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download：Python操作三大主流数据库
在用Python做开发时，你不可避免的会与数据库打交道，这次，带你入门Python操作不同类型数据库的实用技术
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适合人群及技术储备要求
适合想从事Python开发的学生或后端开发者
课程所讲内容非常实用，你可应用于 Python 数据分析方向，Python 后台开发， Python web 方向
完全掌握课程内容，你将可以达到实际工作1-2年的水平
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技术储备要求：
－数据库基础知识－Python基础知识
代码如下：
package main
import (
"fmt"
"math/rand"
"time"
)
var (
Web = fakeSearch("web")
Image = fakeSearch("image")
Video = fakeSearch("video")
)
type Result string
type Search func(query string) Result
func fakeSearch(kind string) Search {
return func(query string) Result {
time.Sleep(time.Duration(rand.Intn(100)) * time.Millisecond)
return Result(fmt.Sprintf("%s result for %q\n", kind, query))
}
}
func Google(query string) (results []Result) {
results = append(results, Web(query))
results = append(results, Image(query))
results = append(results, Video(query))
return
}
func main() {
rand.Seed(time.Now().UnixNano())
start := time.Now()
results := Google("golang")
elapsed := time.Since(start)
fmt.Println(results)
fmt.Println(elapsed)
}
运转结果如下:
[web result for "golang"
image result for "golang"
video result for "golang"
]
153.365484ms
## 谷歌搜索2.0
同时运转网页、图像和视频搜索，并等候一切结果。没有锁，没有条件变量，没有回调。
代码如下，关注Google函数。
package main
import (
"fmt"
"math/rand"
"time"
)
var (
Web = fakeSearch("web")
Image = fakeSearch("image")
Video = fakeSearch("video")
)
type Result string
type Search func(query string) Result
func fakeSearch(kind string) Search {
return func(query string) Result {
time.Sleep(time.Duration(rand.Intn(100)) * time.Millisecond)
return Result(fmt.Sprintf("%s result for %q\n", kind, query))
}
}
func Google(query string) (results []Result) {
c := make(chan Result)
go func() { c <- Web(query) } ()
go func() { c <- Image(query) } ()
go func() { c <- Video(query) } ()
for i := 0; i < 3; i++ {
result := <-c
results = append(results, result)
}
return
}
func main() {
rand.Seed(time.Now().UnixNano())
start := time.Now()
results := Google("golang")
elapsed := time.Since(start)
fmt.Println(results)
fmt.Println(elapsed)
}
## 谷歌搜索2.1 不要等候迟缓的效劳器。没有锁，无条件变量，没有回调。经过select的超时完成，需求把time.After定义的超时通道放在for循环外层。
package main
import (
"fmt"
"math/rand"
"time"
)
var (
Web = fakeSearch("web")
Image = fakeSearch("image")
Video = fakeSearch("video")
)
type Result string
type Search func(query string) Result
func fakeSearch(kind string) Search {
return func(query string) Result {
time.Sleep(time.Duration(rand.Intn(100)) * time.Millisecond)
return Result(fmt.Sprintf("%s result for %q\n", kind, query))
}
}
func Google(query string) (results []Result) {
c := make(chan Result)
go func() { c <- Web(query) } ()
go func() { c <- Image(query) } ()
go func() { c <- Video(query) } ()
timeout := time.After(80 * time.Millisecond)
for i := 0; i < 3; i++ {
select {
case result := <-c:
results = append(results, result)
case <-timeout:
fmt.Println("timed out")
return
}
}
return
}
func main() {
rand.Seed(time.Now().UnixNano())
start := time.Now()
results := Google("golang")
elapsed := time.Since(start)
fmt.Println(results)
fmt.Println(elapsed)
}
## 谷歌搜索3.0 内容从48页到51页。
运用复制的搜索效劳器减少尾部延迟。同样没有锁，没有条件变量，没有回调。
问：我们如何防止由于效劳器运转迟缓而丢弃结果？
答: 复制效劳器。 向多个副本发送恳求，并运用第一个响应。
代码如下：
package main
import (
"fmt"
"math/rand"
"time"
)
var (
Web1 = fakeSearch("web")
Web2 = fakeSearch("web")
Image1 = fakeSearch("image")
Image2 = fakeSearch("image")
Video1 = fakeSearch("video")
Video2 = fakeSearch("video")
)
type Result string
type Search func(query string) Result
func fakeSearch(kind string) Search {
return func(query string) Result {
time.Sleep(time.Duration(rand.Intn(100)) * time.Millisecond)
return Result(fmt.Sprintf("%s result for %q\n", kind, query))
}
}
func Google(query string) (results []Result) {
c := make(chan Result)
go func() { c <- First(query, Web1, Web2) } ()
go func() { c <- First(query, Image1, Image2) } ()
go func() { c <- First(query, Video1, Video2) } ()
timeout := time.After(80 * time.Millisecond)
for i := 0; i < 3; i++ {
select {
case result := <-c:
results = append(results, result)
case <-timeout:
fmt.Println("timed out")
return
}
}
return
}
func First(query string, replicas ...Search) Result {
c := make(chan Result)
searchReplica := func(i int) { c <- replicas[i](query) }
for i := range replicas {
go searchReplica(i)
}
return <-c
}
func main() {
rand.Seed(time.Now().UnixNano())
start := time.Now()
results := Google("golang")
elapsed := time.Since(start)
fmt.Println(results)
fmt.Println(elapsed)
}
执行结果如下：
[image result for "golang"
web result for "golang"
video result for "golang"
]
53.605273ms
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