Advanced OOP Topics

9.3. Advanced OOP Topics#

This notebook covers five topics that extend the core OOP material:

Topic	What it adds
Comparison dunder methods	Value equality, ordering, and hash behavior for custom objects
Operator overloading	Making custom objects work with operators like `+` and `*`
`@dataclass` in depth	Auto-generated `__init__`, `__repr__`, `__eq__`, ordering, frozen instances
Class vs. instance variables	A common source of bugs, explained clearly
Static and class methods	Utility behavior and alternative constructors

9.3.1. Comparison Dunder Methods#

By default, == on a custom object tests identity (same as is), not value equality. Defining __eq__ changes that behavior to value comparison.

class Point:
    def __init__(self, x, y):
        self.x = x
        self.y = y

    def __repr__(self):
        return f'Point({self.x}, {self.y})'

p1 = Point(1, 2)
p2 = Point(1, 2)

print(p1 == p2)   # False — identity check by default
print(p1 is p2)   # False

False
False

Adding __eq__ makes == compare by value instead.

class Point:
    def __init__(self, x, y):
        self.x = x
        self.y = y

    def __repr__(self):
        return f'Point({self.x}, {self.y})'

    def __eq__(self, other):
        if not isinstance(other, Point):
            return NotImplemented
        return self.x == other.x and self.y == other.y

p1 = Point(1, 2)
p2 = Point(1, 2)
p3 = Point(3, 4)

print(p1 == p2)   # now True
print(p1 is p2)   # still False
print(p1 == p3)   # False

# print(p1)

True
False
False

9.3.1.1. Ordering with `lt`#

To support sorting (sorted(), min(), max()), define __lt__ (less than). You don’t need to write all six comparison methods by hand. The @functools.total_ordering decorator derives the missing ordering methods from two definitions you supply:

__eq__: required; the decorator never derives it (equality has different semantics from ordering and is deliberately left to you).
Any one of __lt__, __le__, __gt__, or __ge__: the decorator fills in the remaining three.

You define	Decorator derives
`__eq__`	(nothing; you must always supply this yourself)
one of `__lt__` / `__le__` / `__gt__` / `__ge__`	the other three ordering methods

Using __lt__ is conventional: < reads naturally as “is a less than b?”, which maps cleanly onto sort order, but any of the four works.

For example, if you define __lt__, then a > b becomes b < a, and a >= b becomes not (a < b).

Performance note: because the derived methods add an extra function call layer, @total_ordering is marginally slower than writing all six methods explicitly. The difference is negligible in typical code; only consider hand-writing them if profiling shows comparison is a bottleneck in a tight inner loop.

from functools import total_ordering
import math

@total_ordering
class Point:
    def __init__(self, x, y):
        self.x = x
        self.y = y

    def __repr__(self):
        return f'Point({self.x}, {self.y})'

    def __eq__(self, other):
        if not isinstance(other, Point):
            return NotImplemented
        return self.x == other.x and self.y == other.y

    def __lt__(self, other):                        ### compare distances from the origin
        """Sort by distance from the origin."""
        if not isinstance(other, Point):
            return NotImplemented
        return math.hypot(self.x, self.y) < math.hypot(other.x, other.y)    

points = [Point(3, 4), Point(1, 1), Point(0, 2)]
print(sorted(points))   # sorted by distance from origin
print(min(points))

# __gt__ is derived automatically — no extra code needed
p_near = Point(1, 1)    # distance ≈ 1.41
p_far  = Point(3, 4)    # distance = 5.0

print(p_far > p_near)   # True  — total_ordering derives __gt__ from __lt__
print(p_near > p_far)   # False

[Point(1, 1), Point(0, 2), Point(3, 4)]
Point(1, 1)
True
False

9.3.1.2. Hashability and `hash`#

When you define __eq__, Python automatically sets __hash__ to None, making instances unhashable (cannot be used in sets or as dict keys). Define __hash__ explicitly to restore that ability.

