Polymorphism: subclasses instead of a type field
Let each kind answer for itself, so a new kind adds no new case.
Watch the narrated video
Every step
Problem
Steps 1–3Refactor
Steps 4–8Payoff
Steps 9–10Recap
Steps 11–12The code: before and after
Before
class Rental:
def __init__(self, type: str, days: int) -> None:
self.type = type
self.days = days
def price(self) -> int:
match self.type:
case "economy":
return 30 * self.days
case "suv":
return 50 * self.days + 20
case _:
raise ValueError(self.type)
def free_km(self) -> int:
match self.type:
case "economy":
return 200 * self.days
case "suv":
return 150 * self.days
case _:
raise ValueError(self.type)
def desk() -> None:
city = Rental("economy", 3)
trip = Rental("suv", 3)
print(city.price(), city.free_km()) # 90 600
print(trip.price(), trip.free_km()) # 170 450
desk()After
from abc import ABC, abstractmethod
class Rental(ABC):
def __init__(self, days: int) -> None:
self.days = days
@abstractmethod
def price(self) -> int: ...
@abstractmethod
def free_km(self) -> int: ...
class EconomyRental(Rental):
def price(self) -> int:
return 30 * self.days
def free_km(self) -> int:
return 200 * self.days
class SuvRental(Rental):
def price(self) -> int:
return 50 * self.days + 20
def free_km(self) -> int:
return 150 * self.days
class VanRental(Rental):
def price(self) -> int:
return 70 * self.days
def free_km(self) -> int:
return 100 * self.days
def desk() -> None:
city = EconomyRental(3)
trip = SuvRental(3)
move = VanRental(3)
print(city.price(), city.free_km()) # 90 600
print(trip.price(), trip.free_km()) # 170 450
print(move.price(), move.free_km()) # 210 300
desk()Transcript, step by step
Problem
Step 1: A car rental desk Before
At a car rental desk, a rental knows its type and how many days it runs. Price and free kilometers each match on the type to find the rule for that car. The desk function rents an economy car and an SUV for three days, and prints the price and free kilometers of each.
Step 2: Vans join the fleet Code smell
The fleet now has vans, at seventy dollars a day with a hundred free kilometers a day. The quick way adds a van case to the price match, and then another one to the free kilometers match. The desk rents a van for three days, and the run prints two hundred ten and three hundred.
Step 3: One string, a match per method Code smell
A type field, which Fowler calls a type code, is a string that says what kind of thing an object is. So every method has to match on that string before it can do its job. You can see the type field here, the match in price here, and the match in free kilometers here. Every new kind of car means opening Rental and editing every match in it, so the van's rules end up spread over every method. A deposit method would add a third match, and every later kind would edit all three.
Refactor
Step 4: Refactor first Back it out
Let's take the vans out again. We'll refactor first, so each kind of car gets its own class, and then vans will be one new class.
Step 5: One class per kind Replace Type Code with Subclasses
First, Rental becomes an abstract base class, and each kind of car gets a subclass, a class that extends it. Economy Rental and SUV Rental have only a constructor, which passes its type string and the days up to Rental. The desk now creates the subclasses, and both match statements still work unchanged.
Step 6: Each kind sets its price Replace Conditional with Polymorphism
Next, each subclass overrides price with the line from its own case. Rental's price becomes an abstract method, so the price match is gone. This is polymorphism, because the desk asks every rental the same question, and each subclass answers it its own way. Last lesson pulled shared code up into a superclass, and this one pushes the code that differs down into subclasses.
Step 7: Each kind sets its kilometers Replace Conditional with Polymorphism
Free kilometers gets the same move, and each subclass overrides it with the line from its own case. Rental's free kilometers becomes abstract too, so the second match is gone as well.
Step 8: The type string goes Remove Dead Code
Nothing reads the type field anymore, because no match is left to ask it. So the field and its constructor parameter are deleted, and the subclass constructors go too, because each one only passed a string up.
Payoff
Step 9: Vans, as one class Add vans
Now vans come back, and this time they are one new class, Van Rental. It overrides price with seventy dollars a day, and free kilometers with a hundred a day. Rental, Economy Rental and SUV Rental didn't change, and the desk only gains the two van lines.
Step 10: Each kind answers for itself Payoff
The type field used to sit here, and the two match statements used to sit here and here. All the van's rules sit in one class now, so the next kind of car is one new class too. This is the Open Closed Principle, because Rental is open for extension and closed for change. It fits here because the fleet grows by kinds of car, while the questions the desk asks, price and free kilometers, stay the same. And if a new class leaves out free kilometers, Python raises a type error as soon as you create one, because an abstract method is still open.
Recap
Step 11: Where we started, where we ended Before / after
On the left, Rental keeps a type string, and price and free kilometers each match on it. On the right, Rental holds only the days and declares the two questions as abstract methods. Each kind of car, like Economy Rental, answers both questions in its own class.
Step 12: When to reach for it Takeaways
Reach for it when two or more methods match on the same type field. Skip it when an object changes kind while it lives, like a rental upgraded mid-trip, because an object should keep the class it was created with, so use State or Strategy instead. You'll spot the need by the same match or if chain on one field, repeated in several methods. Python's logging works this way, because Handler leaves emit to its subclasses, and Stream Handler and File Handler each answer it their own way.