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Master Python
🧠 Real-World Analogy — Superpowers
Think of a superhero suit — Tony Stark's Iron Man armor. The suit itself is an object. When Tony raises both hands, repulsors fire. When he raises one hand, a laser fires. Each specific gesture triggers a different ability. These special gestures are like Python's magic methods — special methods that automatically trigger when you use specific Python syntax or built-in functions. They start and end with double underscores (dunder = double underscore): __method__1️⃣ __init__ — Constructor / Initializer
python example
class Book:
def __init__(self, title, author, pages, price):
"""Called automatically when Book(...) is created."""
self.title = title
self.author = author
self.pages = pages
self.price = price
self._id = id(self) # unique Python object id
# We'll add more dunders below...Already covered in detail in constructors.mdx. Here we focus on how it interacts with other magic methods.2️⃣ __str__ and __repr__
Example
class Book:
def __init__(self, title, author, pages, price):
self.title = title
self.author = author
self.pages = pages
self.price = price
def __str__(self):
"""Human-readable: used by print(), str()"""
return f'"{self.title}" by {self.author} ({self.pages} pages) — ₹{self.price}'
def __repr__(self):
"""Developer-readable: used by repr(), REPL, debugging"""
return (f"Book(title={self.title!r}, author={self.author!r}, "
f"pages={self.pages}, price={self.price})")
book = Book("Python Mastery", "WoHoTech", 450, 599)
print(book) # calls __str__
print(str(book)) # calls __str__
print(repr(book)) # calls __repr__
# In a list, repr() is used:
library = [book, Book("Clean Code", "Robert Martin", 464, 799)]
print(library)
Output:
Example
"Python Mastery" by WoHoTech (450 pages) — ₹599
"Python Mastery" by WoHoTech (450 pages) — ₹599
Book(title='Python Mastery', author='WoHoTech', pages=450, price=599)
[Book(title='Python Mastery', ...), Book(title='Clean Code', ...)]
Rule of thumb:__repr__should return a string that, when passed toeval(), recreates the object.__str__is for end users.
3️⃣ __len__ — Length
Example
class Playlist:
def __init__(self, name):
self.name = name
self._songs = []
def add(self, song):
self._songs.append(song)
def __len__(self):
"""Called by len(playlist)"""
return len(self._songs)
def __str__(self):
return f"Playlist '{self.name}' [{len(self)} songs]"
def __bool__(self):
"""Called in boolean context — True if not empty"""
return len(self) > 0
p = Playlist("Chill Vibes")
print(f"Empty: {bool(p)}") # False
print(f"Len: {len(p)}") # 0
p.add("Tum Hi Ho")
p.add("Kal Ho Na Ho")
p.add("Raabta")
print(f"Len: {len(p)}") # 3
print(f"Empty: {bool(p)}") # True
print(p)
Output:
| Empty | False |
| Len | 0 |
| Len | 3 |
| Empty | True |
4️⃣ __add__, __sub__, __mul__ — Arithmetic
python example
class Fraction:
"""Rational number with full arithmetic support."""
from math import gcd as _gcd
def __init__(self, numerator, denominator=1):
if denominator == 0:
raise ZeroDivisionError("Denominator cannot be zero.")
