Reference library

Errors & debugging

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

7 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 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

Errors & debugging easy

How to Validate Input and Raise TypeError in Python

Define a function that checks its argument type and raises a TypeError early with a clear message when given a non-number.

type checking validation typeerror

Python

def validate_number(value):
    if not isinstance(value, (int, float)):
        raise TypeError(f"Expected a number, got {type(value).__name__}")
    return value * 2

if __name__ == "__main__":
    try:
        print(validate_number(5))
        print(validate_number("hello"))
    except TypeError as e:
        print(…

13 0 Open

Errors & debugging easy

How to Validate JSON in Python and Catch JSONDecodeError

A robust Python function that attempts to parse JSON strings and returns a boolean plus either the parsed data or a descriptive error message when decoding fails.

json validation jsondecodeerror

Python

import json

def validate_json(json_string):
    """Try to parse JSON, return (is_valid, data_or_error)."""
    try:
        data = json.loads(json_string)
        return True, data
    except json.JSONDecodeError as e:
        return False, f"Invalid JSON: {e}"

if __name__ == "__main__":
    test_inputs = [
        …

11 0 Open

Errors & debugging easy

How to Validate an Email Address and Raise ValueError in Python

This code defines a validate_email function that checks an email address against a regex pattern and several rules, raising ValueError with a specific reason when invalid.

validation regex errors

Python

import re

def validate_email(email: str) -> str:
    """Validate an email address and return it if valid, otherwise raise ValueError."""
    if not isinstance(email, str):
        raise ValueError("Email must be a string")
    if len(email) > 254:
        raise ValueError("Email length exceeds 254 characters")

    #…

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Errors & debugging easy

How to check for None and raise helpful errors in Python

A defensive function that explicitly validates data, keys, and values — raising descriptive ValueError and KeyError exceptions before returning a result.

none error-handling validation

Python

def get_value(data, key):
    if data is None:
        raise ValueError("data cannot be None")
    if key not in data:
        raise KeyError(f"key '{key}' not found in data")
    result = data[key]
    if result is None:
        raise ValueError(f"value for key '{key}' is None")
    return result


if __name__ == "__…

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Errors & debugging easy

Validate try except ValueError handler for beginners — errors debugging

Learn how to validate user input and handle division errors safely using try/except with ValueError and ZeroDivisionError in Python.

try except valueerror

Python

def divide_numbers(a, b):
    """Divide two numbers, catching division by zero and value errors."""
    try:
        result = a / b
    except ZeroDivisionError:
        print("Error: Cannot divide by zero!")
        return None
    except TypeError:
        print("Error: Both arguments must be numbers!")
        retu…

14 0 Open

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