Piping stdout to Python callable

[Rich Krauter]

Edward Diener wrote:
> Antoon Pardon wrote:
> 
>>Op 2004-08-17, Edward Diener schreef <eldiener at earthlink.net>:
>>
>>>From within a function in my own module I need to take the output
>>>from a Python module "A", which is sending data to stdout and which
>>>can not be changed and which I need to invoke as a top-level module,
>>>and pipe it into another function in my own module so that I can
>>>read it from stdin. Is there an easy way to do this ? The only way I
>>>can presently think to do this is through "system python A.py |
>>>python MyOwnModule.py", which seems a bit laborious having to invoke
>>>python.exe itself twice. Any other solution would be most welcome.
>>
>>What do you mean when you say you need to invoke it as a top-level
>>module?  Do you mean you can't import it at all or that importing it
>>will startup the process of generating output immediatly?
> 
> 
> I mean that it has a "if __name__ == '__main__' line and I need to trigger
> it by calling 'python A.py'.
> 
> 
>>What bothers you with twice invoking the interpreter?
> 
> 
> Nothing practically. Just seems inelegant.
> 
> 
>>In these days
>>a program that is invoked multiples times will generally be only
>>loaded once in memory.
> 
> 
> Shared libraries may be loaded once in memory but python.exe itself gets
> reloaded each time.
> 
> 
>>Are threads an acceptable alternative? Does your MyOwnModule.py needs
>>to write to stdout?
> 
> 
> Threads are acceptable. MyOwnModule.py can do anything, depending on
> parameters, but the idea is that a certain parameter tells it to read from
> stdin on the other end of the pipe.
> 
> I have implemented this via "os.system("python A.py | python MyOwnModule.py
> parameters") and it works fine. I thought there might be a better, more
> elegant way but since the above works without any problem I will stick to it
> unless you can suggest anything better. BTW "A.py" can be any number of
> types of modules which must be invoked by python.exe and writes it's results
> to stdout. I am telling you this lest you suggest that I somehow import and
> parse A.py in order to call directly into it rather than having python.exe
> invoke it. While that might be possible, it would be a real programming
> PITA.
> 
> 

You could do something like this:

$ cat A.py
def test(arg):
     for i in range(1,5):
         print arg*i

$ cat B.py
import A
import sys
import StringIO

sys.stdout = StringIO.StringIO()
A.test(5)
fileobj,sys.stdout = sys.stdout,sys.__stdout__

fileobj.seek(0)
for line in fileobj:
     print line.strip()


Worth the effort to change your code? Probably not. Kinda neat, though.

Rich