from math import gcd
common = gcd(abs(numerator), abs(denominator))
sign = -1 if denominator < 0 else 1
self.num = sign * numerator // common
self.den = sign * denominator // common
def __add__(self, other): # self + other
if isinstance(other, int):
other = Fraction(other)
return Fraction(self.num * other.den + other.num * self.den,
self.den * other.den)
def __radd__(self, other): # other + self (e.g., 1 + Fraction)
return self.__add__(other)
def __sub__(self, other): # self - other
if isinstance(other, int):
other = Fraction(other)
return Fraction(self.num * other.den - other.num * self.den,
self.den * other.den)
def __mul__(self, other): # self * other
if isinstance(other, int):
other = Fraction(other)
return Fraction(self.num * other.num, self.den * other.den)
def __truediv__(self, other): # self / other
if isinstance(other, int):
other = Fraction(other)
return Fraction(self.num * other.den, self.den * other.num)
def __neg__(self): # -self
return Fraction(-self.num, self.den)
def __abs__(self): # abs(self)
return Fraction(abs(self.num), self.den)
def __float__(self): # float(self)
return self.num / self.den
def __str__(self):
if self.den == 1: return str(self.num)
return f"{self.num}/{self.den}"
def __repr__(self):
return f"Fraction({self.num}, {self.den})"
a = Fraction(1, 2) # 1/2
b = Fraction(1, 3) # 1/3
c = Fraction(3, 4) # 3/4
print(f"a = {a}")
print(f"b = {b}")
print(f"a + b = {a + b}") # 1/2 + 1/3 = 5/6
print(f"a - b = {a - b}") # 1/2 - 1/3 = 1/6
print(f"a * b = {a * b}") # 1/2 * 1/3 = 1/6
print(f"a / b = {a / b}") # 1/2 / 1/3 = 3/2
print(f"a + 1 = {a + 1}") # 1/2 + 1 = 3/2
print(f"1 + a = {1 + a}") # uses __radd__
print(f"-a = {-a}")
print(f"abs(-a) = {abs(-a)}")
print(f"float(a) = {float(a)}")
print(f"a + b + c = {a + b + c}") # chained additionOutput:
Example
a = 1/2
b = 1/3
a + b = 5/6
a - b = 1/6
a * b = 1/6
a / b = 3/2
a + 1 = 3/2
1 + a = 3/2
-a = -1/2
abs(-a) = 1/2
float(a) = 0.5
a + b + c = 19/12
5️⃣ __eq__, __lt__ — Comparison (Rich Comparison)
Example
from functools import total_ordering
@total_ordering # auto-generates <=, >, >= from __eq__ and __lt__
class Student:
def __init__(self, name, gpa):
self.name = name
self.gpa = gpa
def __eq__(self, other):
if not isinstance(other, Student):
return NotImplemented
return self.gpa == other.gpa
def __lt__(self, other):
if not isinstance(other, Student):
return NotImplemented
return self.gpa < other.gpa
def __hash__(self): # required when __eq__ is defined
return hash((self.name, self.gpa))
def __str__(self):
return f"{self.name} (GPA: {self.gpa:.2f})"
def __repr__(self):
return f"Student({self.name!r}, {self.gpa})"
students = [
Student("Arjun", 8.9),
Student("Priya", 9.4),
Student("Rahul", 7.5),
Student("Sneha", 9.4),
Student("Vikas", 8.1),
]
print("Unsorted:")
for s in students: print(f" {s}")
sorted_students = sorted(students, reverse=True)
print("\nSorted (high → low GPA):")
for s in sorted_students: print(f" {s}")
best = max(students)
print(f"\nTop student: {best}")
# All @total_ordering generated comparisons work:
a, b = students[0], students[1]
print(f"\n{a} == {b}: {a == b}")
print(f"{a} < {b}: {a < b}")
print(f"{a} > {b}: {a > b}")
print(f"{a} <= {b}: {a <= b}")
print(f"{a} >= {b}: {a >= b}")
# Use in set (requires __hash__)
top_students = {s for s in students if s.gpa >= 9.0}
print(f"\nTop students (GPA ≥ 9.0): {top_students}")
Output:
| Arjun (GPA | 8.90) |
| Priya (GPA | 9.40) |
| Rahul (GPA | 7.50) |
| Sneha (GPA | 9.40) |
| Vikas (GPA | 8.10) |
| Sorted (high | low GPA): |
| Priya (GPA | 9.40) |
| Sneha (GPA | 9.40) |
| Arjun (GPA | 8.90) |
| Vikas (GPA | 8.10) |
| Rahul (GPA | 7.50) |
| Top student | Priya (GPA: 9.40) |
| Arjun (GPA | 8.90) == Priya (GPA: 9.40): False |
| Arjun (GPA | 8.90) < Priya (GPA: 9.40): True |
| Arjun (GPA | 8.90) > Priya (GPA: 9.40): False |
| Arjun (GPA | 8.90) <= Priya (GPA: 9.40): True |
| Arjun (GPA | 8.90) >= Priya (GPA: 9.40): False |
| Top students (GPA ≥ 9.0) | {Student('Priya', 9.4), Student('Sneha', 9.4)} |
6️⃣ __getitem__, __setitem__, __delitem__ — Container Protocol
Example
class Matrix:
"""2D matrix with dict-style access."""
