管道到文件时分析 subprocess.Popen 的输出

Question

How can I analyze the output of my command, which I pipe to file, in real-time while the file is written?

This is what I have so far:

with open('output.log', 'w') as out:
    command = ['pg_dump', 'myDB']
    p = subprocess.Popen(cmd, stdout=out, stderr=subprocess.STDOUT)

    for line in iter(p.stdout.readline, b''):
        sys.stdout.flush()
        print(">>> " + line.rstrip())

But this generates the following error:

Traceback (most recent call last):
  File "pipe-to-file.py", line 95, in <module>
    for line in iter(p.stdout.readline, b''):
AttributeError: 'NoneType' object has no attribute 'readline'

Why is p.stdout equal to None here?

Answer 1

You have to use subprocess.PIPE for the stdout argument in order to get a file object, else it'll be None . That's why p.stdout equals to None in your code.

From the DOC

Use communicate() rather than .stdin.write , .stdout.read or .stderr.read to avoid deadlocks due to any of the other OS pipe buffers filling up and blocking the child process.

If you want to write stdout to a file while analyzing the output then you can use something like this.

with open('log', 'ab+') as out:
    p = subprocess.Popen('ls', stdout=subprocess.PIPE, stderr=subprocess.PIPE)
    std_out, std_error = p.communicate()

    # Do something with std_out
    # ...
    
    # Write to the file
    out.write( std_out )

    # You can use `splitlines()` to iterate over the lines.

    for line in std_out.splitlines():
        print line


        
        
   

Answer 2

p = subprocess.Popen(cmd, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
output,error=p.communicate()

Now you have your output,error.