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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

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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

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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.

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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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