Thanks Alex, now I think I understand much better the fifo/pipe mechanism
and how Python treats them.
For those who are interested, I would like to restate the problem I was
tring to solve and a working solution (inspired by Alex Martelli's code),
feel free to criticize it:
The problem:
I have an external program which takes two matrices (two text files) as
input, and produces an output to stdout, you can take diff as an example.
I want to call this program from within Python, without writing any
temporary file (for the performance reason).
In Bash, this task would be best written as:
#!/bin/bash
diff <(step1) <(step2) | step3
Where step1, step2 and step3 have to be independent external programs.
Now in Python, since there is no exact equivalence of <() magic a.k.a.
process substitution, I figured taht the best solution should be to create
a pair of fifos and do something like this:
#!/bin/bash
mkfifo fifo1 fifo2
step1 > fifo1 &
step2 > fifo2 &
diff step1 step2 | step3
And a working Python equivalent code is:
#!/usr/bin/python
import os
# do step1 and step2 in Python, say we end up with something like these:
s1 = "some string\n second line." # results from step1
s2 = "some string\n a different line." # results from step2
os.mkfifo('fifo1')
os.mkfifo('fifo2')
op = os.popen(' '.join(['diff', 'fifo1', 'fifo2'])) # this step is crucial!
print >> open('fifo1', 'w'), s1
print >> open('fifo2', 'w'), s2
os.unlink('fifo1')
os.unlink('fifo2')
x = op.read()
# Now do something about x
print x
P.S.: process substitution is a Bash hack which uses /dev/fd/<n> to send
(usually more than one) processes output to a program which takes (more
than one) filename as input. Heuristically speaking, this is like a
multiway pipe. You can find more about this mechanism here:
http://www.tldp.org/LDP/abs/html/process-sub.html
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