Question

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")

Answer 1

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.