print readable variables with golang

How to print a map, struct or whatever in a readable way?

With PHP you can to this

echo '<pre>';
print_r($var);
echo '</pre>';

or

header('content-type: text/plain');
print_r($var);

Use the Go fmt package. For example,

package main

import "fmt"

func main() {
	variable := "var"
	fmt.Println(variable)
	fmt.Printf("%#v\n", variable)
	header := map[string]string{"content-type": "text/plain"}
	fmt.Println(header)
	fmt.Printf("%#v\n", header)
}

Output:

var
"var"
map[content-type:text/plain]
map[string]string{"content-type":"text/plain"}

> Package fmt
>
> import "fmt"
>
> Overview
>
> Package fmt implements formatted I/O with functions analogous to C's
> printf and scanf. The format 'verbs' are derived from C's but are
> simpler.

I think in many cases, using "%v" is concise enough:

fmt.Printf("%v", myVar)

From the fmt package's documentation page:

> %v the value in a default format.
> when printing structs, the plus flag (%+v) adds field names
>
> %#v a Go-syntax representation of the value

Here is an example:

package main

import "fmt"

func main() {
	// Define a struct, slice and map
	type Employee struct {
		id   int
		name string
		age  int
	}
	var eSlice []Employee
	var eMap map[int]Employee

	e1 := Employee{1, "Alex", 20}
	e2 := Employee{2, "Jack", 30}
	fmt.Printf("%v\n", e1)
	// output: {1 Alex 20}
	fmt.Printf("%+v\n", e1)
	// output: {id:1 name:Alex age:20}
	fmt.Printf("%#v\n", e1)
	// output: main.Employee{id:1, name:"Alex", age:20}

	eSlice = append(eSlice, e1, e2)
	fmt.Printf("%v\n", eSlice)
	// output: [{1 Alex 20} {2 Jack 30}]
	fmt.Printf("%#v\n", eSlice)
	// output: []main.Employee{main.Employee{id:1, name:"Alex", age:20}, main.Employee{id:2, name:"Jack", age:30}}

	eMap = make(map[int]Employee)
	eMap[1] = e1
	eMap[2] = e2
	fmt.Printf("%v\n", eMap)
	// output: map[1:{1 Alex 20} 2:{2 Jack 30}]
	fmt.Printf("%#v\n", eMap)
	// output: map[int]main.Employee{1:main.Employee{id:1, name:"Alex", age:20}, 2:main.Employee{id:2, name:"Jack", age:30}}
}

You can use fmt.Println() to print. You will need to import the "fmt" package (see the example below). Many data types can be printed out of the box. If you want to get a human-readable print for custom types, you'll need to define a String() string method for that type.

To try the following example, click here: http://play.golang.org/p/M6_KnRJ3Da

package main

import "fmt"

// No `String()` method
type UnstringablePerson struct {
	Age int
	Name string
}

// Has a `String()` method
type StringablePerson struct {
	Age int
	Name string
}

// Let's define a String() method for StringablePerson, so any instances
// of StringablePerson can be printed how we like
func (p *StringablePerson) String() string {
	return fmt.Sprintf("%s, age %d", p.Name, p.Age)
}

func main() {
    // Bobby's type is UnstringablePerson; there is no String() method
    // defined for this type, so his printout will not be very friendly
	bobby := &UnstringablePerson{
		Age: 10,
		Name: "Bobby",
	}

    // Ralph's type is StringablePerson; there *is* a String() method
    // defined for this type, so his printout *will* be very friendly
	ralph := &StringablePerson{
		Age: 12,
		Name: "Ralph",
	}
	fmt.Println(bobby) // prints: &{10 Bobby}
	fmt.Println(ralph) // prints: Ralph, age 12
}

https://github.com/davecgh/go-spew

> Go-spew implements a deep pretty printer for Go data structures to aid in debugging.

fmt.Printf("%v", whatever) 

In Go is like print_r(), var_dump(), var_export() in PHP. (The %v is the important part.)

Good Luck

For debugging I use this:

func printVars(w io.Writer, writePre bool, vars ...interface{}) {
	if writePre {
		io.WriteString(w, "<pre>\n")
	}
	for i, v := range vars {
		fmt.Fprintf(w, "» item %d type %T:\n", i, v)
		j, err := json.MarshalIndent(v, "", "    ")
		switch {
		case err != nil:
			fmt.Fprintf(w, "error: %v", err)
		case len(j) < 3: // {}, empty struct maybe or empty string, usually mean unexported struct fields
			w.Write([]byte(html.EscapeString(fmt.Sprintf("%+v", v))))
		default:
			w.Write(j)
		}
		w.Write([]byte("\n\n"))
	}
	if writePre {
		io.WriteString(w, "</pre>\n")
	}
}

<kbd>playground</kbd>