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comp.lang.python

How to send a var to stdin of an external software

Benjamin Watine

3/13/2008 11:14:00 AM

Hi the list,

I need to send a var to stdin of an external soft ("cat" command for
example).

How can I do this ? I would like a function like that :

theFunction ('cat -', stdin=myVar)

I don't need to get any return value.

Another related question : Is there's a limitation of var size ? I would
have var up to 10 MB.

Thanks !

Ben
9 Answers

Marko Rauhamaa

3/13/2008 12:04:00 PM

0

Benjamin Watine <watine@cines.fr>:

> How can I do this ? I would like a function like that :
>
> theFunction ('cat -', stdin=myVar)
>
> Another related question : Is there's a limitation of var size ? I
> would have var up to 10 MB.

import subprocess
myVar = '*' * 10000000
cat = subprocess.Popen('cat',shell = True,stdin = subprocess.PIPE)
cat.stdin.write(myVar)
cat.stdin.close()
cat.wait()


Marko

--
Marko Rauhamaa mailto:marko@pacujo.net http://pacujo....

Sion Arrowsmith

3/13/2008 2:22:00 PM

0

Benjamin Watine <watine@cines.fr> wrote:
>How can I do this ? I would like a function like that :
>
> theFunction ('cat -', stdin=myVar)
>
>I don't need to get any return value.

http://docs.python.org/lib/no... says this is spelt

myVar = subprocess.Popen(["cat", "-"], stdout=subprocess.PIPE).communicate()[0]

(Probably not obvious how to find this if you've not come across the
backtick notation in shell or Perl.)

--
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"Frankly I have no feelings towards penguins one way or the other"
-- Arthur C. Clarke
her nu becomeþ se bera eadward ofdun hlæddre heafdes bæce bump bump bump

Benjamin Watine

3/13/2008 2:44:00 PM

0

Marko Rauhamaa a écrit :
> Benjamin Watine <watine@cines.fr>:
>
>> How can I do this ? I would like a function like that :
>>
>> theFunction ('cat -', stdin=myVar)
>>
>> Another related question : Is there's a limitation of var size ? I
>> would have var up to 10 MB.
>
> import subprocess
> myVar = '*' * 10000000
> cat = subprocess.Popen('cat',shell = True,stdin = subprocess.PIPE)
> cat.stdin.write(myVar)
> cat.stdin.close()
> cat.wait()
>
>
> Marko
>

Thank you Marko, it's exactly what I need.

And if somebody need it : to get the stdout in a var (myNewVar), not in
the shell :

cat = subprocess.Popen('cat', shell = True, stdin = subprocess.PIPE,
stdout=subprocess.PIPE)
cat.stdin.write(myVar)
cat.stdin.close()
cat.wait()
myNewVar = cat.stdout.read()

Is it correct ?

Ben

Bryan Olson

3/13/2008 4:31:00 PM

0

Benjamin Watine wrote:
> And if somebody need it : to get the stdout in a var (myNewVar), not in
> the shell :
>
> cat = subprocess.Popen('cat', shell = True, stdin = subprocess.PIPE,
> stdout=subprocess.PIPE)
> cat.stdin.write(myVar)
> cat.stdin.close()
> cat.wait()
> myNewVar = cat.stdout.read()
>
> Is it correct ?

No, not really. It is prone to deadlock. The external program might
work by iteratively reading a little input and writing a little
output, as 'cat' almost surely does. If the size of myVar exceeds
the buffer space in cat and the pipes, you get stuck.

Your Python program can block at "cat.stdin.write(myVar)", waiting
for cat to read from its input pipe, while cat blocks at a write
to its output stream, waiting for you to start reading and freeing
up buffer space. Pipe loops are tricky business.

Popular solutions are to make either the input or output stream
a disk file, or to create another thread (or process) to be an
active reader or writer.


--
--Bryan

Bryan Olson

3/13/2008 4:50:00 PM

0

I wrote:
> [...] Pipe loops are tricky business.
>
> Popular solutions are to make either the input or output stream
> a disk file, or to create another thread (or process) to be an
> active reader or writer.

