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NoneType is the data type of the None object in Python, representing the absolute absence of a value. It is a built-in, strictly immutable type that implements the Singleton design pattern at the CPython runtime level. Because it is a singleton, only one instance of NoneType can ever exist in memory during the execution of a Python program. 
type(None)  # <class 'NoneType'>

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Singleton Mechanics and Identity

Because there is only one instance of NoneType, every variable assigned to None points to the exact same memory address. Consequently, evaluating NoneType should always be done using the identity operator (is) rather than the equality operator (==). The is operator checks memory address pointers, making it faster and immune to overridden __eq__ methods in other objects. 
x = None
y = None


# Identity comparison (Recommended)
x is None  # True
x is y     # True


# Value comparison (Suboptimal)
x == None  # True

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Boolean Evaluation

In boolean contexts, NoneType is inherently “falsy”. The CPython implementation of NoneType lacks a __len__ method and implements a __bool__ magic method that strictly returns False. 
bool(None)  # False

if not None:
    print("None evaluates to False")

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Instantiation and Subclassing Constraints

NoneType cannot be subclassed, and developers cannot create new, distinct instances of it. In Python 3, attempting to instantiate the type directly is explicitly prohibited and will raise a TypeError. 

# Extracting the type
NoneType = type(None)


# Attempting to create a new instance
new_instance = NoneType()

# Raises: TypeError: cannot create 'NoneType' instances

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Static Type Hinting

In Python’s static type system (PEP 484), NoneType is an anomaly: it is the only type where the instance value (None) is used to denote the type itself in annotations. Starting in Python 3.10, NoneType was formally added to the types module for explicit type declarations, though using None remains the standard convention. 

# Standard PEP 484 convention
def return_nothing() -> None:
    return None


# Python 3.10+ explicit type definition
import types

def return_nothing_explicit() -> types.NoneType:
    return None

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CPython Implementation Details

At the C level in CPython, NoneType is defined by the _PyNone_Type struct, and the None singleton is a statically allocated object named _Py_NoneStruct. To prevent the Python garbage collector from ever destroying the singleton, its reference count is initialized to a value that is never allowed to drop to zero.
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