. It allows for more control over the formatting, including the ability to specify the order of the placeholders:

package main

import (
    "bytes"
    "text/template"
    "os"
)

func main() {
    type Params struct {
        File string
        Error string
    }

    var msg bytes.Buffer

    params := &Params{
        File: "abc",
        Error: "def",
    }

    tmpl, _ := template.New("errmsg").Parse("File {{.File}} has error {{.Error}}")
    tmpl.Execute(&msg, params)
    msg.WriteTo(os.Stdout)
}

This template-based approach allows for greater flexibility and control over the formatting of the string.

With strings.Replacer

The strings.Replacer type can be used to implement a formatter that allows for placeholders with custom names:

package main

import (
    "fmt"
    "strings"
)

func main() {
    file, err := "/data/test.txt", "file not found"

    log("File {file} had error {error}", "{file}", file, "{error}", err)
}

func log(format string, args ...string) {
    r := strings.NewReplacer(args...)
    fmt.Println(r.Replace(format))
}

This approach is straightforward and easy to use.

Using explicit argument indices

Another option is to use explicit argument indices in the format string, allowing for placeholders to be reused multiple times:

fmt.Printf("File %[1]s had error %[1]s", myfile, err)

This approach is less versatile than the previous ones but can be useful for simple cases.

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