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Errors & debugging

Handle failures gracefully, raise helpful errors, and debug with confidence.

3 matches

How to Assert Preconditions with Descriptive Messages in Python

Use Python's assert statement with a custom message to validate function preconditions and fail fast with clear diagnostics.

assert debugging preconditions

Python

def divide(dividend, divisor):
    assert divisor != 0, f"Divisor must be non-zero, got {divisor!r}"
    return dividend / divisor


if __name__ == "__main__":
    print(divide(10, 2))
    try:
        divide(10, 0)
    except AssertionError as e:
        print(f"AssertionError: {e}")

16 0 Open

Errors & debugging easy

How to Assert an Invariant After a Complex Transformation in Python

Use assert to verify that a multi-step transformation preserves a mathematical invariant, catching regressions early.

assert debugging invariants

Python

def transform_value(value):
    """Apply several transformations to a value."""
    doubled = value * 2
    shifted = doubled + 10
    normalized = shifted / 2
    return int(normalized)

def assert_invariant(value):
    """Assert that the transformation preserves a key invariant."""
    original = value
    transform…

13 0 Open

Errors & debugging easy

How to Test Exceptions in Python with pytest.raises

Learn the pytest.raises pattern to assert that specific exceptions are raised and validate their messages.

pytest testing exceptions

Python

import pytest


def divide(a: int, b: int) -> float:
    if b == 0:
        raise ValueError("Cannot divide by zero")
    return a / b


def test_divide_by_zero_raises():
    with pytest.raises(ValueError, match="Cannot divide by zero"):
        divide(10, 0)


def test_divide_by_zero_raises_exact_match():
    with py…

15 0 Open

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