# Rule of thumb: objects that compare equal must have the same hash.
def __hash__(self):
    return hash((self.x, self.y))

If your object is mutable, do not define __hash__; mutable objects should not be hashed because their value (and their hash) could change after insertion.

from functools import total_ordering
import math

@total_ordering
class Point:
    def __init__(self, x, y):
        self.x = x
        self.y = y

    def __repr__(self):
        return f'Point({self.x}, {self.y})'

    def __eq__(self, other):
        if not isinstance(other, Point):
            return NotImplemented
        return self.x == other.x and self.y == other.y

    def __lt__(self, other):
        if not isinstance(other, Point):
            return NotImplemented
        return math.hypot(self.x, self.y) < math.hypot(other.x, other.y)

    def __hash__(self):
        return hash((self.x, self.y))

p1 = Point(1, 2)
p2 = Point(1, 2)
p3 = Point(3, 4)

point_set = {p1, p2, p3}
print(point_set)          # p1 and p2 are equal — only one appears

lookup = {p1: 'origin-ish', p3: 'far'}
print(lookup[p2])         # works because p2 == p1 and hash(p2) == hash(p1)

{Point(1, 2), Point(3, 4)}
origin-ish

9.3.2. Operator Overloading#

By defining special methods, you can control how Python operators behave on your own types. For every operator there is a corresponding dunder method:

Operator	Method	Example
`+`	`__add__`	`a + b`
`==`	`__eq__`	`a == b`
`<`	`__lt__`	`a < b`
`len()`	`__len__`	`len(a)`

This section focuses on arithmetic operators; comparison operators (__eq__, __lt__, __hash__) are covered above.

Here is __add__ defined on BankAccount. When two accounts are added together, a new merged account is returned:

class BankAccount:
    """BankAccount with operator overloading."""

    def __init__(self, owner="Unknown", balance=0.0):
        self.owner = owner
        self._balance = balance

    @property
    def balance(self):
        return self._balance

    def __str__(self):
        return f"BankAccount(owner={self.owner}, balance=${self._balance:.2f})"

    def __repr__(self):
        return f"BankAccount('{self.owner}', {self._balance})"

    def __add__(self, other):
        """Merge two accounts into one."""
        if not isinstance(other, BankAccount):
            return NotImplemented
        merged_owner = f"{self.owner}&{other.owner}"
        merged_balance = self._balance + other._balance
        return BankAccount(merged_owner, merged_balance)

a = BankAccount("Ava", 12.00)
b = BankAccount("Ben", 8.00)
print(a + b)   # BankAccount(owner=Ava&Ben, balance=$20.00)

BankAccount(owner=Ava&Ben, balance=$20.00)

When Python evaluates a + b, it calls a.__add__(b) automatically. Changing the behavior of an operator so that it works with programmer-defined types is called operator overloading.

9.3.3. `@dataclass`#

@dataclass is a decorator from the standard library that auto-generates common dunder methods (__init__, __repr__, __eq__) for a class based on its annotated fields, eliminating boilerplate.

Dataclasses give type annotations a second job. In a regular class, name: str is mostly a hint for readers and tools. In a dataclass, an annotated class variable also becomes a field that @dataclass uses to generate __init__, __repr__, and comparison behavior.

Here we look at its most useful options.

Option	Effect
`eq=True` (default)	Auto-generates `__eq__` based on fields
`order=True`	Also generates `__lt__`, `__le__`, `__gt__`, `__ge__`
`frozen=True`	Makes instances immutable; also enables `__hash__`
`field(default_factory=...)`	Safe default for mutable fields like lists

When order=True is set, fields are compared in declaration order — here name first, then gpa — so sorted([s1, s2]) places Alice before Bob because 'Alice' < 'Bob' alphabetically.

from dataclasses import dataclass, field

@dataclass(order=True)
class Student:
    name: str
    gpa: float
    courses: list[str] = field(default_factory=list)   # safe mutable default

s1 = Student('Alice', 3.8)
s2 = Student('Bob', 3.5)
s3 = Student('Alice', 3.8)

print(s1 == s3)          # True — same field values
print(sorted([s1, s2]))  # sorted lexicographically by (name, gpa)
s1.courses.append('CS101')
print(s1)

