Learn How Classes Are Created | Metaclasses Fundamentals

When you define a class in Python, a sophisticated process unfolds behind the scenes. Understanding this process is key to grasping the power and flexibility of metaclasses. The sequence begins when Python encounters a class statement. First, Python parses the class definition, identifying its name, base classes, and gathering all methods and attributes defined in the class body. These attributes are collected into a dictionary.

Once these elements are ready, Python determines which metaclass should be used to create the class. By default, this is the built-in type, but you can specify a custom metaclass if needed. The metaclass is then called with three main arguments: the class name as a string, a tuple of base classes, and the dictionary containing the class attributes. The metaclass returns a new class object, which is then bound to the class name in the current namespace.


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# Standard class definition
class Animal:
    species = "Unknown"

    def speak(self):
        return "Some sound"

# Equivalent creation using type
AnimalAlt = type(
    "AnimalAlt",                # Class name
    (),                         # Base classes (empty tuple means no inheritance)
    {
        "species": "Unknown",   # Class attribute
        "speak": lambda self: "Some sound"  # Method
    }
)

# Both classes behave similarly
a1 = Animal()
a2 = AnimalAlt()
print(a1.speak())  # Output: Some sound
print(a2.speak())  # Output: Some sound


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# Collecting class attributes for metaclass
class_attrs = {
    "species": "Unknown",
    "speak": lambda self: "Some sound"
}

# Simulate what Python does internally
class_name = "Animal"
bases = ()
# The metaclass (usually 'type') is called with these arguments
Animal = type(class_name, bases, class_attrs)

# The resulting class has the collected attributes
print(Animal.species)         # Output: Unknown
print(Animal().speak())       # Output: Some sound

This mechanism gives you significant flexibility. By customizing the metaclass, you can intervene in the class creation process, modifying the class attributes, enforcing rules, or even altering inheritance. Metaclasses allow you to control or enhance how classes are constructed, making them a powerful tool for advanced Python programming.

1. What arguments does the metaclass receive when a class is created?

2. Fill in the blanks: The metaclass is called with the class name, base classes, and ____.

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Thanks for your feedback!

Section 1. Chapter 3

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              12345678910111213141516171819202122
            
# Standard class definition
class Animal:
    species = "Unknown"

    def speak(self):
        return "Some sound"

# Equivalent creation using type
AnimalAlt = type(
    "AnimalAlt",                # Class name
    (),                         # Base classes (empty tuple means no inheritance)
    {
        "species": "Unknown",   # Class attribute
        "speak": lambda self: "Some sound"  # Method
    }
)

# Both classes behave similarly
a1 = Animal()
a2 = AnimalAlt()
print(a1.speak())  # Output: Some sound
print(a2.speak())  # Output: Some sound


              123456789101112131415
            
# Collecting class attributes for metaclass
class_attrs = {
    "species": "Unknown",
    "speak": lambda self: "Some sound"
}

# Simulate what Python does internally
class_name = "Animal"
bases = ()
# The metaclass (usually 'type') is called with these arguments
Animal = type(class_name, bases, class_attrs)

# The resulting class has the collected attributes
print(Animal.species)         # Output: Unknown
print(Animal().speak())       # Output: Some sound

1. What arguments does the metaclass receive when a class is created?

2. Fill in the blanks: The metaclass is called with the class name, base classes, and ____.

Everything was clear?

Thanks for your feedback!

Section 1. Chapter 3