Instance Methods

A class with only attributes is still just a fancier dictionary. Methods are functions that live on the class and operate on an instance's own data through self.

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 — Instance Methods

A method is a function defined inside a class

Like __init__, every method takes self as its first parameter, giving it access to that instance's own attributes — no need to pass the data in as an argument, because the method already lives on the object that owns it.

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

    def summary(self):
        return f"{self.name} ({self.region}): pop. {self.population:,}"

k = Country("Kenya", "Africa", 54000000)
print(k.summary())  # "Kenya (Africa): pop. 54,000,000"

What the dot-call actually does

When you write k.summary(), Python translates this into Country.summary(k) behind the scenes — k is automatically supplied as self. This is the exact same mechanism from Session 13's __init__, just used for a different method.

print(k.summary())            # normal way
print(Country.summary(k))     # exactly equivalent — proves it

A method that mutates the instance

Methods can update self's attributes just like the standalone functions from Layer 1 updated dict keys — except now the mutation logic lives with the data it protects.

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

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

k = Country("Kenya", 54000000)
k.grow_population(1000000)
print(k.population)  # 55000000 — mutated in place, no reassignment needed

Standalone function (Layer 1) vs method (Layer 2) — side by side

This is the entire conceptual leap of Layer 2: the data moved inside the object, so the function moved with it.

# Layer 1 style — Session 08
def get_population(country_dict):
    return country_dict["population"]

print(get_population({"population": 54000000}))

# Layer 2 style — this session
class Country:
    def __init__(self, population):
        self.population = population
    def get_population(self):
        return self.population

k = Country(54000000)
print(k.get_population())  # no argument needed — self already has it

4. Lab

What you will build

A file called country_methods.py.

Step-by-step instructions

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

    def summary(self):
        return f"{self.name} ({self.region}): pop. {self.population:,}"

kenya = Country("Kenya", "Africa", 54000000)
print(kenya.summary())

print(kenya.summary() == Country.summary(kenya))  # True

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

    def summary(self):
        return f"{self.name} ({self.region}): pop. {self.population:,}"

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

kenya = Country("Kenya", "Africa", 54000000)
kenya.grow_population(1000000)
print(kenya.summary())

try:
    kenya.grow_population(-500)
except ValueError as e:
    print("Rejected:", e)

5. Expected Files Changed

6. Commit Checkpoint

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

git add country_methods.py docs/sessions/session-14/index.html
git commit -m "session-14: add summary and grow_population instance 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