Class and Static Methods Checkpoint

25 min · Intermediate

Fix the mistakes

The slips that cost marks in this module. Spot each one, then check the fix.

A real class uses all three method types, each for a different job. This checkpoint builds one small design and then tests your choices.

Decide the job of each method

For every piece of logic, ask what it needs. A subscription has a plan and a seat count. Its monthly total needs both, so it is an instance method.

Building from "PRO-5" needs the class only. Checking a plan name needs nothing.

Example 1 · One design, three method types

Python

1class Subscription:2    PRICES = {"BASIC": 500, "PRO": 1200}3
4    def __init__(self, plan, seats):5        if not Subscription.is_valid_plan(plan):6            raise ValueError("unknown plan")7        if seats < 1:8            raise ValueError("seats must be at least 1")9        self.plan = plan10        self.seats = seats11
12    @classmethod13    def from_code(cls, code):14        plan, seats = code.split("-")15        return cls(plan, int(seats))16
17    @staticmethod18    def is_valid_plan(plan):19        return plan in Subscription.PRICES20
21    def monthly_total(self):22        return Subscription.PRICES[self.plan] * self.seats23
24sub = Subscription.from_code("PRO-5")25print(sub.plan, sub.seats, sub.monthly_total())26print(Subscription.is_valid_plan("TRIAL"))

Output

PRO 5 6000
False

What this code does

from_code splits the text and calls cls.
monthly_total uses self data. is_valid_plan needs neither.

Example 2 · One class, three kinds of method

Python

1class Subscription:2    PRICES = {"BASIC": 500, "PRO": 1200}3
4    def __init__(self, plan, seats):5        if not Subscription.is_valid_plan(plan):6            raise ValueError("unknown plan")7        if seats < 1:8            raise ValueError("seats must be at least 1")9        self.plan = plan10        self.seats = seats11
12    @classmethod13    def from_code(cls, code):14        plan, seats = code.split("-")15        return cls(plan, int(seats))16
17    @staticmethod18    def is_valid_plan(plan):19        return plan in Subscription.PRICES20
21    def monthly_total(self):22        return Subscription.PRICES[self.plan] * self.seats23
24
25team = Subscription.from_code("PRO-3")26print(team.monthly_total())27print(Subscription.is_valid_plan("GOLD"))

Output

3600
False

What this code does

from_code needs the class to build an object, so it is a class method.
is_valid_plan needs neither the class nor an object, so it is a static method.
monthly_total reads this object's plan and seats, so it is an instance method.

Each method uses the least it needs: self, cls, or nothing. A reader can tell from the signature what a method can touch.

Keep the rules in one place

The factory calls cls(...), and __init__ checks the plan with the static helper. Every route to a subscription goes through the same two checks.

Python

sub = Subscription.from_code("PRO-5")print(sub.monthly_total())

Example 3 · Both routes are checked

Python

1class Subscription:2    PRICES = {"BASIC": 500, "PRO": 1200}3
4    def __init__(self, plan, seats):5        if not Subscription.is_valid_plan(plan):6            raise ValueError("unknown plan")7        if seats < 1:8            raise ValueError("seats must be at least 1")9        self.plan = plan10        self.seats = seats11
12    @classmethod13    def from_code(cls, code):14        plan, seats = code.split("-")15        return cls(plan, int(seats))16
17    @staticmethod18    def is_valid_plan(plan):19        return plan in Subscription.PRICES20
21    def monthly_total(self):22        return Subscription.PRICES[self.plan] * self.seats23
24for build in (lambda: Subscription("GOLD", 2), lambda: Subscription.from_code("BASIC-0")):25    try:26        build()27    except ValueError as error:28        print("Rejected:", error)

Output

Rejected: unknown plan
Rejected: seats must be at least 1

What this code does

The direct call fails on the plan, the factory call fails on the seat count.
Both errors come from __init__, so the rules cannot be skipped.

Bad input fails in one place with one message, whichever way the object is built.

Questions to ask in any design

Before adding a helper to a class, check these. If the helper is used by several unrelated classes, make it a plain function. If subclasses may change the data it uses, make it a class method. If it reads one object's data, make it an instance method.

