import asyncio  
import httpx  
  
client = httpx.AsyncClient()  
  
  
async def run():  
    r = await client.get("http://127.0.0.1:8000", timeout=3)  
    print(r.text)  
  
  
async def main():  
    async with asyncio.timeout(1):  
        async with asyncio.TaskGroup() as tg:  
            for i in range(2):  
                tg.create_task(run())  
        print("all tasks have completed now.")  
  
  
asyncio.run(main())

Traceback (most recent call last):
  File "C:\Users\jack\PycharmProjects\demo\main.py", line 14, in main
    async with asyncio.TaskGroup() as tg:
  File "C:\Users\jack\.pyenv\pyenv-win\versions\3.11.0b4\Lib\asyncio\taskgroups.py", line 121, in __aexit__
    raise propagate_cancellation_error
  File "C:\Users\jack\.pyenv\pyenv-win\versions\3.11.0b4\Lib\asyncio\taskgroups.py", line 96, in __aexit__
    await self._on_completed_fut
asyncio.exceptions.CancelledError

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "C:\Users\jack\PycharmProjects\demo\main.py", line 20, in <module>
    asyncio.run(main())
  File "C:\Users\jack\.pyenv\pyenv-win\versions\3.11.0b4\Lib\asyncio\runners.py", line 187, in run
    return runner.run(main)
           ^^^^^^^^^^^^^^^^
  File "C:\Users\jack\.pyenv\pyenv-win\versions\3.11.0b4\Lib\asyncio\runners.py", line 120, in run
    return self._loop.run_until_complete(task)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\jack\.pyenv\pyenv-win\versions\3.11.0b4\Lib\asyncio\base_events.py", line 650, in run_until_complete
    return future.result()
           ^^^^^^^^^^^^^^^
  File "C:\Users\jack\PycharmProjects\demo\main.py", line 13, in main
    async with asyncio.timeout(1):
  File "C:\Users\jack\.pyenv\pyenv-win\versions\3.11.0b4\Lib\asyncio\timeouts.py", line 98, in __aexit__
    raise TimeoutError
TimeoutError

Process finished with exit code 1

package main

import (
	"context"
	"crypto/md5"
	"fmt"
	"io/ioutil"
	"log"
	"os"
	"path/filepath"

	"golang.org/x/sync/errgroup"
)

// Pipeline demonstrates the use of a Group to implement a multi-stage
// pipeline: a version of the MD5All function with bounded parallelism from
// https://blog.golang.org/pipelines.
func main() {
	m, err := MD5All(context.Background(), ".")
	if err != nil {
		log.Fatal(err)
	}

	for k, sum := range m {
		fmt.Printf("%s:\t%x\n", k, sum)
	}
}

type result struct {
	path string
	sum  [md5.Size]byte
}

// MD5All reads all the files in the file tree rooted at root and returns a map
// from file path to the MD5 sum of the file's contents. If the directory walk
// fails or any read operation fails, MD5All returns an error.
func MD5All(ctx context.Context, root string) (map[string][md5.Size]byte, error) {
	// ctx is canceled when g.Wait() returns. When this version of MD5All returns
	// - even in case of error! - we know that all of the goroutines have finished
	// and the memory they were using can be garbage-collected.
	g, ctx := errgroup.WithContext(ctx)
	paths := make(chan string)

	g.Go(func() error {
		defer close(paths)
		return filepath.Walk(root, func(path string, info os.FileInfo, err error) error {
			if err != nil {
				return err
			}
			if !info.Mode().IsRegular() {
				return nil
			}
			select {
			case paths <- path:
			case <-ctx.Done():
				return ctx.Err()
			}
			return nil
		})
	})

	// Start a fixed number of goroutines to read and digest files.
	c := make(chan result)
	const numDigesters = 20
	for i := 0; i < numDigesters; i++ {
		g.Go(func() error {
			for path := range paths {
				data, err := ioutil.ReadFile(path)
				if err != nil {
					return err
				}
				select {
				case c <- result{path, md5.Sum(data)}:
				case <-ctx.Done():
					return ctx.Err()
				}
			}
			return nil
		})
	}
	go func() {
		g.Wait()
		close(c)
	}()

	m := make(map[string][md5.Size]byte)
	for r := range c {
		m[r.path] = r.sum
	}
	// Check whether any of the goroutines failed. Since g is accumulating the
	// errors, we don't need to send them (or check for them) in the individual
	// results sent on the channel.
	if err := g.Wait(); err != nil {
		return nil, err
	}
	return m, nil
}

type Group struct {
	cancel func()       // 封装context cancel 方法
	wg sync.WaitGroup   // 使用 waitGroup 同步
	sem chan token      // 使用 channel 限制数量
	errOnce sync.Once   // 只执行一次
	err     error       // 保存执行的错误信息
}


// Wait blocks until all function calls from the Go method have returned, then
// returns the first non-nil error (if any) from them.
func (g *Group) Wait() error {
	g.wg.Wait()
	if g.cancel != nil {
		g.cancel()
	}
	return g.err
}

// Go calls the given function in a new goroutine.
// It blocks until the new goroutine can be added without the number of
// active goroutines in the group exceeding the configured limit.
//
// The first call to return a non-nil error cancels the group; its error will be
// returned by Wait.
func (g *Group) Go(f func() error) {
	if g.sem != nil {
		g.sem <- token{}
	}

	g.wg.Add(1)
	go func() {
		defer g.done()

		if err := f(); err != nil {
			g.errOnce.Do(func() {
				g.err = err
				if g.cancel != nil {
					g.cancel()
				}
			})
		}
	}()
}