Read from File, or STDIN

I’ve written a command line utility that uses getopt for parsing arguments given on the command line. I would also like to have a filename be an optional argument, such as it is in other utilities like grep, cut etc. So, I would like it to have the following usage

tool -d character -f integer [filename]

How can I implement the following?

• if a filename is given, read from the file.
• if a filename is not given, read from STDIN.

Answer

In the simplest terms:

import sys
# parse command line
if file_name_given:
    inf = open(file_name_given)
else:
    inf = sys.stdin

At this point you would use inf to read from the file. Depending on whether a filename was given, this would read from the given file or from stdin.

When you need to close the file, you can do this:

if inf is not sys.stdin:
    inf.close()

However, in most cases it will be harmless to close sys.stdin if you’re done with it.