This started as a python-list thread. On that thread, Peter Otten verified the behavior on Linux, so it is not specific to the Windows pythonw executable. He also verified on 3.2 (which gets security fixes only) and 3.4. I just checked 2.7. On that thread, I verified a) the three lines submitted by themselves as a file cause the same error message, as they should ("must refer to pre-existing bindings in an enclosing scope ") and b) Python in interactive mode syntax-checks lines as entered, even for compound statements. I just verified that Idle behaves the same when started with -n (no subprocess):
C:\Programs\Python33>python -m idlelib.idle -n

Summary: Idle acts as if it syntax-checks the cumulative input instead of just the line entered. (But I have not looked as the code.)

The three lines constitute a test if they can be run from a test file.