Design Patterns: Builder

Hello again!

In my last post, I was showing you a basic example on how to implement and use the "Abstract Factory" pattern.

This time, it is Builder's patterns turn. I will describe the intent, and a simple use case where it naturally fits well (I hope).

Intent:

Separate the construction of a complex object from its representation so that the same construction process can create different representations.

As an example is best than tones of lines, lets create a requirement:

"We need to create a small representation on how Ice creams will be built, and who is going to deal with the specifics of how to create/handle ice creams"

The code that uses the builder pattern to solve the requirement would look like this:

import abc

class InvalidIceCreamException(Exception):

pass

class IceCreamChashier:

def __init__(self):

self._ice_cream_builder = None

def construct(self, builder):

self._ice_cream_builder = builder

self._ice_cream_builder.build_type()

self._ice_cream_builder.build_flavors()

class IceCreamBuilder(metaclass=abc.ABCMeta):

def __init__(self):

self._ice_cream = IceCream()

@abc.abstractmethod

def build_type(self):

pass

@abc.abstractmethod

def build_flavors(self):

pass

@property

def ice_cream(self):

return self._ice_cream

@property

def ice_type(self):

return self.ice_cream.inner_type

@property

def flavors(self):

return self.ice_cream.list_flavors()

class IceCream(object):

GELATO = 'gelato'

CONE = 'cone'

SORBET = 'sorbet'

DONDURMA = 'dondurma'

TYPES = [GELATO, CONE, SORBET, DONDURMA]

def __init__(self):

self._type = None

self._flavors = set()

@property

def inner_type(self):

return self._type

def list_flavors(self):

return self._flavors

def set_type(self, ice_type):

if ice_type not in self.TYPES:

raise InvalidIceCreamException("This is an invalid icecream!")

self._type = ice_type

def set_flavor(self, flavor):

self._flavors.add(flavor)

class VanillaGelatoIceCreamBuilder(IceCreamBuilder):

def build_type(self):

self._ice_cream.set_type(IceCream.GELATO)

def build_flavors(self):

self._ice_cream.set_flavor('vanilla')

self._ice_cream.set_flavor('chocolate') #added just because... yes! Internal politics.

def main():

vanillaBuilder = VanillaGelatoIceCreamBuilder()

cashier = IceCreamChashier()

cashier.construct(vanillaBuilder)

print('Type of ice cream: {}'.format(vanillaBuilder.ice_type))

print('Flavors of ice cream: {}'.format(vanillaBuilder.flavors))

if __name__ == "__main__":

main()

So we can see that the complex object (IceCream) is hidden in the IceCreamBuilder abstraction, and we provide a small API to access to its main data, which is the ice cream type, and the flavors needed

Thats it!!