Master the skills of using Golang packages

In-depth understanding of how to use Golang packages requires specific code examples

In the Go language, a package is the basic unit for organizing and managing code. A package can contain multiple Go source files (with .go extension), which together form a functional unit. The use of packages allows us to better organize code, improve code reusability, and facilitate team collaboration. This article will introduce how to deeply understand the use of Golang packages and provide specific code examples.

1. Import of package

In Go language, if you want to use code in another package, you must import this package first. Packages can be imported through the import keyword. You can import standard library packages or custom packages. For example:

import (
    "fmt"
    "log"
    "github.com/example/mypackage"
)

When importing a package, you can also give the imported package an alias. For example:

import (
    "fmt"
    mylog "log"
    mypkg "github.com/example/mypackage"
)

Using aliases can avoid package name conflicts and better express the intent of the code.

2. Visibility of packages

In the Go language, identifiers (variables, functions, types, etc.) in a package can be exported (visible) or not Exported. If the identifier starts with an uppercase letter, then it is exported and can be used by other packages; if it starts with a lowercase letter, then it is unexported and can only be used inside the current package.

For example, suppose we have a package named mypackage, which defines a function and a variable:

package mypackage

func Add(a, b int) int {
    return a + b
}

var Name string = "mypackage"

Use this package in another package:

package main

import (
    "fmt"
    "github.com/example/mypackage"
)

func main() {
    sum := mypackage.Add(2, 3)
    fmt.Println(sum) // 输出：5

    fmt.Println(mypackage.Name) // 编译错误，Name是未导出的
}

3. Package initialization

In the Go language, the initialization of the package will be performed automatically when the program is executed. The initialization operation of a package will only be performed once, no matter how many times the package is imported. Packages are initialized in dependency order. For example, if package A depends on package B, then package B will be initialized before package A is initialized.

The initialization operation of a package mainly includes:

• Initialize package-level variables
• Execute package-level init function

Call The order is as follows:

1. First initialize the package-level variables and initialize them in the order of declaration.
2. Then execute the package-level init function in the order of declaration.

package mypackage

import "fmt"

var Var1 = 10
var Var2 = calculateVar2()

func calculateVar2() int {
    return Var1 * 2
}

func init() {
    fmt.Println("mypackage initialization")
    fmt.Println(Var1, Var2)
}

func Add(a, b int) int {
    return a + b
}

Use this package in the main program:

package main

import (
    "github.com/example/mypackage"
)

func main() {
    sum := mypackage.Add(2, 3)
    println(sum) // 输出：5
}

Run this program and you will get the following output:

mypackage initialization
10 20
5

4. Test of the package

In the Go language, we can write test code for each package, and the test code can be placed in the same package as the normal code file. The file name of the test code must end with _test.go so that the Go language testing tool can recognize it.

The name of the test function must start with Test and receive a *testing.T type parameter. For example:

package mypackage

import (
    "testing"
)

func TestAdd(t *testing.T) {
    sum := Add(2, 3)
    if sum != 5 {
        t.Errorf("Add(2, 3) = %d; want 5", sum)
    }
}

Execute the go test command in the root directory of the package to run all tests of the package:

$ go test

If the test passes, it will output:

PASS
ok      mypackage   0.003s

If If a test fails, failure information will be output.

In summary, to deeply understand the use of Golang packages, you need to pay attention to the following points:

• You can give the package an alias when importing it to avoid package name conflicts.
• The visibility rules of the package should be clear, and exported identifiers can be used by other packages.
• The initialization of packages is performed in dependency order.
• You can write test code for each package and run the test through the go test command.

I hope that through the introduction of this article, readers will have a deeper understanding of how to use Golang packages and be able to use them flexibly in actual development.

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