subprocess.call is not waiting

Question

with open('pf_d.txt', 'w+') as outputfile:
        rc = subprocess.call([pf, 'disable'], shell=True, stdout=outputfile, stderr=outputfile)
        print outputfile.readlines()

output.readlines() is returning [] even though the file is written with some data. Something is wrong here.

looks like subprocess.call() is not blocking and the file is being written after the read function. How do i solve this?

Answer 1

The with open('pf_d.txt', 'w+') as outputfile: construct is called context manager. In this case, the resource is a file represented by the handle/file object outputfile. The context manager makes sure that the file is closed when the context is left. Closing implicates flushing, and re-opening the file after that will show you all its contents. So, one option to solve your issue is to read your file after it has been closed:

with open('pf_d.txt', 'w+') as outputfile:
    rc = subprocess.call(...)

with open('pf_d.txt', 'r') as outputfile:
    print outputfile.readlines()

Another option is to re-use the same file object, after flushing and seeking:

with open('pf_d.txt', 'w+') as outputfile:
    rc = subprocess.call(...)
    outputfile.flush()
    outputfile.seek(0)
    print outputfile.readlines()

A file handle is always represented by a file pointer, indicating the current position in the file. write() forwards this pointer to the end of the file. seek(0) moves it back to the beginning, so that a subsequent read() startes from the beginning of the file.