Updating State via Methods

Direct attribute assignment lets anyone set invalid state from anywhere. This session channels every state change through named, validating methods instead.

1. Learning Objective

2. Pre-Coding Quiz

Answer these before reading the concept explanation or writing any code. It is fine to get these wrong — that is the point. You need 4/5 to proceed to the lab.

3. The Concept — Controlled State Updates

From direct assignment to a named method

Session 14 already introduced grow_population as a validating method. This session generalizes that pattern: every state change should go through a method, never a bare attribute assignment from outside the class.

class Country:
    def __init__(self, name, region, population):
        self.name = name
        self.region = region
        self.population = population

    def grow_population(self, amount):
        if amount < 0:
            raise ValueError(f"amount must be non-negative, got {amount}")
        self.population += amount

    def set_population(self, value):
        if value < 0:
            raise ValueError(f"population must be non-negative, got {value}")
        self.population = value  # only reachable if the value passed validation

Naming a method to communicate intent

A generic update(field, value) method either needs complicated per-field logic or skips validation entirely. Specific method names — set_population, grow_population, set_capital — each validate exactly one clear case.

# Avoid: a generic method that has to guess what "field" even means
def update(self, field, value):
    setattr(self, field, value)  # no validation possible here — dangerous

# Prefer: specific, self-documenting, individually validated methods
def set_capital(self, new_capital):
    if not new_capital:
        raise ValueError("capital cannot be empty")
    self.capital = new_capital

Extending the pattern to CountryExplorer

The same discipline applies to the collection-level state from Session 16 — an add_country method guarantees every item in the list is actually a valid Country.

class CountryExplorer:
    def __init__(self, countries):
        self.countries = countries

    def add_country(self, country):
        if not isinstance(country, Country):
            raise TypeError(f"expected a Country instance, got {type(country).__name__}")
        self.countries.append(country)

explorer = CountryExplorer(countries=[])
explorer.add_country(Country(name="Kenya", region="Africa", population=54000000))
print(len(explorer.countries))  # 1

# explorer.add_country("not a country")  # raises TypeError — caught before corrupting the list

Why this actually solves Session 20's risk

Once population can only change through grow_population or set_population, tracing an unexpected value means checking those two call sites — not the entire codebase.

4. Lab

What you will build

A file called controlled_state.py.

Step-by-step instructions

# controlled_state.py
class Country:
    def __init__(self, name, region, population, capital=None):
        self.name = name
        self.region = region
        self.population = population
        self.capital = capital

    def grow_population(self, amount):
        if amount < 0:
            raise ValueError(f"amount must be non-negative, got {amount}")
        self.population += amount

class Country:
    def __init__(self, name, region, population, capital=None):
        self.name = name
        self.region = region
        self.population = population
        self.capital = capital

    def grow_population(self, amount):
        if amount < 0:
            raise ValueError(f"amount must be non-negative, got {amount}")
        self.population += amount

    def set_population(self, value):
        if value < 0:
            raise ValueError(f"population must be non-negative, got {value}")
        self.population = value

    def set_capital(self, new_capital):
        if not new_capital:
            raise ValueError("capital cannot be empty")
        self.capital = new_capital

k = Country(name="Kenya", region="Africa", population=54000000)

try:
    k.set_population(-5)
except ValueError as e:
    print("Rejected:", e)

print("Population still valid:", k.population)  # unchanged — still 54000000

try:
    k.set_capital("")
except ValueError as e:
    print("Rejected:", e)

class CountryExplorer:
    def __init__(self, countries):
        self.countries = countries

    def add_country(self, country):
        if not isinstance(country, Country):
            raise TypeError(f"expected a Country instance, got {type(country).__name__}")
        self.countries.append(country)

explorer = CountryExplorer(countries=[])
explorer.add_country(k)
print(len(explorer.countries))

try:
    explorer.add_country("not a country")
except TypeError as e:
    print("Rejected:", e)

print("Collection still valid, length:", len(explorer.countries))  # still 1

5. Expected Files Changed

6. Commit Checkpoint

Once the lab is complete and you can explain every line, make this exact commit:

git add controlled_state.py docs/sessions/session-21/index.html
git commit -m "session-21: channel all state changes through validating methods"

7. Code Review Checklist

Go through your code line by line and check each item:

8. Post-Coding Quiz

Same 5 questions. Take it again now that you have written and run the code. You need 4/5 to mark this session complete.

9. Reflection Questions

Think through these after the post-quiz. No right answer — they are for discussion.

10. What Breaks If This Knowledge Is Missing?

11. What We Learned