Python Properties

Properties let you access methods like attributes. Instead of: - person.get_name() - person.set_name("Sarah") You can write: - person.name - person.name = "Sarah" But still run validation behind the scenes. ## Why properties matter They help you: - validate inputs - create read-only attributes - compute values dynamically - keep your API clean ## Basic property (getter + setter) ```python class Person: def __init__(self, name): self._name = name # underscore = internal @property def name(self): return self._name @name.setter def name(self, value): if not value or not value.strip(): raise ValueError("Name cannot be empty") self._name = value.strip() p = Person("Tom") print(p.name) p.name = " Sarah " print(p.name) ``` Expected output: ``` Tom Sarah ``` ## Read-only property ```python import math class Circle: def __init__(self, radius): self.radius = radius @property def area(self): return math.pi * (self.radius ** 2) c = Circle(5) print(round(c.area, 2)) ``` Here, `area` has no setter, so it cannot be directly changed. ## Computed property with two-way conversion ```python class Temperature: def __init__(self, celsius): self._celsius = celsius @property def celsius(self): return self._celsius @property def fahrenheit(self): return (self._celsius * 9/5) + 32 @fahrenheit.setter def fahrenheit(self, value): self._celsius = (value - 32) * 5/9 t = Temperature(25) print(t.fahrenheit) t.fahrenheit = 86 print(round(t.celsius, 2)) ``` Expected output: ``` 77.0 30.0 ``` ## Real-world example: bank account validation ```python class BankAccount: def __init__(self, balance): self.balance = balance @property def balance(self): return self._balance @balance.setter def balance(self, value): if value < 0: raise ValueError("Balance cannot be negative") self._balance = value acc = BankAccount(1000) acc.balance = 1200 print(acc.balance) # acc.balance = -10 # would raise error ``` ## Graph: property access ```mermaid flowchart LR A[Read: obj.name] --> B[@property getter] C[Write: obj.name = value] --> D[@name.setter] B --> E[Return data] D --> F[Validate + store] ``` ## Key points - Properties look like attributes, behave like methods - Great for validation and computed values - Keeps public API stable even if internals change