How to access additional file streams of the unix shell in Python?

Question

How can I access the additional file streams like the comm command in Python?

comm -23 <(sort -n Asub|uniq) <(sort -n A|uniq)

I know that I can access stdin via sys.stdin, but how to access the other input stream?

Answer 1

Thanks for asking this question, as I actually didn't understand the behavior of <() myself. It appears after some digging, however, that what it actually does it creates a temporary virtual file descriptor that it pipes the information from the subcommand into, then returns the name of that file descriptor. To see what I mean, look at this python program:

import sys
for arg in sys.argv:
    print('|{}|'.format(repr(arg)))

When it is invoked like python3 thing.py <(cat a.txt) <(cat b.txt), you should see that the output is something like:

|'thing.py'|
|'/proc/self/fd/11'|
|'/proc/self/fd/12'|

So finally, to answer your question, what you need to do to read the data from that subprocess is to simply open that file descriptor as you would any other file. For example:

with open(sys.argv[1]) as f:                                                                        
    for line in f:                                                                                    
        print(line.strip())

Which gives me an output like:

A
B
C

(Matching the contents of a.txt)

Hope that helps!