python - Calling list() twice on reversed() returns an empty list the second time

Question

I don't understand the result of this code:

aa = 'hello, world'
bb = reversed(aa)
print(bb)
print(list(bb))
print(bb)
dd = list(bb)
print(dd)
print(''.join(dd))

The result:

<reversed object at 0x025C8C90>
['d', 'l', 'r', 'o', 'w', ' ', ',', 'o', 'l', 'l', 'e', 'h']
<reversed object at 0x025C8C90>
[]

Why is dd []?

Answer 1

That's because reversed creates an iterator, which is already spent when you're calling list(bb) for the second time.

aa = 'hello, world'
bb = reversed(aa)    # Creates an iterator, not a list
print(bb)            # Prints "<reversed object at 0x10c619b90>" (that's the iterator)
print(list(bb))      # Pops items from the iterator to create a list, then prints the list
print(bb)            # At this point the iterator is spent, and doesn't contain elements anymore
dd = list(bb)        # Empty iterator -> Empty list ([])
print(dd)
print(''.join(dd))
print('----------')

To fix that, just change reversed(aa) by list(reversed(aa)).