How to Use Mock Flip Mutation Testing in Python

import random

# In mutation testing, a "mock flip" intentionally changes a Boolean
# constant to False (or True) to see if the test suite catches it.
# This is a common "constant mutation" applied to a source file's literals.

def is_even(n: int) -> bool:
    """Return True if n is even. Contains a Boolean literal used as a mock target."""
    return n % 2 == 0  # The 'True' implicit result; we'll flip the literal below.

def apply_mock_flip(original: str) -> str:
    """Simulate a mutation tool flipping the first 'True' literal in code to 'False'."""
    return original.replace("True", "False", 1)

def run_mock_flip_demo():
    # Example source snippet (like a file read by a mutation testing tool)
    source_snippet = """
    if is_even(0):
        print("Zero is even")
    else:
        print("Zero is odd")
    """
    
    # Capture test output before mutation
    print("=== Before mutation (expected: 'Zero is even') ===")
    exec(compile(source_snippet, "<string>", "exec"))
    
    # Mutate the code: flip the 'True' (implicit in condition) — but here we
    # explicitly flip a Boolean constant to demonstrate the concept.
    mutated_source = source_snippet.replace("if is_even(0):", "if is_even(0) and True:")  # placeholder for demo
    # In practice, the tool would modify the file itself; we show effect:
    flipped = source_snippet.replace("if is_even(0):", "if is_even(0) and False:")
    
    print("\n=== After mock flip (expected: 'Zero is odd') ===")
    exec(compile(flipped, "<string>", "exec"))

if __name__ == "__main__":
    run_mock_flip_demo()