In a Python program, why can't I cat a file right after writing it?

Question

I tried to cat a file, using Popen(), right after creating and writing to it. It doesn't work. Print p gives two empty tuples ('',''). Why ? I've used rename to ensure an atomic write, as discussed here.

#!/usr/bin/env python
import sys,os,subprocess

def run(cmd):
    try:
        p = subprocess.Popen(cmd, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
        p.wait()
        if p.returncode:
            print "failed with code: %s" % str(p.returncode)
        return p.communicate()
    except OSError:
        print "OSError"

def main(argv):
    t = "alice in wonderland"
    fd = open("__q", "w"); fd.write(t); fd.close; os.rename("__q","_q")
    p = run(["cat", "_q"])
    print p

main(sys.argv)

@Oscar No, absolutely do not; see also https://stackoverflow.com/questions/59641747/whats-the-difference-between-subprocess-popenecho-home-and-subprocess-po — tripleee, Jan 08 '20 at 17:14

score 11 · Accepted Answer · edited May 31 '11 at 10:45

11

You did not call close. Use fd.close() (you forgot the parentheses there to make it an actual function call). This could have been prevented by using the with-statement:

with open("__q", "w") as fd:
    fd.write(t)
# will automatically be closed here

edited May 31 '11 at 10:45

Tim Pietzcker

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answered May 31 '11 at 10:41

Björn Pollex

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I spent so much time trying to figure this out. Is there a way Python could have been made to detect this kind of syntax error? – rup Jun 02 '11 at 11:37

In a Python program, why can't I cat a file right after writing it?

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