foo.py

def foo():
    print 23
    # Now an empty line. In a REPL, Python does not know if the user
    # finished the definition of the function foo, or if it will
    # continue. So it assumes the empty line terminates the current
    # open code block.

    # In the REPL, this would now cause a "bad indentation" error,
    # because the definition of foo() ended above. In an actual
    # program, this works.
    print 17