Python Class Method

Knowing a few fundamental Python methods help make better code.

str:

The str method is used to provide a string representation of an object. It is called by the str() function and when the object is used in string contexts (e.g., using print()). Overriding str allows you to define a human-readable string representation of your objects.

class Person:
    def __init__(self, name, age):
        self.name = name
        self.age = age

    def __str__(self):
        return f"Person: {self.name}, {self.age} years old"

person = Person("Alice", 25)
print(person)  # Output: Person: Alice, 25 years old

repr:

The repr method is used to provide a string representation of an object that is primarily used for debugging and representation in the interactive console. It is called by the repr() function and provides a detailed and unambiguous representation of the object.

class Person:
    def __init__(self, name, age):
        self.name = name
        self.age = age

    def __repr__(self):
        return f"Person({self.name}, {self.age})"

person = Person("Alice", 25)
print(repr(person))  # Output: Person(Alice, 25)

len:

The len method is used to define the length of an object. It is called by the len() function and should return an integer representing the length of the object. This method allows objects of a class to support the built-in len() function.

class MyList:
    def __init__(self):
        self.items = []

    def __len__(self):
        return len(self.items)

my_list = MyList()
my_list.items = [1, 2, 3, 4]
print(len(my_list))  # Output: 4

getitem and setitem:

The getitem and setitem methods are used to define the behavior of accessing elements of an object using indexing (e.g., obj[index]). getitem is called when retrieving an item by index, and setitem is called when assigning a value to an item by index.

class MyList:
    def __init__(self):
        self.items = []

    def __getitem__(self, index):
        return self.items[index]

    def __setitem__(self, index, value):
        self.items[index] = value

my_list = MyList()
my_list.items = [1, 2, 3, 4]
print(my_list[2])  # Output: 3
my_list[2] = 5
print(my_list[2])  # Output: 5

del:

The del method is used to define the behavior when an object is about to be destroyed or garbage collected. It is called when the object’s reference count reaches zero, indicating that there are no more references to the object. The del method can be used to release resources, close files, or perform any necessary cleanup actions.

class MyClass:
    def __del__(self):
        print("Object destroyed")

obj = MyClass()
del obj  # Output: Object destroyed

enter and exit (Context Managers):

The enter and exit methods are used to define a context manager, allowing you to control the setup and teardown of resources within a with statement. The enter method is called when entering the context, and the exit method is called when exiting the context, even if an exception occurs. Context managers are commonly used to manage file opening and closing, acquiring and releasing locks, and other resource management scenarios.

class FileHandler:
    def __init__(self, filename):
        self.filename = filename

    def __enter__(self):
        self.file = open(self.filename, 'r')
        return self.file

    def __exit__(self, exc_type, exc_value, exc_traceback):
        self.file.close()

with FileHandler('example.txt') as f:
    for line in f:
        print(line)

getattr and setattr:

The getattr and setattr methods are used to define custom behavior when accessing or assigning attribute values of an object. getattr is called when accessing an attribute that doesn’t exist, and setattr is called when assigning a value to an attribute. These methods can be used to implement dynamic attribute access, validation, or interception of attribute-related operations.

class Person:
    def __getattr__(self, name):
        return f"Attribute '{name}' doesn't exist"

    def __setattr__(self, name, value):
        if name == 'age' and value < 0:
            raise ValueError("Age must be a positive number")
        else:
            self.__dict__[name] = value

person = Person()
print(person.name)  # Output: Attribute 'name' doesn't exist
person.age = 25
print(person.age)  # Output: 25
person.age = -5  # Raises ValueError