Dunder Methods

• Magic methods.
• Special methods in Python that are surrounded by double underscores.

• are not meant to be invoked directly by the user but are internally invoked by the Python interpreter to perform specific actions.

  E.g. when you wanna specify how two objects should be added together, you can define a __add__ method for your classes.

  ```py class Account: def __init__(self, balance: float): self.balance = balance class Credit: def __init__(self, amount: float): self.amount = amount def __add__(self, account: Account) -> float: return account.balance + self.amount my_account = Account(100) my_credit = Credit(20) print(my_credit + my_account) ```

__init__

• The constructor method.
• Called when an object is instantiated.
• Used to initialize the object’s attributes.
• Will be used for things like performing any necessary setup or initialization tasks as well.

class Animal:
    """
    Create an animal

    Attributes:
        name: The name of the animal.
        species: The species of the animal.
    """

    def __init__(self, name: str, species: str):
        self.name = name
        self.species = species

[!TIP]

In python we have in fact two method which will be called when an object is instantiated, __new__ and __init__. __new__ is called first and it’s responsible for creating the object, while __init__ is called second and it’s responsible for initializing the object.

__str__

• The string representation of an object.
• Used to provide a human-readable string representation of an object.

class BankAccount:
    """
    Create a bank account

    Attributes:
        account_number: The account number.
        balance: The balance of the account.
    """

    def __init__(self, account_number: int, balance: float):
        self.account_number = account_number
        self.balance = balance

    def __str__(self):
        return f"Account Number: {self.account_number}, Balance: {self.balance}"

my_account = BankAccount(123456789, 1000.0)
print(my_account)

YouTube/Aparat

• https://youtu.be/Po_2bCv8_uc.
• https://aparat.com/v/fxld834.

__iter__ & __next__

If you need to loop over your class and wanted to define how it should do it you need to implement __iter__ and __next__.

Not a Good Solution	Generators	__iter__ & __next__
```py class Person(object): def __init__(self, name: str, age: int): self.name = name self.age = age self._data_attributes = ["name", "age"] def __iter__(self): return self def __next__(self): if len(self._data_attributes) == 0: raise StopIteration return getattr(self, self._data_attributes.pop(0)) p1 = Person("John", 30) for i in p1: print(i) ``` **Note**, this solution is modifying `_data_attributes` during iteration (using `pop(0)`), which ain't good practice since we might want to use it somewhere else in that class.	```py from typing import Iterator class Person(object): def __init__(self, name: str, age: int): self.name = name self.age = age self._data_attributes = ["name", "age"] def __iter__(self) -> Iterator[str | int]: # return (getattr(self, attr) for attr in self._data_attributes) for attr_name in self._data_attributes: yield getattr(self, attr_name) ```	```py class Person(object): def __init__(self, name: str, age: int): self.name = name self.age = age self._data_attributes = ["name", "age"] self._index = 0 # Track iteration state def __iter__(self): self._index = 0 # Reset for new iterations return self def __next__(self): if self._index >= len(self._data_attributes): raise StopIteration attr_name = self._data_attributes[self._index] self._index += 1 return getattr(self, attr_name) ```

[!TIP]

If you need to define how you’re dataclass should be converted to a dictionary you can use __iter__ method, this is similar to __str__ method:

from dataclasses import dataclass


@dataclass
class User:
    def __init__(self):
        self.name = "something"
        self.age = 123

    def __iter__(self):
        yield "name", self.name
        yield "age", self.age

temp = User()
print(dict(temp))