Linux/Python: setting shell variable to value returned from python function

From stackoverflow.com.  This works with python 2.4:

$ f=$(python foo.py)
$ echo $f
FOOO

Where foo.py is:

def foo():
  print "FOOO"

if __name__ == '__main__':
   foo()

There was a recommendation to use sys.stdout.write("%s\n", fooPy()) instead of print(), but I got an error about sys.stdout.write() expecting one not two input variables when I tried it. If I use sys.stdout.write("%s\n" % foo()) or print(foo()) in the main clause then the output is:

$ python foo.py
FOOO
None
$ w=$(python foo.py)
$ echo $w
FOOO None

With

import sys
def foo():
  sys.stdout.write("FOOO")
if __name__ == '__main__':
   foo()

I get

$ python foo.py
FOOO$
$ w=$(python foo.py)
$ echo $w
FOOO
$