You can read about the changes in What’s New In Python 3.0. You should read it thoroughly when you move from 2.x to 3.x since a lot has been changed.
The whole answer here are quotes from the documentation.
Views And Iterators Instead Of Lists
Some well-known APIs no longer return lists:
filter()return iterators. If you really need a list, a quick fix is e.g.
list(map(...)), but a better fix is often to use a list comprehension (especially when the original code uses lambda), or rewriting the code so it doesn’t need a list at all. Particularly tricky is
map()invoked for the side effects of the function; the correct transformation is to use a regular
forloop (since creating a list would just be wasteful).
functools.reduce()if you really need it; however, 99 percent of the time an explicit
forloop is more readable.