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The assert statement is a built-in debugging construct that tests a boolean expression. If the expression evaluates to True, program execution continues uninterrupted. If the expression evaluates to False, the interpreter raises an AssertionError exception, optionally passing a specified error message to the exception’s constructor to be displayed in the traceback.

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Syntax

assert expression [, assertion_message]
  • expression: Any valid Python expression that can be evaluated in a boolean context.
  • assertion_message (Optional): A string or object passed to the AssertionError constructor to provide context about the failure.

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Internal Mechanics

The assert statement is intrinsically tied to Python’s built-in __debug__ constant. By default, __debug__ is True. When the interpreter encounters an assert statement, it translates it into the following equivalent logic: 
if __debug__:
    if not expression:
        raise AssertionError(assertion_message)

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The -O (Optimize) Flag and Security Implications

Because assertions are controlled by the __debug__ constant, they can be globally disabled. If the Python interpreter is invoked with the -O (optimize) command-line switch, __debug__ is set to False. 
python -O script.py
When run in optimized mode, the Python compiler completely ignores assert statements. They are stripped from the generated bytecode, meaning they incur zero runtime performance overhead. Critical Implication: Because assert statements are stripped in optimized mode, they must never be used for data validation, security checks, or expressions with side effects. Relying on assertions for core application logic or access control introduces severe bugs and security vulnerabilities when the code is executed in production environments using the -O flag.

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Syntactic Pitfall: The Tuple Trap

Because assert is a statement and not a function, using parentheses around both the expression and the message evaluates them as a single tuple. In Python, a non-empty tuple always evaluates to True in a boolean context. Consequently, the assertion will never fail, rendering it useless. Modern Python (3.8+) explicitly catches this mistake during compilation and emits a warning: SyntaxWarning: assertion is always true, perhaps remove parentheses?. Incorrect: 

# Evaluates as a tuple: (False, "Value must be positive")

# Since the tuple is not empty, it evaluates to True. The assert passes.
assert(x > 0, "Value must be positive")
Correct: 

# Evaluates the expression, passes the string to the AssertionError
assert x > 0, "Value must be positive"
If parentheses are required for line continuation of a long expression, they must wrap only the expression, not the expression and the message together: 
assert (
    complex_condition_one 
    and complex_condition_two
), "Detailed error message"
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