[ Python 常見問題 ] Saving stdout from subprocess.Popen to file

Source From Here
Question
I'm writing a Python script that uses subprocess.Popen to execute two programs (from compiled C code) which each produce stdout. The script gets that output and saves it to a file. Because the output is sometimes large enough to overwhelm subprocess.PIPE, causing the script to hang, I send the stdout directly to the log file. I want to have my script write something to the beginning and end of the file, and between the two subprocess.PIPE calls. However, when I look at my log file, anything I wrote to the log file from the script is all together at the top of the file, followed by all the executable stdout. How can I interleave my added text to the file?

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1. def run(cmd, logfile):  
2.     p = subprocess.Popen(cmd, shell=True, universal_newlines=True, stdout=logfile)  
3.     return p  
4.   
5. def runTest(path, flags, name):  
6.     log = open(name, "w")  
7.     print >> log, "Calling executable A"  
8.     a_ret = run(path + "executable_a_name" + flags, log)  
9.     print >> log, "Calling executable B"  
10.     b_ret = run(path + "executable_b_name" + flags, log)  
11.     print >> log, "More stuff"  
12.     log.close()  

How-To
You could call .wait() on each Popen object in order to be sure that it's finished and then call log.flush(). Maybe something like this:

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1. def run(cmd, logfile):  
2.     p = subprocess.Popen(cmd, shell=True, universal_newlines=True, stdout=logfile)  
3.     ret_code = p.wait()  
4.     logfile.flush()  
5.     return ret_code  

Supplement
* How do I redirect stdout to a file when using subprocess.call in python?

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1. with open('out-file.txt', 'w') as f:  
2.     subprocess.call(['program'], stdout=f)