In our everyday life, the builder design pattern is used in fast-food restaurants. The same procedure is always used to prepare a burger and the packaging (box and paper bag), even if there are many different kinds of burgers (classic, cheeseburger, and more) and different packages (small-sized box, medium-sized box, and so forth). The difference between a classic burger and a cheeseburger is in the representation, and not in the construction procedure. In this case, the director is the cashier who gives instructions about what needs to be prepared to the crew, and the builder is the person from the crew that takes care of the specific order.

We can also find software examples:

• The HTML example that was mentioned at the beginning of the chapter is actually used by django-widgy (https://wid.gy/), a third-party tree editor for Django that can be used as a content...

We use the builder pattern when we know that an object must be created in multiple steps, and different representations of the same construction are required. These requirements exist in many applications such as page generators (for example, the HTML page generator mentioned in this chapter), document converters, and user interface (UI) form creators (j.mp/pipbuild).

Some online resources mention that the builder pattern can also be used as a solution to the telescopic constructor problem. The telescopic constructor problem occurs when we are forced to create a new constructor for supporting different ways of creating an object. The problem is that we end up with many constructors and long parameter lists, which are hard to manage. An example of the telescopic constructor is listed at the Stack Overflow website (j.mp/sobuilder). Fortunately, this problem does not exist...

Let's see how we can use the builder design pattern to make a pizza-ordering application. The pizza example is particularly interesting because a pizza is prepared in steps that should follow a specific order. To add the sauce, you first need to prepare the dough. To add the topping, you first need to add the sauce. And you can't start baking the pizza unless both the sauce and the topping are placed on the dough. Moreover, each pizza usually requires a different baking time, depending on the thickness of its dough and the topping used.

We start by importing the required modules and declaring a few Enum parameters (j.mp/pytenum) plus a constant that is used many times in the application. The STEP_DELAY constant is used to add a time delay between the different steps of preparing a pizza (prepare the dough, add the sauce, and so on) as follows:

from enum...

In this chapter, we have seen how to use the builder design pattern. We use the builder pattern for creating an object in situations where using the factory pattern (either a factory method or an abstract factory) is not a good option. A builder pattern is usually a better candidate than a factory pattern when the following applies:

• We want to create a complex object (an object composed of many parts and created in different steps that might need to follow a specific order).
• Different representations of an object are required, and we want to keep the construction of an object decoupled from its representation.
• We want to create an object at one point in time, but access it at a later point.

We saw how the builder pattern is used in fast-food restaurants for preparing meals, and how two third-party Django packages, django-widgy, and django-query-builder, use it for generating...