def __init__(self, rows, cols, default=0):
self.rows = rows
self.cols = cols
self._data = [[default] * cols for _ in range(rows)]
def _validate(self, row, col):
if not (0 <= row < self.rows and 0 <= col < self.cols):
raise IndexError(f"Index ({row},{col}) out of bounds "
f"for {self.rows}×{self.cols} matrix.")
def __getitem__(self, key):
"""m[row, col] or m[row]"""
if isinstance(key, tuple):
row, col = key
self._validate(row, col)
return self._data[row][col]
return self._data[key]
def __setitem__(self, key, value):
"""m[row, col] = value"""
if isinstance(key, tuple):
row, col = key
self._validate(row, col)
self._data[row][col] = value
else:
self._data[key] = value
def __delitem__(self, key):
"""del m[row, col] — resets to 0"""
if isinstance(key, tuple):
row, col = key
self._validate(row, col)
self._data[row][col] = 0
def __len__(self):
return self.rows * self.cols
def __str__(self):
lines = []
for row in self._data:
lines.append(" | " + " ".join(f"{v:4}" for v in row) + " |")
return "\n".join(lines)
m = Matrix(3, 3)
# Set values
m[0, 0] = 1; m[0, 1] = 2; m[0, 2] = 3
m[1, 0] = 4; m[1, 1] = 5; m[1, 2] = 6
m[2, 0] = 7; m[2, 1] = 8; m[2, 2] = 9
print("Matrix:")
print(m)
print(f"\nm[1, 1] = {m[1, 1]}") # 5
print(f"m[0] = {m[0]}") # [1, 2, 3]
del m[1, 1] # reset to 0
print(f"\nAfter del m[1,1]:")
print(m)
Output:
Example
Matrix:
| 1 2 3 |
| 4 5 6 |
| 7 8 9 |
m[1, 1] = 5
m[0] = [1, 2, 3]
After del m[1,1]:
| 1 2 3 |
| 4 0 6 |
| 7 8 9 |
7️⃣ __iter__ and __next__ — Iterator Protocol
Example
class NumberRange:
"""Custom range object — iterable and iterator."""
def __init__(self, start, stop, step=1):
self.start = start
self.stop = stop
self.step = step
def __iter__(self):
"""Return iterator object (self is its own iterator)."""
self._current = self.start
return self
def __next__(self):
"""Return next value, or raise StopIteration."""
if (self.step > 0 and self._current >= self.stop) or \
(self.step < 0 and self._current <= self.stop):
raise StopIteration
value = self._current
self._current += self.step
return value
def __len__(self):
if self.step > 0:
return max(0, (self.stop - self.start + self.step - 1) // self.step)
return max(0, (self.start - self.stop - self.step - 1) // (-self.step))
def __contains__(self, item):
if self.step > 0:
return self.start <= item < self.stop and (item - self.start) % self.step == 0
return self.stop < item <= self.start and (self.start - item) % (-self.step) == 0
def __repr__(self):
return f"NumberRange({self.start}, {self.stop}, {self.step})"
# for loop
evens = NumberRange(0, 20, 2)
print("Even numbers:", list(evens))
# in operator
print(f"10 in evens: {10 in evens}") # True
print(f"7 in evens: {7 in evens}") # False
# len
print(f"len(evens): {len(evens)}") # 10
# Countdown
countdown = NumberRange(5, 0, -1)
print("Countdown:", end=" ")
for n in countdown:
print(n, end=" ")
print("🚀")
# list comprehension
squares = [x**2 for x in NumberRange(1, 6)]
print(f"Squares: {squares}")
Output:
| Even numbers | [0, 2, 4, 6, 8, 10, 12, 14, 16, 18] |
| 10 in evens | True |
| 7 in evens | False |
| len(evens) | 10 |
| Countdown | 5 4 3 2 1 🚀 |
| Squares | [1, 4, 9, 16, 25] |
8️⃣ __call__ — Make Objects Callable
Example
class Multiplier:
"""A callable object — works like a function!"""
