1. Start IDLE.
2. In the IDLE shell that pops up, write a line of
Python code and press enter.
3. Pretend you made a spelling mistake in step two. So
you press up and try to fix it but you can't because
the cursor is moved and the IDLE shell doesn't work at
all like the normal CPython shell. In fact, there is no
easy way at all to get back to the last line of input
for correcting spelling mistakes. 
I think there should be one. Maybe ALT+Up could do what
Up does in the CPython shell?

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IDLE is a full screen editor, unlike the CPython interactive
interpreter.  So arrow up/down are reserved for cursor
motion.

However, IDLE has a special facility to recall command
history.  See Help / IDLE Help - Tip Section - Python Shell
Window - Command History.  There are several ways to
recall the command history.

If you like you can rebind <history-previous> to
Alt-Up Arrow etc. using Options / Configure IDLE / Keys.
Default is Alt-p.