Classes & Objects

1# ============================================================
2# Basic class definition
3# ============================================================
4class BankAccount:
5    """A simple bank account with deposit and withdrawal."""
6
7    # Class attribute — shared by all instances
8    bank_name = "Python National Bank"
9    _total_accounts = 0
10
11    def __init__(self, owner: str, balance: float = 0.0):
12        """Initialize a new account."""
13        self.owner = owner            # Public instance attribute
14        self._balance = balance       # "Private" by convention
15        self.__id = id(self)          # Name-mangled (harder to access)
16        BankAccount._total_accounts += 1
17
18    def deposit(self, amount: float) -> float:
19        """Deposit money into the account."""
20        if amount <= 0:
21            raise ValueError("Deposit amount must be positive")
22        self._balance += amount
23        return self._balance
24
25    def withdraw(self, amount: float) -> float:
26        """Withdraw money from the account."""
27        if amount <= 0:
28            raise ValueError("Withdrawal amount must be positive")
29        if amount > self._balance:
30            raise ValueError("Insufficient funds")
31        self._balance -= amount
32        return self._balance
33
34    def get_balance(self) -> float:
35        """Return current balance."""
36        return self._balance
37
38    def __str__(self) -> str:
39        """Human-readable string representation."""
40        return f"Account({self.owner}: ${self._balance:,.2f})"
41
42    def __repr__(self) -> str:
43        """Developer-friendly representation."""
44        return f"BankAccount(owner={self.owner!r}, balance={self._balance})"
45
46    @classmethod
47    def get_total_accounts(cls) -> int:
48        """Return total number of accounts created."""
49        return cls._total_accounts
50
51    @classmethod
52    def from_dict(cls, data: dict) -> "BankAccount":
53        """Alternative constructor from dictionary."""
54        return cls(owner=data["owner"], balance=data.get("balance", 0))
55
56    @staticmethod
57    def validate_amount(amount: float) -> bool:
58        """Validate that an amount is a positive number."""
59        return isinstance(amount, (int, float)) and amount > 0
60
61
62# ============================================================
63# Using the class
64# ============================================================
65# Create instances
66alice = BankAccount("Alice", 1000)
67bob = BankAccount("Bob")
68
69# Instance methods
70alice.deposit(500)
71print(alice)  # Account(Alice: $1,500.00)
72print(repr(alice))  # BankAccount(owner='Alice', balance=1500)
73
74# Class method
75print(BankAccount.get_total_accounts())  # 2
76
77# Alternative constructor
78data = {"owner": "Charlie", "balance": 750}
79charlie = BankAccount.from_dict(data)
80
81# Static method
82print(BankAccount.validate_amount(100))   # True
83print(BankAccount.validate_amount(-50))   # False
84
85# ============================================================
86# Class vs Instance attributes
87# ============================================================
88class Dog:
89    species = "Canis familiaris"  # Class attribute
90
91    def __init__(self, name, breed):
92        self.name = name    # Instance attribute
93        self.breed = breed  # Instance attribute
94
95fido = Dog("Fido", "Labrador")
96buddy = Dog("Buddy", "Poodle")
97
98# Both share the class attribute
99print(fido.species)   # "Canis familiaris"
100print(buddy.species)  # "Canis familiaris"
101
102# Modifying class attribute affects all instances
103Dog.species = "Canis lupus familiaris"
104print(fido.species)   # "Canis lupus familiaris"
105
106# But assigning to instance creates a new instance attribute!
107fido.species = "Custom"     # Creates instance attribute
108print(fido.species)         # "Custom" (instance attribute)
109print(buddy.species)        # "Canis lupus familiaris" (still class attribute)
110print(Dog.species)          # "Canis lupus familiaris" (class attribute unchanged)
111
112# ============================================================
113# Name mangling (__double_underscore)
114# ============================================================
115class Secret:
116    def __init__(self):
117        self.__hidden = "secret value"
118
119s = Secret()
120# print(s.__hidden)          # AttributeError!
121print(s._Secret__hidden)     # "secret value" (mangled name)
122
123# Name mangling prevents accidental override in subclasses
124class Parent:
125    def __init__(self):
126        self.__value = "parent"
127
128class Child(Parent):
129    def __init__(self):
130        super().__init__()
131        self.__value = "child"  # This is _Child__value, NOT _Parent__value
132
133c = Child()
134print(c._Parent__value)  # "parent"
135print(c._Child__value)   # "child"
136
137# ============================================================
138# @classmethod vs @staticmethod
139# ============================================================
140class Date:
141    def __init__(self, year, month, day):
142        self.year = year
143        self.month = month
144        self.day = day
145
146    @classmethod
147    def from_string(cls, date_string):
148        """Parse 'YYYY-MM-DD' string. Uses cls so subclasses work."""
149        year, month, day = map(int, date_string.split("-"))
150        return cls(year, month, day)  # cls, not Date!
151
152    @classmethod
153    def today(cls):
154        """Create a Date for today."""
155        import datetime
156        t = datetime.date.today()
157        return cls(t.year, t.month, t.day)
158
159    @staticmethod
160    def is_valid(year, month, day):
161        """Check if date is valid. No class/instance access needed."""
162        return 1 <= month <= 12 and 1 <= day <= 31
163
164    def __str__(self):
165        return f"{self.year:04d}-{self.month:02d}-{self.day:02d}"
166
167d = Date.from_string("2024-06-15")
168print(d)  # 2024-06-15
169print(Date.is_valid(2024, 13, 1))  # False