Logical NOT

The not operator is a unary logical operator that performs boolean negation. It evaluates the truthiness of a single operand and returns the inverted boolean value: True if the operand is considered false, and False if the operand is considered true.

not expression

Evaluation Mechanics

Unlike Python’s and and or operators, which can return the operands themselves, the not operator strictly guarantees a return type of bool. It forces the operand into a boolean context using Python’s internal truth value testing.

1. Truthiness: If the operand evaluates to True (truthy), not returns False.
2. Falsiness: If the operand evaluates to False (falsy), not returns True.

In Python, the following standard values are inherently falsy:

• Constants defined to be false: None and False.
• Zero of any numeric type: 0, 0.0, 0j, Decimal(0), Fraction(0, 1).
• Empty sequences and collections: '', (), [], {}, set(), range(0).

All other values are considered truthy by default.

# Standard boolean negation
print(not True)   # False
print(not False)  # True


# Negation of falsy objects
print(not None)   # True
print(not 0)      # True
print(not [])     # True


# Negation of truthy objects
print(not 1)      # False
print(not "Data") # False
print(not [1, 2]) # False

Object-Level Implementation

Under the hood, not x is evaluated by calling bool(x) and inverting the result. The bool() built-in determines truthiness by looking for specific magic methods (dunder methods) on the object’s class:

1. Python first checks for the __bool__() method. If it exists, not returns the inverse of what __bool__() returns.
2. If __bool__() is not defined, Python falls back to __len__(). If __len__() returns 0, the object is falsy (so not returns True). If it returns a non-zero integer, the object is truthy (so not returns False).
3. If neither method is defined, the object is considered truthy by default, and not will return False.

Operator Precedence

The not operator has a lower precedence than non-boolean operators (like arithmetic, bitwise, and comparison operators) but a higher precedence than the logical operators and and or.

# Evaluated as: not (5 > 2)
not 5 > 2  # False


# Evaluated as: (not True) and False
not True and False  # False

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