OOP & classes
Classes, instances, methods, dataclasses, and object-oriented design in Python.
Composable Predicates with the &, |, ~ Operators in Python
Define a reusable Predicate class that combines boolean checks with & (AND), | (OR), and ~ (NOT) operators.
class Predicate:
def __init__(self, func, name=None):
self.func = func
self.name = name or getattr(func, "__name__", "predicate")
def __call__(self, value):
return self.func(value)
def __and__(self, other):
return Predicate(lambda v: self(v) and other(v), f"({self.name} AN…How to Implement the Command Pattern with Undo in Python
Python code demonstrating the Command design pattern with undo and redo support using action objects and a history manager.
class Command:
def execute(self):
raise NotImplementedError
def undo(self):
raise NotImplementedError
class AddTextCommand(Command):
def __init__(self, document, text):
self.document = document
self.text = text
def execute(self):
self.document.append(self.tex…How to Implement the State Pattern in Python
Implement the State design pattern in Python by delegating behavior to state objects, letting a media player change actions dynamically without if-else chains.
class State:
def play(self, player): pass
def pause(self, player): pass
def stop(self, player): pass
class PlayingState(State):
def play(self, player):
return "Already playing"
def pause(self, player):
player.state = PausedState()
return "Pausing playback"
def stop(self…How to Lazy Load an Expensive Attribute with a Proxy in Python
This code shows a Proxy class that lazily loads an ExpensiveResource only when first accessed, caching it for subsequent uses.
class ExpensiveResource:
def __init__(self, name):
self.name = name
print(f"Expensive resource '{name}' created (e.g., DB connection)")
def use(self):
return f"Using {self.name}"
class Proxy:
def __init__(self, name):
self._name = name
self._resource = None
@p…How to Use abstractmethod in Python
Define an abstract base class with abstract methods to enforce a common interface across subclasses.
import abc
class Shape(abc.ABC):
@abc.abstractmethod
def area(self):
"""Calculate area of the shape."""
@abc.abstractmethod
def perimeter(self):
"""Calculate perimeter of the shape."""
class Rectangle(Shape):
def __init__(self, width, height):
self.width = width
s…How to implement a Facade class to simplify subsystem calls in Python
Use a Facade class to wrap complex subsystem interactions behind a simple start() method, hiding the details and providing a clean interface.
class CPU:
def freeze(self):
print("CPU: freezing")
def jump(self, position):
print(f"CPU: jumping to {position}")
def execute(self):
print("CPU: executing")
class Memory:
def load(self, position, data):
print(f"Memory: loading '{data}' at {position}")
class HardDr…Implement the Strategy Pattern with Interchangeable Algorithm Classes in Python
Uses abstract base classes to define a SortStrategy interface, then swaps between BubbleSort and QuickSort at runtime.
from abc import ABC, abstractmethod
from typing import List
class SortStrategy(ABC):
@abstractmethod
def sort(self, data: List[int]) -> List[int]:
pass
class BubbleSort(SortStrategy):
def sort(self, data: List[int]) -> List[int]:
result = data[:]
n = len(result)
for i in…Unit of Work Pattern: Track Changes, Commit, and Rollback in Python
This code defines a UnitOfWork class that tracks operations (add) and supports commit to apply changes and rollback to revert them, using a dataclass-based logger.
from dataclasses import dataclass, field
from typing import Any, Callable, List, Tuple
@dataclass
class UnitOfWork:
log: List[Tuple[str, Callable, tuple, dict]] = field(default_factory=list)
def track(self, operation: str, fn: Callable, *args, **kwargs):
self.log.append((operation, fn, args, kwargs)…Visitor Pattern in Python: Double Dispatch Demo
Demonstrates the Visitor design pattern with double dispatch so operations on Dog and Cat objects are selected at runtime without modifying their classes.
class Animal:
def accept(self, visitor):
visitor.visit(self)
class Dog(Animal):
def speak(self):
return "Woof!"
class Cat(Animal):
def speak(self):
return "Meow!"
class SoundVisitor:
def visit(self, animal):
if isinstance(animal, Dog):
return self.visit_do…Browse by section
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OOP & classes — Python code examples
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