Python script output in file

[Albert-Jan Roskam]

-----------------------------
On Thu, Mar 19, 2015 12:16 AM CET Steven D'Aprano wrote:

>On Thu, 19 Mar 2015 07:22 am, Albert-Jan Roskam wrote:
>
>> 
>> ----------------------------
>> On Wed, Mar 18, 2015 7:06 PM CET Rustom Mody wrote:
>> 
>>On Wednesday, March 18, 2015 at 8:12:12 PM UTC+5:30, Robert Clove wrote:
>>> ./my_eth_script.pl eth0 M > a.txt
>>> 
>>> How can i run this command with subprocess.popen
>>
>>Something like this I guess?
>>
>>> proc = Popen("cat", shell=True, stdout=open(inname, "w"),
>>> stdin=open(outname,"r"))
>> 
>> 
>> How will the file object associated with in name be closed? (When is
>> close() method called?
>
>Depends on the interpreter.
>
>CPython (the one you are probably using) will notice immediately the proc
>object goes out of scope and is garbage-collected. That will free up the
>Popen stdout argument, which will garbage collect the file object, which
>will close the inname file immediately.
>
>Assuming there are no circular references involved. If there are, then the
>file won't be closed until the second garbage collector gets to run. It
>will break the circular reference and close the file, but there is no
>promise as to how often it will run. The user can configure how often it
>runs and even whether it runs at all.
>
>On the other hand, Jython will garbage-collect the file object, but the file
>itself may remain open until your code exits, and only be closed on
>interpreter-shutdown. Since the operator system probably has limits on how
>many files you can have open at once, you might actually run out.
>
>For quick and dirty scripts, it doesn't matter, but best practice is to
>explicitly close your files as soon as possible, preferably with a `with`
>statement:
>
>
># I think inname and outname are reversed...
>with open(inname, "w") as outfile, open(outname, "r") as infile:
>    proc = Popen("cat", shell=True, stdout=outfile, stdin=infile)
>    # do stuff with proc
>    # when finished, save the stuff you care about
>    result = ...
>
>print(result)
>
>
>Outside of the `with` block, proc's stdout and stdin will be automatically
>closed, even if the block exits with an exception.

Thanks, this is indeed what I was thinking about. I had no idea about the GC differences between cpython and jython (reference counting vs true, java GC). I am amazed that jython is just a version 2.3, is it tries to look like cpython 2.3! But perhaps they're about to jump to 3.4