Example 4 · A subclass keeps its type

Python

1class Subscription:2    PRICES = {"BASIC": 500, "PRO": 1200}3
4    def __init__(self, plan, seats):5        if not Subscription.is_valid_plan(plan):6            raise ValueError("unknown plan")7        if seats < 1:8            raise ValueError("seats must be at least 1")9        self.plan = plan10        self.seats = seats11
12    @classmethod13    def from_code(cls, code):14        plan, seats = code.split("-")15        return cls(plan, int(seats))16
17    @staticmethod18    def is_valid_plan(plan):19        return plan in Subscription.PRICES20
21    def monthly_total(self):22        return Subscription.PRICES[self.plan] * self.seats23
24class TeamSubscription(Subscription):25    pass26
27team = TeamSubscription.from_code("BASIC-10")28print(type(team).__name__, team.monthly_total())

Output

TeamSubscription 5000

What this code does

from_code is inherited and uses cls, so it builds a TeamSubscription.
10 seats at 500 each give 5000.

Finish up

Mark complete & continue

Next: Single Inheritance and Method Overriding

1class Subscription:2 PRICES = {"BASIC": 500, "PRO": 1200}3 4 def __init__(self, plan, seats):5 if not Subscription.is_valid_plan(plan):6 raise ValueError("unknown plan")7 if seats < 1:8 raise ValueError("seats must be at least 1")9 self.plan = plan10 self.seats = seats11 12 @classmethod13 def from_code(cls, code):14 plan, seats = code.split("-")15 return cls(plan, int(seats))16 17 @staticmethod18 def is_valid_plan(plan):19 return plan in Subscription.PRICES20 21 def monthly_total(self):22 return Subscription.PRICES[self.plan] * self.seats23 24sub = Subscription.from_code("PRO-5")25print(sub.plan, sub.seats, sub.monthly_total())26print(Subscription.is_valid_plan("TRIAL"))

1class Subscription:2 PRICES = {"BASIC": 500, "PRO": 1200}3 4 def __init__(self, plan, seats):5 if not Subscription.is_valid_plan(plan):6 raise ValueError("unknown plan")7 if seats < 1:8 raise ValueError("seats must be at least 1")9 self.plan = plan10 self.seats = seats11 12 @classmethod13 def from_code(cls, code):14 plan, seats = code.split("-")15 return cls(plan, int(seats))16 17 @staticmethod18 def is_valid_plan(plan):19 return plan in Subscription.PRICES20 21 def monthly_total(self):22 return Subscription.PRICES[self.plan] * self.seats23 24 25team = Subscription.from_code("PRO-3")26print(team.monthly_total())27print(Subscription.is_valid_plan("GOLD"))

1class Subscription:2 PRICES = {"BASIC": 500, "PRO": 1200}3 4 def __init__(self, plan, seats):5 if not Subscription.is_valid_plan(plan):6 raise ValueError("unknown plan")7 if seats < 1:8 raise ValueError("seats must be at least 1")9 self.plan = plan10 self.seats = seats11 12 @classmethod13 def from_code(cls, code):14 plan, seats = code.split("-")15 return cls(plan, int(seats))16 17 @staticmethod18 def is_valid_plan(plan):19 return plan in Subscription.PRICES20 21 def monthly_total(self):22 return Subscription.PRICES[self.plan] * self.seats23 24for build in (lambda: Subscription("GOLD", 2), lambda: Subscription.from_code("BASIC-0")):25 try:26 build()27 except ValueError as error:28 print("Rejected:", error)

1class Subscription:2 PRICES = {"BASIC": 500, "PRO": 1200}3 4 def __init__(self, plan, seats):5 if not Subscription.is_valid_plan(plan):6 raise ValueError("unknown plan")7 if seats < 1:8 raise ValueError("seats must be at least 1")9 self.plan = plan10 self.seats = seats11 12 @classmethod13 def from_code(cls, code):14 plan, seats = code.split("-")15 return cls(plan, int(seats))16 17 @staticmethod18 def is_valid_plan(plan):19 return plan in Subscription.PRICES20 21 def monthly_total(self):22 return Subscription.PRICES[self.plan] * self.seats23 24class TeamSubscription(Subscription):25 pass26 27team = TeamSubscription.from_code("BASIC-10")28print(type(team).__name__, team.monthly_total())