Unlock Python’s Magic: Master Dunder Methods for Cleaner, Powerful Code

Python’s dunder (double‑underscore) methods are built‑in functions that let you control how objects behave, making your code more concise, intuitive, and powerful.

What Are Dunder Methods?

Dunder methods such as __str__, __repr__, __add__, and others let you customize object behavior to match your specific needs.

Want your object to print like a regular string? Implement __str__.

Need to compare two instances? Implement __eq__.

Want your object to support the + operator? Implement __add__.

Ugly Print Output ( __str__ & __repr__ )

Without custom methods, printing an instance shows a memory address:

Define a simple class:

class Book:
    def __init__(self, title, author):
        self.title = title
        self.author = author

Printing an instance now yields the default representation:

b = Book("1984", "George Orwell")
print(b)

Implement __str__ and __repr__ to improve output:

class Book:
    def __init__(self, title, author):
        self.title = title
        self.author = author

    def __str__(self):
        return f'"{self.title}" by {self.author}'

    def __repr__(self):
        return f'Book(title="{self.title}", author="{self.author}")'

Now printing the object gives a readable string:

b = Book("1984", "George Orwell")
print(b)

"1984" by George Orwell

Make Objects Behave Like Numbers ( __add__ , __sub__ )

Define a Wallet class that can be added:

wallet1 = Wallet( 50 )
wallet2 = Wallet( 30 )
total = wallet1 + wallet2  # should add balances

Implement __add__:

class Wallet:
    def __init__(self, balance):
        self.balance = balance

    def __add__(self, other):
        if isinstance(other, Wallet):
            return Wallet(self.balance + other.balance)
        return NotImplemented

Using it:

w1 = Wallet(50)
w2 = Wallet(30)
w3 = w1 + w2
print(w3.balance)  # 80

The same approach works for subtraction ( __sub__), multiplication ( __mul__), and comparisons ( __lt__, __gt__).

Compare Objects ( __eq__ , __gt__ , __lt__ )

By default, Python compares object identities, not their contents. Implement __eq__ to compare balances:

class Wallet:
    def __init__(self, balance):
        self.balance = balance

    def __eq__(self, other):
        if isinstance(other, Wallet):
            return self.balance == other.balance
        return False

Now:

w1 = Wallet(50)
w2 = Wallet(50)
print(w1 == w2)  # True

Make Objects Iterable ( __iter__ , __next__ )

To iterate over a custom container, implement __iter__ and __next__:

class TransactionHistory:
    def __init__(self, transactions):
        self.transactions = transactions
        self.index = 0

    def __iter__(self):
        return self

    def __next__(self):
        if self.index >= len(self.transactions):
            raise StopIteration
        transaction = self.transactions[self.index]
        self.index += 1
        return transaction

Using it:

history = TransactionHistory(["Deposit $100", "Withdraw $50", "Deposit $200"])
for transaction in history:
    print(transaction)

Deposit $100
Withdraw $50
Deposit $200

Final Thoughts

Dunder methods are not just a curiosity; they let your classes feel like built‑in Python types. Without __str__, objects print as garbage; without __eq__, Python cannot determine equality; without __iter__, you cannot loop over them. Mastering these methods makes your code more powerful and expressive.