[Python-Dev] A question about the subprocess implementation (original) (raw)

Vinay Sajip vinay_sajip at yahoo.co.uk
Sun Jan 8 03:06:33 CET 2012

Mike Meyer <mwm mired.org> writes:

Since the only reason they exist is so you can access your end of a pipe, setting them to anything would seem to be a bug. I'd argue that their existence is more a pola violation than them having the value None. But None is easier than a call to hasattr.

I don't follow your reasoning, re. why setting them to a handle used for subprocess output would be a bug - it's logically the same as the PIPE case. For example, I might have a pipe (say, constructed using os.pipe()) whose write end is intended for the subprocess to output to, and whose read end I want to hand off to some other code to read the output from the subprocess. However, if that other code does a read() on that pipe, it will hang until the write handle for the pipe is closed. So, once the subprocess has terminated, I need to close the write handle. The actual reading might be done not in my code but in some client code of my code. While I could use some other place to store it, where's the problem in storing it in proc.stdout or proc.stderr?

You can close the object you passed in if it wasn't PIPE. If you passed in PIPE, the object has to be exposed some way, otherwise you can't close it.

Yes, I'm not disputing that I need to keep track of it - just that proc.stdout seems a good place to keep it. That way, the closing code can be de-coupled from the code that sets up the subprocess. A use case for this is when you want the subprocess and the parent to run concurrently/asynchronously, so the proc.wait() and subsequent processing happens at a different time and place to the kick-off.

Regards,

Vinay Sajip

More information about the Python-Dev mailing list