Generation behavior in built-ins

Amongst the built-in types, the generation behavior is now quite common. This is a major difference between Python 2 and Python 3. A lot of functions such as map, zip, and filter have been transformed so that they return objects that behave like iterables. The idea behind this change is that if you need to make a list of those results you can always wrap the call in a list() class, and you're done. On the other hand, if you just need to iterate and want to keep the impact on memory as light as possible, you can use those functions safely.

Another notable example is the range function. In Python 2 it returns a list, and there is another function called xrange that returns an object that you can iterate on, which generates the numbers on the fly. In Python 3 this function has gone, and range now behaves like it.

But this concept in general is now quite widespread. You can find it in the open() function, which is used to operate on file objects (we'll see it in...