Among 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 Chapter 7, Files and Data Persistence), but also in enumerate, in the dictionary keys, values, and items methods, and several other places.

It all makes sense: Python's aim is to try to reduce the memory footprint by avoiding wasting space wherever possible, especially in those functions and methods that are used extensively in most situations.

Do you remember at the beginning of this chapter? I said that it makes more sense to optimize the performances of code that has to deal with a lot of objects, rather than shaving off a few milliseconds from a function that we call twice a day.