使用python的子流程从iperf3获取实时输出

Question

这是：Getting realtime output using subprocess的后续内容

我正在尝试使用子流程实时捕获iperf3的输出（在windows上使用python 3.6）。目标是保持iperf3会话持续运行，并获取数据以更新实时绘图

我根据引用的问题创建了一个实现（请参见本文末尾的代码），但代码仍在等待iperf3会话完成的第一个“readline”调用

输出和期望的行为

我的代码返回输出：

Iperf test
Popen returns after: 0.012966156005859375 seconds
Readline 0 returned after: 3.2275266647338867 seconds, line was: Connecting to host 10.99.99.21, port 5201
Readline 1 returned after: 3.2275266647338867 seconds, line was: [  4] local 10.99.99.7 port 55563 connected to 10.99.99.21 port 5201
Readline 2 returned after: 3.2275266647338867 seconds, line was: [ ID] Interval           Transfer     Bandwidth
Readline 3 returned after: 3.2275266647338867 seconds, line was: [  4]   0.00-0.50   sec  27.4 MBytes   458 Mbits/sec
Readline 4 returned after: 3.2275266647338867 seconds, line was: [  4]   0.50-1.00   sec  29.0 MBytes   486 Mbits/sec
Exited

输出显示，第一个readline调用直到3秒后iperf会话完成后才返回。所需的行为是，readline调用0、1和2几乎立即返回，而readline调用#3在大约0.5秒后返回，只要iperf3完成第一个0.5秒的报告间隔

代码

import subprocess
import time

if __name__ == "__main__":
    print('Iperf test')
    tref = time.time()

    reads_to_capture = 5
    times = [0] * reads_to_capture
    lines = [''] * reads_to_capture

    interval = 0.5
    ip = '10.99.99.21' # Iperf server IP address
    process = subprocess.Popen(f'iperf3 -c {ip} -f m -i {interval} -t 3', encoding = 'utf-8',
            stdout=subprocess.PIPE)

    print(f'Popen returns after: {time.time() - tref} seconds')

    cnt = 0

    while True:
        output = process.stdout.readline()
        if cnt < reads_to_capture: # To avoid flooding the terminal, only print the first 5
            times[cnt] = time.time() - tref
            lines[cnt] = output
            cnt = cnt + 1
        if output == '': 
            rc = process.poll()
            if rc is not None:
                break

    rc = process.poll()

    for ii in range(reads_to_capture):
        print(f'Readline {ii} returned after: {times[ii]} seconds, line was: {lines[ii].strip()}')

    print('Exited')