Builder

1 Gamma Categorization

Design patterns are split into 3 categories
Creational patterns: deal with creation of patterns
Structural Patterns
concerned with structures. e.g. class members
stress on good API design
Behavioral patterns
no central theme; all different

2 Builder pattern motivation

Some objects are simple. Can be created in a single initializer.
Other require a lot of ceremony
having 10 init args not productive
instead, opt for piecewise construction

When piecewise object construction is complicated, provide API for it.

3 Builder

example of HTML builder

class PersonBuilder:
    def __init__(self, person=Person()):
        self.person = person

    @property
    def works(self):
        return PersonJobBuilder(self.person)

  @property
    def lives(self):
        return PersonAddressBuilder(self.person)

pb = PersonBuilder()
pb = person\
                    .lives.at('london')
                .works.at("google")

Base class jumps from one builder to other.

Super useful but it directly violates open-close principle
need to go back to personbuilder to modify it

5 Builder interface

we can also do it without violating open-close principle
inherit builders from the upper one.

Summary

Builder is separate component for building an object
to make builder fluent, return self from every single method of builder.