True
[Student(name='Alice', gpa=3.8, courses=[]), Student(name='Bob', gpa=3.5, courses=[])]
Student(name='Alice', gpa=3.8, courses=['CS101'])

9.3.3.1. Frozen Dataclasses#

frozen=True prevents attribute mutation after creation and automatically provides a correct __hash__, making instances usable in sets and as dict keys.

from dataclasses import dataclass

@dataclass(frozen=True)
class Color:
    r: int
    g: int
    b: int

red = Color(255, 0, 0)
print(red)
print(hash(red))   # hashable

palette = {red, Color(0, 255, 0), Color(0, 0, 255)}
print(palette)

try:
    red.r = 128    # raises FrozenInstanceError
except Exception as e:
    print(type(e).__name__, e)

Color(r=255, g=0, b=0)
4091835460043580556
{Color(r=0, g=255, b=0), Color(r=255, g=0, b=0), Color(r=0, g=0, b=255)}
FrozenInstanceError cannot assign to field 'r'

9.3.4. Class vs. Instance Variables#

A class variable is defined directly in the class body, outside any method. It is shared across all instances. An instance variable is set on self inside a method and belongs only to that one object.

Confusing the two is one of the most common OOP bugs in Python.

class Dog:
    species = 'Canis lupus familiaris'   # class variable — shared by all dogs

    def __init__(self, name):
        self.name = name                  # instance variable — unique per dog

d1 = Dog('Rex')
d2 = Dog('Fido')

print(d1.species)    # 'Canis lupus familiaris'
print(d2.species)    # same — shared
print(d1.name)       # 'Rex'
print(d2.name)       # 'Fido'

Canis lupus familiaris
Canis lupus familiaris
Rex
Fido

9.3.4.1. The Mutation Trap#

Assigning to a class variable via an instance creates a new instance variable that shadows the class variable — it does not change the class variable for all instances.

But mutating a mutable class variable (like a list) does affect all instances, because no new variable is created.

class Counter:
    count = 0            # class variable
    history = []         # mutable class variable — danger zone

    def __init__(self, name):
        self.name = name

a = Counter('a')
b = Counter('b')

# Reassignment via instance — creates a new instance variable on `a` only
a.count = 99
print(a.count)           # 99  — instance variable on a
print(b.count)           # 0   — class variable unchanged
print(Counter.count)     # 0

# Mutation via instance — modifies the shared class-level list
a.history.append('event')
print(b.history)         # ['event'] — b sees the change!
print(Counter.history)   # ['event']

99
0
0
['event']
['event']

Rule of thumb:

Use class variables for constants or data that truly belongs to the class (e.g., species, MAX_SIZE).
Use instance variables (set in __init__) for data that belongs to individual objects.
Never use a mutable class variable as a default container — use field(default_factory=list) with @dataclass, or set the list in __init__.

9.3.5. Static and Class Methods#

Not every method needs an instance. Python provides two decorators for methods that are attached to the class itself rather than an instance:

Decorator	First parameter	Typical use
`@staticmethod`	(none)	Utility function logically grouped with the class
`@classmethod`	`cls` (the class itself)	Alternative constructors / factory methods

Both decorators come up naturally alongside class variables, because all three belong to the class rather than any one instance.

9.3.5.1. Static Methods#

A static method is a regular function that lives inside a class for organizational reasons. It receives neither self nor cls, so it cannot access instance or class state directly.

