What does the '.' (dot or period) in a Go

问题:

In the Go tutorial, and most of the Go code I've looked at, packages are imported like this:

import (
    "fmt"
    "os"
    "launchpad.net/lpad"
    ...
)

But in http://bazaar.launchpad.net/~niemeyer/lpad/trunk/view/head:/session_test.go, the gocheck package is imported with a . (period):

import (
    "http"
    . "launchpad.net/gocheck"
    "launchpad.net/lpad"
    "os"    
)

What is the significance of the . (period)?

回答1:

It allows the identifiers in the imported package to be referred to in the local file block without a qualifier.

If an explicit period (.) appears instead of a name, all the package's exported identifiers will be declared in the current file's file block and can be accessed without a qualifier.

Assume we have compiled a package containing the package clause package math, which exports function Sin, and installed the compiled package in the file identified by "lib/math". This table illustrates how Sin may be accessed in files that import the package after the various types of import declaration.

Import declaration          Local name of Sin

import   "lib/math"         math.Sin
import M "lib/math"         M.Sin
import . "lib/math"         Sin

Ref: http://golang.org/doc/go_spec.html#Import_declarations

回答2:

Here's an analogy for those coming from Python:

• Go's import "os" is roughly equivalent to Python's import os
• Go's import . "os" is roughly equivalent to Python's from os import *

In both languages, using the latter is generally frowned upon but there can be good reasons for doing it.

回答3:

This should only be used in testing.

Here is some documentation in golang's wiki

If you've generated some mock code such as with mockgen and it imports your package code, and then your testing package also imports your package code, you get a circular dependency (Something golang chooses to let the user to decide how to resolve).

However, if inside your testing package you use dot notation on the package you're testing then they are treated as the same package and there is no circular dependency to be had!