CODEWITHSUNDEEP
pythonpython-2.7
user765443
user765443

Reputation: 1892

capture Python script output

I have written very small script and try to capture output of the script. I have written multiple time similar way but never had issue. Could I have input. I think I am doing very silly mistake 

numpy_temp = """
import numpy
import sys
a, b, c = numpy.polyfit(%s,%s, 2)
print a, b, c""" %(x, y)
fp_numpy = open("numpy_temp.py", "w")
fp_numpy.write(numpy_temp)
cmd = "/remote/Python-2.7.2/bin/python numpy_temp.py "
proc = subprocess.Popen(cmd, stdout = subprocess.PIPE,
            stderr = subprocess.PIPE, shell = True)
out, err = proc.communicate()
print "out", out

Upvotes: 1

Views: 1272

Answers (4)

abarnert
abarnert

Reputation: 365807

You're never actually closing fp_numpy, so the script can be empty or incomplete at the time you try to run it.

It isn't guaranteed to be empty, but it's very likely. When I try this on two different *nix computers with 7 different versions of Python, it's empty every time… (The fact that, after your script finishes, the file gets closed, and therefore flushed, makes the problem harder to debug.)

The best way to fix this is to use a with statement, so it's impossible to forget to close the file:

with open("numpy_temp.py", "w") as fp_numpy:
    fp_numpy.write(numpy_temp)

But beyond that, you've got another problem. If the generated script raises an exception, it will print nothing to stdout, and will dump a traceback to stderr, which you will read and ignore. It's very hard to debug problems when you're ignoring the errors… I don't know what you're passing for x and y, but if you're, say, passing a numpy.array instead of a string that evaluates to one, you could easily get an exception and never see it. Either send stderr to stdout, or print "err", err at the end.

And finally, you really shouldn't be using a command string and shell=True here, because you end up with an extra level of indirection for no good reason, which can also make things harder to debug. Just do this:

cmd = ["/remote/Python-2.7.2/bin/python", "numpy_temp.py"]
proc = subprocess.Popen(cmd, stdout = subprocess.PIPE,
                        stderr = subprocess.PIPE)

Upvotes: 2

axblount
axblount

Reputation: 2662

I don't know why you'd want to write a script to capture the output of a script (maybe you have a good reason). this sort of task is much easier using the shell.

Create your numpy_temp.py and then pipe it's output to a file.

/remote/Python-2.7.2/bin/python numpy_temp.py > output.txt

To append the data to an existing file use

/remote/Python-2.7.2/bin/python numpy_temp.py >> output.txt

Upvotes: 0

iChux
iChux

Reputation: 2386

Check out this method

see.py

for line in range(6):
    print line

me@pubx:~$ python see.py > all

Upvotes: 0

dav
dav

Reputation: 1219

use print out not print "out".

Upvotes: 0

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