Errors & debugging
Handle failures gracefully, raise helpful errors, and debug with confidence.
Catch RecursionError and Fail Gracefully in Python
Wrap a recursive function call in a try-except block to catch RecursionError and print a graceful failure message instead of crashing.
def compute_factorial_recursive(n):
"""Compute factorial recursively, raising RecursionError for deep recursion."""
if n == 0:
return 1
return n * compute_factorial_recursive(n - 1)
if __name__ == "__main__":
try:
result = compute_factorial_recursive(10000)
print(f"Factorial c…Catch ValueError and print friendly message in Python
Wrap an int() call in a try/except block and print a friendly message when ValueError is raised.
try:
number = int("not_a_number")
except ValueError:
print("That's not a valid number. Please enter digits only.")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.
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}")How to Catch ValueError in Python (try except)
Handle invalid numeric input by catching ValueError in a try/except block and returning a friendly error message.
def parse_number(text):
try:
number = int(text)
return f"Parsed number: {number}"
except ValueError as error:
return f"Error: '{text}' is not a valid number ({error})"
if __name__ == "__main__":
examples = ["42", "hello", "3.14", "100"]
for item in examples:
print(pars…How to Handle ValueError When Converting Strings to Integers in Python
Convert text to an integer with a try-except block that catches ValueError and prints beginner-friendly error messages.
def parse_number(text):
"""Convert text to an integer, showing beginner-friendly error handling."""
try:
number = int(text)
print(f"Successfully parsed: {number}")
return number
except ValueError as e:
print(f"Error: '{text}' is not a valid number.")
print(f"Debuggin…How to Handle ValueError with try except in Python
Shows a beginner-friendly try/except pattern that catches ValueError when converting text to an integer, prints a helpful message, and returns None instead of crashing.
def parse_number(text):
try:
return int(text)
except ValueError:
print(f"ValueError: '{text}' is not a valid integer.")
return None
if __name__ == "__main__":
user_input = "abc"
result = parse_number(user_input)
print(f"Parsing '{user_input}' returned: {result}")
vali…How to Record Last N Errors with a Ring Buffer in Python
Use collections.deque with maxlen to keep only the most recent N error messages while discarding older entries automatically.
import collections
class ErrorRecorder:
def __init__(self, size):
self.buffer = collections.deque(maxlen=size)
def record_error(self, message):
self.buffer.append(message)
def get_errors(self):
return list(self.buffer)
if __name__ == "__main__":
recorder = ErrorRecorder(3)
…How to Return Success or Error as a Tuple in Python (Result Type Pattern)
Use a (bool, value) tuple as a lightweight Result type to return either a successful result or a descriptive error message from a Python function.
def divide(dividend: float, divisor: float) -> tuple[bool, float | str]:
"""Return (True, result) on success, (False, error_message) on failure."""
if divisor == 0:
return False, "Error: Division by zero"
return True, dividend / divisor
if __name__ == "__main__":
# Success case
success, r…How to Serialize an Exception to a JSON-Safe Dict in Python
Convert any Python exception into a JSON-safe dictionary with type, message, and the last few traceback lines for logging.
import json
import traceback
from typing import Any
def exception_to_dict(exc: Exception) -> dict[str, Any]:
"""Convert an exception into a JSON-safe dictionary."""
return {
"type": type(exc).__name__,
"message": str(exc),
"traceback": traceback.format_exc().strip().split("\n")[-3:],
…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.
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…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.
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(…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.
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 = [
…How to attach a request ID to exception messages in Python
This code shows how to enrich exception messages with contextual request IDs using context variables, making error logs more traceable across concurrent requests.
import logging
from contextvars import ContextVar
request_id_var = ContextVar("request_id", default="unknown")
def add_request_id(exc: Exception) -> Exception:
exc.args = (f"request_id={request_id_var.get()} | {exc.args[0]}" if exc.args else f"request_id={request_id_var.get()}",) + exc.args[1:]
return exc
d…How to catch ValueError in Python and print a friendly message
This code defines a function that safely converts text to an integer, catches ValueError, and prints a friendly message instead of crashing.
def parse_number(text):
try:
return int(text)
except ValueError:
print("Oops! That's not a valid number.")
return None
if __name__ == "__main__":
result = parse_number("abc")
if result is None:
print("Parsing failed.")
else:
print(f"Parsed value: {result}")How to handle ZeroDivisionError in Python
Wrap a division operation in try/except to return None or a friendly message instead of crashing when dividing by zero.
def safe_divide(a, b):
"""Return a/b if possible, else None when dividing by zero."""
try:
return a / b
except ZeroDivisionError:
return None
def safe_divide_with_message(a, b):
"""Return a how-to message on divide-by-zero error."""
try:
return a / b
except ZeroDivisio…Log to stderr with Python logging basicConfig
Configure Python's logging module to send all log messages to standard error (stderr) instead of the default stderr, with a readable timestamped format.
import logging
def main():
logging.basicConfig(
level=logging.DEBUG,
format="%(asctime)s — %(name)s — %(levelname)s — %(message)s",
stream=__import__("sys").stderr,
)
logger = logging.getLogger("example")
logger.debug("Debug message")
logger.info("Info message")
logger.…Redact secrets from log message formatter in Python
Build a custom logging.Formatter that masks passwords, API keys, and credit card numbers in log output.
import re
import logging
class RedactingFormatter(logging.Formatter):
"""Formatter that masks sensitive data in log messages."""
SENSITIVE_PATTERNS = [
(re.compile(r'password[=:]\s*\S+', re.IGNORECASE), 'password=[REDACTED]'),
(re.compile(r'api[_-]?key[=:]\s*\S+', re.IGNORECASE), 'api_key…Browse by section
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Errors & debugging — Python code examples
What you will find here
This page collects errors & debugging snippets — short, copy-ready Python you can paste into our free online IDE and run without installing anything. Each sample includes a plain-English explanation and the full source code.
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Samples are quick reference — one concept per page. For step-by-step teaching, use our Python tutorials. To test yourself, try quizzes or coding challenges. Clean up style with the Python formatter.