Redirect command to input of another in Python

Question

I would like to replicate this in python:

gvimdiff <(hg cat file.txt) file.txt

(hg cat file.txt outputs the most recently committed version of file.txt)

I know how to pipe the file to gvimdiff, but it won't accept another file:

$ hg cat file.txt | gvimdiff file.txt -
Too many edit arguments: "-"

Getting to the python part...

# hgdiff.py
import subprocess
import sys
file = sys.argv[1]
subprocess.call(["gvimdiff", "<(hg cat %s)" % file, file])

When subprocess is called it merely passes <(hg cat file) onto gvimdiff as a filename.

So, is there any way to redirect a command as bash does? For simplicity's sake just cat a file and redirect it to diff:

diff <(cat file.txt) file.txt

Answer 1

It can be done. As of Python 2.5, however, this mechanism is Linux-specific and not portable:

import subprocess
import sys

file = sys.argv[1]
p1 = subprocess.Popen(['hg', 'cat', file], stdout=subprocess.PIPE)
p2 = subprocess.Popen([
    'gvimdiff',
    '/proc/self/fd/%s' % p1.stdout.fileno(),
    file])
p2.wait()

That said, in the specific case of diff, you can simply take one of the files from stdin, and remove the need to use the bash-alike functionality in question:

file = sys.argv[1]
p1 = subprocess.Popen(['hg', 'cat', file], stdout=subprocess.PIPE)
p2 = subprocess.Popen(['diff', '-', file], stdin=p1.stdout)
diff_text = p2.communicate()[0]

Answer 2

There is also the commands module:

import commands

status, output = commands.getstatusoutput("gvimdiff <(hg cat file.txt) file.txt")

There is also the popen set of functions, if you want to actually grok the data from a command as it is running.

Answer 3

This is actually an example in the docs:

p1 = Popen(["dmesg"], stdout=PIPE)
p2 = Popen(["grep", "hda"], stdin=p1.stdout, stdout=PIPE)
output = p2.communicate()[0]

which means for you:

import subprocess
import sys

file = sys.argv[1]
p1 = Popen(["hg", "cat", file], stdout=PIPE)
p2 = Popen(["gvimdiff", "file.txt"], stdin=p1.stdout, stdout=PIPE)
output = p2.communicate()[0]

This removes the use of the linux-specific /proc/self/fd bits, making it probably work on other unices like Solaris and the BSDs (including MacOS) and maybe even work on Windows.

Answer 4

It just dawned on me that you are probably looking for one of the popen functions.

from: http://docs.python.org/lib/module-popen2.html

popen3(cmd[, bufsize[, mode]]) Executes cmd as a sub-process. Returns the file objects (child_stdout, child_stdin, child_stderr).

namaste, Mark