stdin and stdout decision in Python

The easiest automatic decision between defined input file or stdin and output file or stdout I have come up with in Python is:

import sys

# If intab (input) is defined, read it; if not assume stdin
if intab:
    f = open(intab, "r")
else:
    f = sys.stdin

# If otab (output) is defined, read it; if not assume stdout
if otab:
    fout = open(otab, "w")
else:
    fout = sys.stdout

# Do whatever you want 
for line in f:
    print(line)
    write.fout(line)

# And close files, just to be safe
if f is not sys.stdin:
    f.close()

if fout is not sys.stdout:
    fout.close()

There might be something easier but this is simple and obvious.