python subprocess can’t print last line with PIPE

Question

p =Popen('cmd.exe',shell=True,stdin=PIPE,stdout=PIPE,bufsize=0)

def run():
    global p
    while True:
        line = p.stdout.read(1)
        if not line:  #空则跳出
            break
        sys.stdout.write(line.decode("GBK"))
    sys.stdout.write("look up!!! EXIT ===")   #跳出

when I run, i got

Microsoft Windows [Version 10.0.22000.613]
(c) Microsoft Corporation. All rights reserved.

But when I use stdout=sys.stdout,I got

Microsoft Windows [Version 10.0.22000.613]
(c) Microsoft Corporation. All rights reserved.

C:\Windows>

why I can't get last line with PIPE,i want to know if process end or timeout by last line