python - What is the difference if I don't use stdout=subprocess.PIPE in subprocess.Popen()?

Question

I recently noted in Python the subprocess.Popen() has an argument:

stdout=None(default)

I also saw people using stdout=subprocess.PIPE.

What is the difference? Which one should I use?

Another question would be, why the wait() function can't wait until the process is really done sometimes? I used:

a = sp.Popen(....,shell=True)
a.wait()
a2 = sp.Popen(...,shell=True)
a2.wait()

sometimes the a2 command is executed before the command a is done.

Answer 1

stdout=None means, the stdout-handle from the process is directly inherited from the parent, in easier words it basically means, it gets printed to the console (same applies for stderr).

Then you have the option stderr=STDOUT, this redirects stderr to the stdout, which means the output of stdout and stderr are forwarded to the same file handle.

If you set stdout=PIPE, Python will redirect the data from the process to a new file handle, which can be accessed through p.stdout (p beeing a Popen object). You would use this to capture the output of the process, or for the case of stdin to send data (constantly) to stdin. But mostly you want to use p.communicate, which allows you to send data to the process once (if you need to) and returns the complete stderr and stdout if the process is completed!

One more interesting fact, you can pass any file-object to stdin/stderr/stdout, e.g. also a file opened with open (the object has to provide a fileno() method).

To your wait problem. This should not be the case! As workaround you could use p.poll() to check if the process did exit! What is the return-value of the wait call?

Furthermore, you should avoid shell=True especially if you pass user-input as first argument, this could be used by a malicious user to exploit your program! Also it launches a shell process which means additional overhead. Of course there is the 1% of cases where you actually need shell=True, I can't judge this with your minimalistic example.