Or asynchronous I/O. On Unix-like systems, you can select() on
the underlying file descriptors. (MS-Windows async mechanisms are
not as well exposed by the Python standard library.)


--
--Bryan

Benjamin Watine

3/14/2008 11:38:00 AM

0

Bryan Olson a écrit :
> I wrote:
>> [...] Pipe loops are tricky business.
>>
>> Popular solutions are to make either the input or output stream
>> a disk file, or to create another thread (or process) to be an
>> active reader or writer.
>
> Or asynchronous I/O. On Unix-like systems, you can select() on
> the underlying file descriptors. (MS-Windows async mechanisms are
> not as well exposed by the Python standard library.)
>

Hi Bryan

Thank you so much for your advice. You're right, I just made a test with
a 10 MB input stream, and it hangs exactly like you said (on
cat.stdin.write(myStdin))...

I don't want to use disk files. In reality, this script was previously
done in bash using disk files, but I had problems with that solution
(the files wasn't always cleared, and sometimes, I've found a part of
previous input at the end of the next input.)

That's why I want to use python, just to not use disk files.

Could you give me more information / examples about the two solutions
you've proposed (thread or asynchronous I/O) ?

Thank you !

Ben

Floris Bruynooghe

3/14/2008 2:47:00 PM

0

On Mar 14, 11:37 am, Benjamin Watine <wat...@cines.fr> wrote:
> Bryan Olson a écrit :
>
> > I wrote:
> >> [...] Pipe loops are tricky business.
>
> >> Popular solutions are to make either the input or output stream
> >> a disk file, or to create another thread (or process) to be an
> >> active reader or writer.
>
> > Or asynchronous I/O. On Unix-like systems, you can select() on
> > the underlying file descriptors. (MS-Windows async mechanisms are
> > not as well exposed by the Python standard library.)
>
> Hi Bryan
>
> Thank you so much for your advice. You're right, I just made a test with
> a 10 MB input stream, and it hangs exactly like you said (on
> cat.stdin.write(myStdin))...
>
> I don't want to use disk files. In reality, this script was previously
> done in bash using disk files, but I had problems with that solution
> (the files wasn't always cleared, and sometimes, I've found a part of
> previous input at the end of the next input.)
>
> That's why I want to use python, just to not use disk files.
>
> Could you give me more information / examples about the two solutions
> you've proposed (thread or asynchronous I/O) ?

The source code of the subprocess module shows how to do it with
select IIRC. Look at the implementation of the communicate() method.

Regards
Floris

Bryan

3/14/2008 10:07:00 PM

0

Floris Bruynooghe wrote:
> Benjamin Watine wrote:
> > Could you give me more information / examples about the two solutions
> > you've proposed (thread or asynchronous I/O) ?
>
> The source code of the subprocess module shows how to do it with
> select IIRC. Look at the implementation of the communicate() method.

And here's a thread example, based on Benjamin's code:

import subprocess
import thread

def readtobox(pipe, box):
box.append(pipe.read())

cat = subprocess.Popen('cat', shell=True, stdin=subprocess.PIPE,
stdout=subprocess.PIPE)

myVar = str(range(1000000)) # arbitrary test data.

box = []
thread.start_new_thread(readtobox, (cat.stdout, box))
cat.stdin.write(myVar)
cat.stdin.close()
cat.wait()
myNewVar = box[0]

assert myNewVar == myVar
print len(myNewVar), "bytes piped around."


--
--Bryan

Bryan

3/14/2008 10:33:00 PM

0

I wrote:
> And here's a thread example, based on Benjamin's code:
[...]

Doh! Race condition. Make that:

import subprocess
import thread
import Queue

def readtoq(pipe, q):
q.put(pipe.read())

cat = subprocess.Popen('cat', shell=True, stdin=subprocess.PIPE,
stdout=subprocess.PIPE)

myVar = str(range(1000000)) # arbitrary test data.

q = Queue.Queue()
thread.start_new_thread(readtoq, (cat.stdout, q))
cat.stdin.write(myVar)
cat.stdin.close()
cat.wait()
myNewVar = q.get()

assert myNewVar == myVar
print len(myNewVar), "bytes piped around."


--
--Bryan