def __init__(self, factor):
self.factor = factor
def __call__(self, value):
return value * self.factor
def __repr__(self):
return f"Multiplier(×{self.factor})"
double = Multiplier(2)
triple = Multiplier(3)
percent = Multiplier(0.01)
print(double(5)) # 10
print(triple(7)) # 21
print(percent(1500)) # 15.0
# Use in map/filter — just like a function!
numbers = [1, 2, 3, 4, 5]
print(list(map(double, numbers))) # [2, 4, 6, 8, 10]
# More practical: Memoized function decorator
class Memoize:
def __init__(self, func):
self.func = func
self.cache = {}
def __call__(self, *args):
if args not in self.cache:
self.cache[args] = self.func(*args)
return self.cache[args]
def __repr__(self):
return f"Memoize({self.func.__name__}, {len(self.cache)} cached)"
@Memoize
def fibonacci(n):
if n < 2: return n
return fibonacci(n-1) + fibonacci(n-2)
print(fibonacci(10)) # 55
print(fibonacci(30)) # 832040
print(fibonacci(50)) # computed instantly due to memoization
print(fibonacci) # Memoize(fibonacci, ... cached)
9️⃣ __enter__ and __exit__ — Context Manager
python example
class DatabaseTransaction:
"""Context manager for database transactions."""
def __init__(self, db_name):
self.db_name = db_name
self._operations = []
self._committed = False
def __enter__(self):
print(f"🔓 BEGIN TRANSACTION on '{self.db_name}'")
return self # returned as 'as' variable
def execute(self, sql):
self._operations.append(sql)
print(f" 📝 Queued: {sql}")
def __exit__(self, exc_type, exc_val, exc_tb):
if exc_type is None:
# No exception — commit
print(f" ✅ COMMIT: {len(self._operations)} operations committed.")
self._committed = True
else:
# Exception occurred — rollback
print(f" ❌ ROLLBACK: {exc_type.__name__}: {exc_val}")
self._operations.clear()
print(f"🔒 END TRANSACTION on '{self.db_name}'")
return True # suppress exception (return False to propagate)
# Successful transaction
with DatabaseTransaction("users_db") as tx:
tx.execute("INSERT INTO users (name) VALUES ('Arjun')")
tx.execute("UPDATE balance SET amount = 5000 WHERE user_id = 1")
print()
# Failed transaction — auto rollback
with DatabaseTransaction("orders_db") as tx:
tx.execute("INSERT INTO orders VALUES (101, 'Laptop')")
raise ValueError("Payment gateway timeout!")
tx.execute("UPDATE inventory SET qty = qty - 1") # never reached
print("\nProgram continues after handled exception ✅")Output:
| 📝 Queued | INSERT INTO users (name) VALUES ('Arjun') |
| 📝 Queued | UPDATE balance SET amount = 5000 WHERE user_id = 1 |
| ✅ COMMIT | 2 operations committed. |
| 📝 Queued | INSERT INTO orders VALUES (101, 'Laptop') |
| ❌ ROLLBACK | ValueError: Payment gateway timeout! |
🏆 Complete Example — ShoppingCart
Example
from functools import total_ordering
@total_ordering
class CartItem:
def __init__(self, name, price, qty=1):
self.name = name
self.price = price
self.qty = qty
@property
def total(self): return self.price * self.qty
def __eq__(self, other): return self.total == other.total
def __lt__(self, other): return self.total < other.total
def __str__(self): return f"{self.name:25s} ₹{self.price:>7.2f} × {self.qty:>3} = ₹{self.total:>9.2f}"
def __repr__(self): return f"CartItem({self.name!r}, {self.price}, {self.qty})"
class ShoppingCart:
def __init__(self, customer):
self.customer = customer
self._items = []
def __len__(self): # len(cart)
return len(self._items)
def __bool__(self): # if cart:
return len(self) > 0
def __iter__(self): # for item in cart:
return iter(self._items)
def __getitem__(self, idx): # cart[0]
return self._items[idx]
def __contains__(self, name): # "item" in cart
return any(i.name == name for i in self._items)
def __add__(self, other): # cart1 + cart2 (merge carts)
merged = ShoppingCart(f"{self.customer}+{other.customer}")
merged._items = self._items + other._items
return merged
def __str__(self):
if not self:
return f"🛒 {self.customer}'s cart is empty."
