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Wed, Aug 23, 2017 11:36 pm

Determining a file's type from within a Python script

I needed a way to determine a file's type within a Python script when I can't rely on the file's extension to determine the file format. I'll be running the script on a MacBook Pro laptop running the OS X El Capitan operating system. OS X/macOS, like Linux, comes with the file command, so I could run that command at a shell prompt to have the utility check the magic number in the files I'm interested in, but I want to do some additional processing of the files within the Python script, so I want to perform the format check within Python. Python provides the subprocess module that provides the capability to "spawn new processes, connect to their input/output/error pipes, and obtain their return codes." So I can call the file utility from within Python using that module. To get the results from running a shell command, you use suprocess.Popen(). You can then set a variable to hold the results of .communicate() and print the contents of that variable as shown below. The script expects the name of the file to be checked to be provided as an argument on the command line.

#!/usr/bin/python

import subprocess as sub, sys

try:
   sys.argv[1]
except IndexError:
   print "Error - missing input file name! Usage ./filetype.py infile"
   sys.exit(1)
else:
   fileName = sys.argv[1]

p = sub.Popen(['file',fileName],stdout=sub.PIPE,stderr=sub.PIPE)
output, errors = p.communicate()
print output

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