Python · Classes and OOP
Methods
The full text of this chapter, to read or revisit. To practise, do the exercises.
▶ Do the exercisesExercise 1 — What are Methods?
You know how to store data in objects with attributes. But objects can also do things! A method is a function defined inside a class. Just like `"hello".upper()` calls the `upper()` method on a string, you can create your own methods.
class Codemon:
def __init__(self, name, level=1):
self.name = name
self.level = level
def greet(self):
print(f"Hi! I'm {self.name}!")
myCdm = Codemon("Aquachu")
myCdm.greet()
`greet` is a method. Just like `__init__`, it takes `self` as the first parameter. We call it with `myCdm.greet()` — Python automatically passes `myCdm` as `self`.
Exercise 2 — Methods use self
Inside a method, `self` gives you access to all the object's attributes. This is how methods know which Codémon they're working with.
class Codemon:
def __init__(self, name, level=1):
self.name = name
self.level = level
def level_up(self):
self.level += 1
print(f"{self.name} is now level {self.level}!")
myCdm = Codemon("Pyrochu", 10)
myCdm.level_up()
myCdm.level_up()
`level_up()` reads `self.name` and modifies `self.level`. The changes are permanent — the object keeps the new level.
Exercise 3 — Methods with Parameters
Methods can take extra parameters, just like regular functions. The only difference is that `self` is always first.
class Codemon:
def __init__(self, name, level=1):
self.name = name
self.level = level
def attack(self, target):
print(f"{self.name} attacks {target.name}!")
target.level -= 1
a = Codemon("Aquachu", 10)
p = Codemon("Pyrochu", 8)
a.attack(p)
print(f"{p.name}: Level {p.level}")
Here `attack` takes another Codémon object as a parameter. One object can interact with another!
Exercise 4 — Define a Method
Complete the method that heals the Codémon by a given amount.
class Codemon:
def __init__(self, name, hp=100):
self.name = name
self.hp = hp
def heal(____, amount):
self.____ += amount
myCdm = Codemon("Aquachu", 50)
myCdm.heal(30)
print(myCdm.hp)
Options: self hp amount name myCdm
Hint : Every method's first parameter is `self`. The method needs to modify the HP attribute.
Exercise 5 — Calling a Method
What does this code print?
class Codemon:
def __init__(self, name, hp=100):
self.name = name
self.hp = hp
def take_damage(self, amount):
self.hp -= amount
myCdm = Codemon("Florachu")
myCdm.take_damage(30)
print(myCdm.hp)
Options
• 100
• 70
• 30
• Error
Hint : `take_damage(30)` subtracts 30 from the starting HP of 100.
Exercise 6 — Method with Parameter
Complete the method to transfer HP from one Codémon to another.
class Codemon:
def __init__(self, name, hp=100):
self.name = name
self.hp = hp
def give_hp(self, target, amount):
self.hp -= amount
____.hp += ____
a = Codemon("Aquachu")
b = Codemon("Pyrochu", 50)
a.give_hp(b, 20)
print(a.hp, b.hp)
Options: target amount self hp 20
Hint : The `target` is the other Codémon receiving the HP. The `amount` is how much to add.
Exercise 7 — Methods that Return Values
Methods can also return values, just like functions. Use the `return` keyword.
class Codemon:
def __init__(self, name, level=1, hp=100):
self.name = name
self.level = level
self.hp = hp
def power(self):
return self.level * 10 + self.hp
myCdm = Codemon("Rockachu", 15, 120)
print(f"Power: {myCdm.power()}")
`power()` calculates and returns a value. It doesn't modify the object — it just computes something from its attributes.
Exercise 8 — Return from Method
What does this code print?
class Codemon:
def __init__(self, name, level=1):
self.name = name
self.level = level
def is_evolved(self):
return self.level >= 20
myCdm = Codemon("Metachu", 25)
print(myCdm.is_evolved())
Options
• True
• False
• 25
• Error
Hint : `is_evolved()` checks if the level is 20 or more and returns a boolean.
Exercise 9 — The __str__ Method
When you `print()` an object, Python shows something ugly like `<__main__.Codemon object>`.
class Codemon:
def __init__(self, name, element, level=1):
self.name = name
self.element = element
self.level = level
myCdm = Codemon("Aquachu", "Water", 15)
print(myCdm)
You can fix this by defining the `__str__` method — it tells Python how to convert your object to a string.
class Codemon:
def __init__(self, name, element, level=1):
self.name = name
self.element = element
self.level = level
def __str__(self):
return f"{self.name} ({self.element}) - Lv.{self.level}"
myCdm = Codemon("Aquachu", "Water", 15)
print(myCdm)
Now `print(myCdm)` shows something readable! `__str__` must return a string (not print it).
Exercise 10 — Define __str__
Complete the `__str__` method to display the Codémon nicely.
class Codemon:
def __init__(self, name, hp=100):
self.name = name
self.hp = hp
def ____(self):
return f"{self.name}: {self.hp} HP"
myCdm = Codemon("Pyrochu", 85)
print(____)
Options: __str__ __init__ myCdm self str
Hint : The special method for string representation starts and ends with double underscores. To print an object, you pass the variable to `print()`.
Exercise 11 — Methods Vocabulary
Match each piece of code with its description.
Items to match
• `def method(self):`
• `def method(self, x):`
• `self.attribute`
• `return value`
• `__str__(self)`
Definitions
• A method with no extra parameters
• A method with one extra parameter
• Access the object's data inside a method
• Give back a result from a method
• Controls how print() displays the object
Exercise 12 — Codémon Battle Methods
Add methods to the Codémon class:
1. `attack(self, target)` — reduces target's HP by `self.level * 2`. Print `"X attacks Y!"`
2. `is_alive(self)` — returns `True` if HP > 0
3. `__str__(self)` — returns `"Name (Lv.X) - HP: Y"`
4. Create 2 Codémon, have them attack each other, then print both
Starter code
class Codemon:
def __init__(self, name, level=1, hp=100):
self.name = name
self.level = level
self.hp = hp
# Add attack method
# Add is_alive method
# Add __str__ method
# Create 2 Codémon and battle!