如何使子进程在python中可见

Question

I have a python script that does 2 things sequentially:

1. generate a bunch of files and then
2. move files around.

I'm moving the files by iterating through all the files in the folder with:

for filename in os.listdir("."):
    if filename.endswith(".rmp"):

where the .rmp files are exactly the ones generated by the first subprocess.
So right now running it once only gets the .rmp files in the folder and the second step do not see them for whatever reason, but it moves them correctly on second running as expected.
So how do I get it to recognize all the files in the very first try. Why are they not visible in the first place?

here's the code

var = os.getcwd()
pipe = subprocess.Popen(["perl", "./runtest.pl", var])


for filename in os.listdir("."):
    if filename.endswith(".rmp"):
        print "woopee"

Answer 1

I think you're not closing the files. That might delay a write and cause os.listdir not to get all the files.

One nice way to ensure you're always closing the files is with:

with open('filename.rmp', 'w') as f:
  f.write(...)

This is nice because it works well with exceptions so that you're sure you don't forget them opened.

Once everything is closed they should show up in os.listdir(.)

Answer 2

When you create a subprocess, it executes in parallel with your current process. Since your process continues running while your perl script runs, it's a race between your program reading the directory and the perl process writing its files (this is your race condition ).

To resolve it, you should add a call to .wait() :

var = os.getcwd()
p = subprocess.Popen(["perl", "./runtest.pl", var])
p.wait() # wait for perl to exit

for filename in os.listdir("."):
    if filename.endswith(".rmp"):
        print "woopee"

I'd also add that this would be a good opportunity to use the glob module:

from glob import glob
var = os.getcwd()
p = subprocess.Popen(["perl", "./runtest.pl", var])
p.wait()

for filename in glob('*.rpm'):
    print "woopee"

Answer 3

Looks like you don't need any communication with the subprocess, all you need is just run a subprocess and wait for exit. subprocess.call() is more convenient:

var = os.getcwd()
retcode = subprocess.call(["perl", "./runtest.pl", var])
if retcode != 0:
    # do some error handling, the subprocess returned nonzero exit code

# process the file generated by subprocess