lines = [f"🛒 {self.customer}'s Cart ({len(self)} items)"]
lines.append("─" * 55)
for item in sorted(self._items, reverse=True):
lines.append(f" {item}")
lines.append("─" * 55)
lines.append(f" {'TOTAL':25s} {'':>22} ₹{self.subtotal:>9.2f}")
return "\n".join(lines)
@property
def subtotal(self):
return sum(i.total for i in self._items)
def add(self, item):
self._items.append(item)
def remove(self, name):
self._items = [i for i in self._items if i.name != name]
# ── Build a cart ────────────────────────────────────────
cart = ShoppingCart("Arjun Sharma")
cart.add(CartItem("Python Mastery Book", 599, 2))
cart.add(CartItem("USB-C Hub", 1299, 1))
cart.add(CartItem("Laptop Stand", 1499, 1))
cart.add(CartItem("Mechanical Keyboard", 3499, 1))
cart.add(CartItem("Mouse Pad XL", 399, 1))
print(cart)
print(f"\n'USB-C Hub' in cart: {'USB-C Hub' in cart}")
print(f"cart[0]: {cart[0]}")
print(f"Most expensive: {max(cart)}")
# Merge two carts
cart2 = ShoppingCart("Priya")
cart2.add(CartItem("Python Stickers Pack", 99, 5))
cart2.add(CartItem("Notepad", 149, 3))
merged = cart + cart2
print(f"\nMerged cart has {len(merged)} items, subtotal ₹{merged.subtotal:,.2f}")
Output:
| 'USB-C Hub' in cart | True |
| cart[0] | Python Mastery Book ₹ 599.00 × 2 = ₹ 1198.00 |
| Most expensive | Mechanical Keyboard ₹ 3499.00 × 1 = ₹ 3499.00 |
🇮🇳 Hindi Explanation (हिंदी में समझें)
Magic methods kya hain? Magic methods (dunder methods) are special methods that Python calls automatically when you perform specific operations — like using+, callinglen(), or running aforloop. They let you define how your objects behave with Python's built-in syntax.
What is the difference between__str__and__repr__?__str__provides user-readable output (what shows up in print).__repr__provides developer-oriented technical output — ideally a string that can recreate the object viaeval().
Why do we use__iter__and__next__? By defining these, you can use your class objects inforloops,list(),sum()and everywhere else.__iter__returns the iterator,__next__yields one value at a time, and raisesStopIterationat the end.
@total_orderingkya karta hai? If you only define__eq__and__lt__,@total_orderingautomatically generates<=,>,>=for you. Very convenient!
❓ Interview Questions
Q1: What are dunder/magic methods? Give 5 examples.
Answer: Magic methods are special methods with double underscores (__method__) that Python calls automatically for specific operations:
__init__: object construction__str__: string representation (print())__len__: length (len())__add__: addition (+)__iter__: iteration (forloop)
Q2: What is the difference between __str__ and __repr__?
Answer:
__str__: User-friendly string — used byprint(),str(). Should be readable.__repr__: Developer-facing, unambiguous string — used byrepr(), REPL, list display. Should ideally recreate the object viaeval().
If __str__ is not defined, Python falls back to __repr__.
Q3: When do you need to define __hash__ along with __eq__?
