Python3: write to file if filename is given as argument, else write to stdout.

write_to_file_or_stdout.py

import argparse
import datetime
import sys

"""
This script writes the current timestamp to the file given a cli argument.
If no filename is given, the timestamp is written to stdout.
"""

if __name__ == "__main__":
    parser = argparse.ArgumentParser()
    parser.add_argument('output_file', nargs='?', default=sys.stdout)
    args = parser.parse_args()

    timestamp = datetime.datetime.now()

    if isinstance(args.output_file, str):  # write to file
        with open(args.output_file, 'w') as output_file:
            print(timestamp, file=output_file)
    else:  # write to STDOUT
        print(timestamp, file=args.output_file)

Caveat: print(some_string, file=output_file) appends a newline at the end of the file!
Alternative: use output_file.write(some_string)