What does an underscore in front of an import statement mean?

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.

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")

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) }

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.

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.