A common use-case is a validation or parsing helper that supports other methods without depending on object state:

class BankAccount:
    """Bank account used to demonstrate static and class methods."""

    def __init__(self, owner="Unknown", balance=0.0):
        self.owner = owner
        self._balance = balance

    @property
    def balance(self):
        return self._balance

    def deposit(self, amount):
        self._balance += amount
        return self

    def withdraw(self, amount):
        if amount > self._balance:
            raise ValueError("Insufficient funds")
        self._balance -= amount
        return self

    def __str__(self):
        return f"BankAccount(owner={self.owner}, balance=${self._balance:.2f})"

    def __repr__(self):
        return f"BankAccount('{self.owner}', {self._balance})"

    # -- static method ------------------------------------------------
    @staticmethod
    def parse_record(s):
        """Parse an 'owner:balance' string into raw values."""
        owner, balance = s.split(":")
        return owner, float(balance)

Because parse_record is a static method, it has no self or cls parameter. It is a utility helper that can be called on the class directly:

owner, balance = BankAccount.parse_record("Taylor:348.00")
acct = BankAccount(owner, balance)
print(acct)   # BankAccount(owner=Taylor, balance=$348.00)

BankAccount(owner=Taylor, balance=$348.00)

9.3.5.2. Class Methods#

A class method receives the class as its first argument (cls). This makes it better than a static method when subclasses are involved: cls(...) creates an instance of the actual subclass, not the hardcoded parent class.

Here is from_string rewritten as a class method:

class BankAccount:
    """Bank account — class-method version of from_string."""

    def __init__(self, owner="Unknown", balance=0.0):
        self.owner = owner
        self._balance = balance

    @property
    def balance(self):
        return self._balance

    def deposit(self, amount):
        self._balance += amount
        return self

    def withdraw(self, amount):
        if amount > self._balance:
            raise ValueError("Insufficient funds")
        self._balance -= amount
        return self

    def __str__(self):
        return f"BankAccount(owner={self.owner}, balance=${self._balance:.2f})"

    def __repr__(self):
        return f"BankAccount('{self.owner}', {self._balance})"

    # ── class method ────────────────────────────────────────────
    @classmethod
    def from_string(cls, s):
        """Create an instance from an 'owner:balance' string."""
        owner, balance = s.split(":")
        return cls(owner, float(balance))

    @classmethod
    def zero_balance(cls, owner):
        """Return an account with a zero balance."""
        return cls(owner, 0.0)

acct1 = BankAccount.from_string("Taylor:348.00")
print(acct1)  # BankAccount(owner=Taylor, balance=$348.00)

BankAccount(owner=Taylor, balance=$348.00)

# Alternative constructors — both work on subclasses automatically
acct1 = BankAccount.from_string("Casey:94.50")
acct2 = BankAccount.zero_balance("Rin")
print(acct1)   # BankAccount(owner=Casey, balance=$94.50)
print(acct2)   # BankAccount(owner=Rin, balance=$0.00)

BankAccount(owner=Casey, balance=$94.50)
BankAccount(owner=Rin, balance=$0.00)

When to use which:

	`@staticmethod`	`@classmethod`
Receives class?	No	Yes (`cls`)
Subclass-safe?	No — hardcodes class name	Yes — `cls(...)` creates the right type
Typical use	Pure utility / validation helper	Alternative constructors

Prefer @classmethod for constructors; use @staticmethod only for helpers that truly need no access to the class.

### Exercise: Class Methods
#   Create a subclass ExtendedBankAccount(BankAccount) and add a class
#   method from_balance_str(cls, owner, s) that parses a dollar string
#   like "12.34" and returns a new account.
#   Test:
#       print(ExtendedBankAccount.from_balance_str("Kai", "12.34"))
### Your code starts here.


### Your code ends here.

BankAccount(owner=Kai, balance=$12.34)

9.3.6. Summary#

Topic	Key takeaway
`__eq__` / `__lt__` / `__hash__`	Define these to make objects sortable and hashable; remember the mutability rule for `__hash__`
`@dataclass`	Use `order=True` for sorting, `frozen=True` for hashable immutable objects, `field(default_factory=...)` for mutable defaults
Class vs. instance variables	Keep mutable state in instance variables; treat class variables as shared constants
Static / class methods	`@staticmethod` for pure helpers; `@classmethod` for alternative constructors (subclass-safe)
Operator overloading	Define `__add__`, `__eq__`, etc. to give your objects natural operator syntax