Builder¶

This pattern separates the construction of complex objects from their representations, thus enabling the emergence of different representations in one construction process.

We use the Builder pattern when:

• The algorithm for creating a complex object should be independent of the component parts of this object and the way of their compilation.
• The construction process must take into account the different representations of the object being constructed.

Design¶

Participants:

• Builder - abstract interface for creating target objects
• ConcreteBuilder - a specific builder that creates and combines the components of the created object
• Director - manages the creation of the object
• Product - the generated complex object

Cooperation:

• The client creates a Director object and configures it with the desired ConcreteBuilder object
• Director tells ConcreteBuilder to build a part of the product
• The ConcreteBuilder processes the requests from the Director and adds parts to the product
• The customer picks up the product from ConcreteBuilder

Consequences:

• Enabling changes to the internal product representation
• Separating the constructing code from the representation
• Better control of the design process

Implementation example¶

class Cook:
    '''
    Director - manages the creation of the object
    '''
    def __init__(self):
        self._builder = None

    def prepare(self, builder):
        self._builder = builder
        self._builder.prepare_dough()
        self._builder.add_extras()
        self._builder.bake()

class PizzaBuilder:
    '''
    Builder - abstract interface for creating target objects
    '''
    def __init__(self):
        self.pizza = Pizza()

    def prepare_dough(self): pass

    def add_extras(self): pass

    def bake(self): pass

class MargeritaBuilder(PizzaBuilder):
    '''
    ConcreteBuilder - a specific builder that creates and combines the components of the created object
    '''
    def prepare_dough(self): pass

    def add_extras(self): pass

    def bake(self): pass

class PepperoniBuilder(PizzaBuilder):
    def prepare_dough(self): pass

    def add_extras(self): pass

    def bake(self): pass

class Pizza:
    '''
    the generated complex object
    '''
    pass

def main():
    cook = Cook()
    # we choose a builder
    baking = PepperoniBuilder()
    cook.prepare(baking)
    pizza = baking.pizza