CODEWITHSUNDEEP
pythonfilesubprocess
r36363
r36363

Reputation: 599

Subprocess file output needs to close before reading

I'm trying to use a subprocess to write the output to a data file, and then parse through it in order to check for some data in it. However, when I need to do the reading through the file's lines, I always get a blank file unless I close the file and then reopen it. While it works, I just don't like having to do this and I want to know why it happens. Is it an issue with subprocess, or another intricacy of the file mode?

dumpFile=open(filename,"w+")
dump = subprocess.Popen(dumpPars,stdout=dumpFile)
dump.wait()

At this point, if I try to read the file, I get nothing. However, it works fine by doing these commands after:

dumpFile.close()
dumpFile=open(filename,"r")

Upvotes: 0

Views: 251

Answers (3)

LSerni
LSerni

Reputation: 57388

If you want to read what you've already written, either close and reopen the file, or "rewind" it - seek to offset 0.

If you want to read the file while it is being written, you can do so (don't even need to write it to disk), see this other question Capture output from a program

Upvotes: 0

mgilson
mgilson

Reputation: 309891

You probably need to seek back to the beginning of the file, otherwise the file pointer will be at the end of the file when you try to read it:

 dumpFile.seek(0)

However, if you don't need to actually store dumpFile, it's probably better to do something like:

dump = = subprocess.Popen(dumpPars,stdout=subprocess.PIPE)
stdoutdata,_ = dump.communicate()  #now parse stdoutdata

unless your command produces large volumes of data.

Upvotes: 1

orlp
orlp

Reputation: 117681

The with statement automatically closes the file after the block ends:

with open(filename, "w+") as dumpFile:
    dump = subprocess.Popen(dumpPars, stdout=dumpFile)
    dump.wait()

with open(filename, "r") as dumpFile:
    # dumpFile reading code goes here

Upvotes: 2

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