Factory Pattern

The Idea

Scattered SomeClass() calls hard-wire every caller to a concrete type. A factory takes a key and returns an object honouring a shared interface. Choosing moves to one function, and unknown keys fail there with a clear message instead of anywhere.

Real-World Example

A car hire desk. You booked "estate, automatic" and you are handed whichever car in today's fleet meets that class. You never name a model, and the depot can retire one and buy another without telling you.

The Code

class Estate:  seats, boot = 5, 550      # three products, one shared shape
class Van:     seats, boot = 3, 3000
class Compact: seats, boot = 4, 300

FLEET = {"estate": Estate, "van": Van, "compact": Compact}

def hire(car_class):                     # callers name a class, never a constructor
    if car_class not in FLEET:
        raise ValueError("no such class: " + car_class)
    return FLEET[car_class]()

car = hire("van")
print(type(car).__name__, car.seats, car.boot)   # Van 3 3000
print(type(hire("estate")).__name__)             # Estate

class Cat:
  def sound(self): return "meow"

class Cow:
  def sound(self): return "moo"

MAKERS = {"cat": Cat, "cow": Cow}

def make(key): return MAKERS[key]()

print(make("cow").sound(), type(make("cat")).__name__)