Why doesn't list.reverse return a list?

list.reverse is an inplace operation, so it will change the list and return None. You should be using reversed function, like this

"".join(reversed(rst))

I would personally recommend using slicing notation like this

rst[::-1]

For example,

rst = "cabbage"
print "".join(reversed(rst))   # egabbac
print rst[::-1]                # egabbac

Have you tried the following?

"".join(s for s in reversed(st))

reversed returns a reverse iterator. Documentation is here

It fails because lst.reverse() reverses a list in place and returns None (and you cannot iterate over None). What you are looking for is (for example) reversed(lst) which creates a new list out of lst which is reversed.

Note that if you want to reverse a string then you can do that directly (without lists):

>>> st = "This is Ok"
>>> st[::-1]
"kO si sihT"