Strange behaviour with reversed()

[Steven D'Aprano]

I don't understand how reversed() is operating. I've read the description 
in the docs:

reversed(seq)
Return a reverse iterator. seq must be an object which supports the 
sequence protocol (the __len__() method and the __getitem__() method with 
integer arguments starting at 0). New in version 2.4. 

and help(reversed) but neither gives any insight to what happens when you 
use reversed() on a sequence, then modify the sequence.


This works as I expected:

>>> L = list("abc")
>>> RL = reversed(L)
>>> del L
>>> list(RL)
['c', 'b', 'a']

This suggests that reversed() makes a copy of the list:

>>> L = list("abc")
>>> RL = reversed(L)
>>> L.append("d")
>>> list(RL)
['c', 'b', 'a']

This suggests that reversed() uses a reference to the original list:

>>> RL = reversed(L)
>>> L[0] = 'e'
>>> list(RL)
['d', 'c', 'b', 'e']

And these examples suggests that reversed() is confused, or at least 
confusing:

>>> RL = reversed(L)
>>> del L[2]
>>> list(RL)
[]

>>> L = list("abc")
>>> RL = reversed(L)
>>> L.insert(0, "d")
>>> L
['d', 'a', 'b', 'c']
>>> list(RL)
['b', 'a', 'd']


Does anyone know what reversed() is actually doing?



-- 
Steven.