While writing this chapter, I discovered that it can be very difficult, and extremely educational to come up with good examples where specific design patterns should be used. Instead of going over current or old projects to see where you can apply these patterns, as I've suggested in previous chapters, think about the patterns and different situations where they might come up. Try to think outside your own experiences. If your current projects are in the banking business, consider how you'd apply these design patterns in a retail or point-of-sale application. If you normally write web applications, think about using design patterns while writing a compiler.

Start with the iterator pattern. We've been looking at iterators all through this book, including the special comprehension and generator syntaxes. Consider places where you'd want to implement the iterator pattern from scratch; what objects would you want to implement __iter__ or __next__ on?

Look at the decorator pattern and come up with some good examples of when to apply it. Focus on the pattern itself, not the Python syntax we discussed; it's a bit more general than the actual pattern. The special syntax for decorators is, however, something you might want to look for places to apply in existing projects too.

What are some good areas to use the observer pattern? Why? Think about not only how you'd apply the pattern, but how you would implement the same task without using observer? What do you gain, or lose, by choosing to use it?

Consider the difference between the strategy and state patterns. Implementation-wise, they look very similar, yet they have different purposes. Can you think of cases where the patterns could be interchanged? Would it be reasonable to redesign a state-based system to use strategy instead, or vice-versa? How much different would the design actually be?

The template pattern is such an obvious application of inheritance to reduce duplicate code that you may have used it before, without knowing its name. Try to think of at least half a dozen different scenarios where it would be useful. If you can do that, you'll be finding places for it in your daily coding all the time.