What does an underscore in front of an import statement mean?
GoImportGo Problem Overview
I saw this example from sqlite3 on GitHub :
import (
"database/sql"
"fmt"
_ "github.com/mattn/go-sqlite3"
"log"
"os"
)
and cannot seem to find what the underscore in front of an import statement means.
Go Solutions
Solution 1 - Go
Short answer:
It's for importing a package solely for its side-effects.
From the Go Specification:
> To import a package solely for its side-effects (initialization), use the blank identifier as explicit package name: > > import _ "lib/math"
In sqlite3
In the case of go-sqlite3, the underscore import is used for the side-effect of registering the sqlite3 driver as a database driver in the init() function, without importing any other functions:
sql.Register("sqlite3", &SQLiteDriver{})
Once it's registered in this way, sqlite3 can be used with the standard library's sql interface in your code like in the example:
db, err := sql.Open("sqlite3", "./foo.db")
Solution 2 - Go
While other answers described it completely, for "Show me The Code" people, this basically means: create package-level variables and execute the init function of that package.
And (if any) the hierarchy of package-level variables & init functions of packages that, this package has imported.
The only side effect that a package can make, without being actually called, is by creating package-level variables (public or private) and inside it's init function.
Note: There is a trick to run a function before even init function. We can use package-level variables for this by initializing them using that function.
func theVeryFirstFunction() int {
log.Println("theVeryFirstFunction")
return 6
}
var (
Num = theVeryFirstFunction()
)
func init() { log.Println("init", Num) }
Solution 3 - Go
https://golang.org/doc/effective_go.html#blank
It's either a work in progress, or imported for side effects. In this case, I believe it's for the side effects, as described in the doc.
Solution 4 - Go
Let's say you have an Animal package. And your main file wants to use that Animal package to call a method called Speak but there are many different types of animals and each animal implemented their own common Talk method. So let's say you want to call a method Speak implemented in the Animal's package which internally calls Talk method implemented in each of the animal's package. So in this case you just want to do an import _ "dog" which will actually call the init method defined inside the dog package which actually registers a Talk method with the Animal package which it too imports.