I think Idle should consistently display astral chars with their \U escape.  It sometimes does, just not always.

>>> s='\U0001f680'
>>> s
'\U0001f680'
>>> str(s)
'\U0001f680'
>>> repr(s)
"'\U0001f680'"
>>> print(s) # gives error above.
>>> print(str(s))  #ditto

I thought that implicit print of expression and overt print of the same expression were supposed to be the same.

#21084 is also about this general issue.