Answer: When you define __eq__, Python automatically sets __hash__ = None, making objects unhashable (can't be used in sets or as dict keys). To allow hashing, explicitly define __hash__. A good rule: hash() should be consistent with equality: a == b ⟹ hash(a) == hash(b).
Q4: What does __call__ do? Give a use case.
Answer: __call__ makes an instance callable like a function (obj(args)). Use cases: callable objects with state (memoization, counters), function decorators as classes, strategy pattern objects.
Q5: Explain the context manager protocol (__enter__, __exit__).
Answer:
__enter__: Called whenwithblock starts. Returns the object bound toas.__exit__(exc_type, exc_val, exc_tb): Called when block exits (normally or via exception). ReturnTrueto suppress the exception;False/Noneto propagate it.
Use cases: file handling, database transactions, lock management.
Q6: What is @total_ordering?
Answer: A class decorator from functools. If you define __eq__ and one of (__lt__, __le__, __gt__, __ge__), @total_ordering automatically generates the remaining comparison methods. Saves writing 4 extra methods.
Q7: What is __contains__? How does it relate to __iter__?
Answer: __contains__ is called when you use item in obj. If not defined, Python falls back to iterating with __iter__ and checking each element. Define __contains__ for O(1) membership testing (e.g., hash-based lookup) instead of O(n) linear scan.
Q8: What happens if __iter__ returns self vs a new iterator?
Answer:
- Returns
self: The object is both iterable AND iterator. Can only be iterated once (callingforloop again restarts only if__iter__resets state). - Returns new object: Iterable produces fresh iterators each time — multiple independent loops work correctly.
Best practice for containers: __iter__ returns iter(self._data) (fresh iterator each call).
📊 Complete Magic Methods Reference
| Category | Method | Triggered by |
|---|---|---|
| Construction | __new__, __init__, __del__ | Class(), garbage collection |
| Representation | __str__, __repr__, __format__ | print(), repr(), f-strings |
| Numeric | __int__, __float__, __bool__, __abs__, __neg__ | int(), float(), bool(), abs(), unary - |
| Arithmetic | __add__, __sub__, __mul__, __truediv__, __mod__, __pow__ | +, -, *, /, %, ** |
| Comparison | __eq__, __ne__, __lt__, __le__, __gt__, __ge__ | ==, !=, <, <=, >, >= |
| Container | __len__, __getitem__, __setitem__, __delitem__, __contains__ | len(), obj[k], obj[k]=v, del obj[k], in |
| Iteration | __iter__, __next__ | for, iter(), next() |
| Context | __enter__, __exit__ | with statement |
| Callable | __call__ | obj(args) |
| Attribute | __getattr__, __setattr__, __delattr__ | obj.x, obj.x=v, del obj.x |
| Hashing | __hash__ | hash(), dict keys, sets |
🧪 Practice Exercises
- Money Class: Implement
Money(amount, currency)with__add__,__sub__,__mul__,__eq__,__lt__,__str__,__repr__. RaiseTypeErrorfor cross-currency ops. - Inventory System: Build
Inventorywith__getitem__(by name),__setitem__,__delitem__,__contains__,__iter__,__len__,__str__. - Polynomial: Create a
Polynomialclass with coefficient list. Implement__add__,__mul__,__call__(evaluate at x),__str__(format nicely like "3x² + 2x + 1"). - Lazy File Reader: Implement
LazyFileReader(filename)as an iterator —__iter__opens the file,__next__yields one line at a time, closes onStopIteration. - Rate Limiter (callable): Create a
RateLimiter(max_calls, period)callable class that allows at mostmax_callsinvocations perperiodseconds, raisingRuntimeErrorotherwise.
🎉 Congratulations! You've completed the entire OOP section of the Python Master Course. What's Next? → File I/O, Exception Handling, Decorators, Generators, and Python Standard Library!
Exam Focus
Revise definitions, diagrams, examples, and short-answer points for Magic Methods (Dunder Methods) in Python.
Interview Use
Prepare one clear explanation, one practical example, and one common mistake for this Python Master Course topic.
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