> I wonder why "print(1, file=sys.stderr)" returns '1' instead of '1\n'.

I suppose that you mean "returns '1\n' instead of '1'". This is a
major change between Python 2 and Python 3. Use print(1, end=' ') if
you want the same behaviour. See:
http://docs.python.org/dev/whatsnew/3.0.html#print-is-a-function

You can also use the print() as a function in Python 2 using "from
__future__ import print_function":
http://docs.python.org/dev/whatsnew/2.6.html#pep-3105-print-as-a-function

I never liked "print expr," because it looks like a typo or an ugly
hack. It's easy to add a comma by mistake.