subprocess -- broken pipe error

[Steve Holden]

7stud wrote:
> Why doesn't the following program write to the file?
> 
> driver.py
> -------
> import subprocess as sub
> 
> p = sub.Popen(["python", "-u", "test1.py"], stdin=sub.PIPE,
> stdout=sub.PIPE)
> 
> 
> p.stdin.write("text3")
> 
> while True:
>     pass
> -------
> 
> test1.py:
> ---------
> import sys
> 
> data = sys.stdin.read()
> 
Let me ask you a question: what conditions have to be true to this 
statement to terminate?

A: the Python driver.py process has to close its output file. Since it 
doesn't do this (instead writing a little bit of output then going into 
an infinite loop) the whole thing just sits there consuming CPU time.


> f = open("aaa.txt", "w")
> f.write(data + "\n")
> f.close()
> -----------
> 
> 
> After I hit Ctrl+C to end the program and look in the file, the text
> wasn't written to the file.  But, if I change driver.py to the
> following it works:
> 
> driver.py:
> ----------
> import subprocess as sub
> 
> p = sub.Popen(["python", "-u", "test1.py"], stdin=sub.PIPE,
> stdout=sub.PIPE)
> 
> 
> p.stdin.write("text3")
> -------
> 
> Ok. So that looks like the data is caught in a buffer--even though the
> pipes should be unbuffered by default.  But this doesn't work:
> 
Who told you pipes should be unbuffered by default, and what difference 
does that make anyway?

The reason it works now is that your program closes its standard output 
by default when it terminates.

> driver.py
> ----------
> import subprocess as sub
> 
> p = sub.Popen(["python", "-u", "test1.py"], stdin=sub.PIPE,
> stdout=sub.PIPE)
> 
> p.stdin.write("text4")
> p.stdin.flush()
> 
> while True:
>     pass
> -------
> 
> It just hangs, and then when I hit Ctrl+C and look in the file, the
> data isn't in there.
> 
Of course it does, for the reasons mentioned above. file.read() only 
returns when it has consumed *all* the data from the file (which means 
the write must close the file for the reader to be able to return).

regards
  Steve
-- 
Steve Holden        +1 571 484 6266   +1 800 494 3119
Holden Web LLC/Ltd           http://www.holdenweb.com
Skype: holdenweb      http://del.icio.us/steve.holden
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