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Separate Evens and Odds into Two Lists in Python
Split a list of numbers into two lists containing even and odd numbers using a simple loop and the modulo operator.
def separate_evens_odds(numbers):
evens = []
odds = []
for num in numbers:
if num % 2 == 0:
evens.append(num)
else:
odds.append(num)
return evens, odds
if __name__ == "__main__":
nums = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
evens, odds = separate_evens_odds(nu…Truncate List Keeping Last N Elements in Python
Return a new list containing only the last N elements from a sequence, handling edge cases like zero or oversized counts.
def truncate(seq, keep_last_n):
"""Return a new list keeping only the last n elements."""
if keep_last_n <= 0:
return []
return list(seq)[-keep_last_n:]
if __name__ == "__main__":
data = [10, 20, 30, 40, 50, 60]
print(truncate(data, 3))
print(truncate(data, 0))
print(truncate(data…Add Type Hints to Function Parameters and Return in Python
Add type hints to function parameters and return values in Python for clearer, more maintainable code using the typing module.
from typing import List, Optional, Dict
def average(numbers: List[float]) -> float:
return sum(numbers) / len(numbers)
def full_name(first: str, last: Optional[str] = "") -> str:
return f"{first} {last}".strip()
def build_user(name: str, age: int, email: Optional[str] = None) -> Dict[str, object]:
us…Chain Generators with yield from in Python
Combine multiple generators into one seamless sequence using the `yield from` delegation syntax in Python.
def numbers():
yield 1
yield 2
yield 3
def letters():
yield 'a'
yield 'b'
yield 'c'
def combined():
yield from numbers()
yield from letters()
if __name__ == "__main__":
print(list(combined()))Create a retry decorator with max attempts in Python
A decorator that retries a function up to a specified number of times when it raises an exception, with an optional delay between attempts.
import functools
import time
def retry(max_attempts, delay=0.1):
"""Retry a function up to max_attempts times on exception."""
def decorator(func):
@functools.wraps(func)
def wrapper(*args, **kwargs):
for attempt in range(1, max_attempts + 1):
try:
…How to Compose Two Functions into a Single Callable in Python
Combine two Python functions into a single callable using a compose helper, then apply the chained call.
def add_one(x):
return x + 1
def double(x):
return x * 2
def compose(f, g):
return lambda x: f(g(x))
add_then_double = compose(double, add_one)
double_then_add = compose(add_one, double)
result1 = add_then_double(5)
result2 = double_then_add(5)
print(f"add_one then double(5) = {result1}")
print(f"doub…How to Use the if __name__ == '__main__' Guard in Python
This code defines reusable functions and uses the standard main guard to run them only when the script is executed directly, not when imported.
def greet(name: str) -> str:
"""Return a friendly greeting."""
return f"Hello, {name}!"
def get_planet() -> str:
"""Return the name of our planet."""
return "Earth"
if __name__ == "__main__":
user = "Dorothy"
print(greet(user))
print(f"We live on {get_planet()}.")How to Validate Function Arguments in Python
Shows how to manually check argument types and values in a Python function, raising clear TypeError and ValueError messages.
def calculate_area(length: float, width: float) -> float:
"""Calculate the area of a rectangle with manual type validation."""
if not isinstance(length, (int, float)) or isinstance(length, bool):
raise TypeError(f"length must be a number, got {type(length).__name__}")
if not isinstance(width, (int,…Python Function Default Parameters Explained with Examples
Learn how to define Python functions with default parameter values and call them with fewer arguments than declared.
def greet(name, greeting="Hello", punctuation="!"):
return f"{greeting}, {name}{punctuation}"
def calculate_area(length, width=1, unit="sq units"):
area = length * width
return f"Area: {area} {unit}"
if __name__ == "__main__":
print(greet("Alice"))
print(greet("Bob", "Hi"))
print(greet("Charl…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 Mock a Failing Dependency to Test Error Paths in Python
Inject a fake HTTP client that raises a connection error to test how code handles dependency failures without touching the network.
import requests
def fetch_user(user_id):
url = f"https://api.example.com/users/{user_id}"
response = requests.get(url, timeout=5)
response.raise_for_status()
return response.json()
def get_user_name(user_id, http_client):
try:
user_data = http_client(user_id)
return user_data["nam…How to Raise a Custom Exception with Extra Context in Python
Define a custom exception that carries extra context fields and raise it to provide richer error information.
class InsufficientFundsError(Exception):
def __init__(self, balance, amount):
self.balance = balance
self.amount = amount
super().__init__(f"Withdrawal of ${amount} failed: balance ${balance} is insufficient")
def withdraw(balance, amount):
if amount > balance:
raise Insuffici…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 Use Optional Return in Python Instead of Raising Exceptions
A Python function returns None for missing dictionary keys instead of raising KeyError, enabling graceful lookup handling with type hints.
from typing import Optional
def find_user(users: dict, user_id: int) -> Optional[dict]:
"""
Look up a user by ID. Returns the user dict if found,
otherwise returns None instead of raising KeyError.
"""
return users.get(user_id)
def main() -> None:
users = {
1: {"name": "Alice", "ema…How to Use a Fallback Path with FileNotFoundError in Python
Read a primary file and fall back to a backup file when the first is missing, returning an empty string if both fail.
import pathlib
def read_config(path):
primary = pathlib.Path(path)
fallback = pathlib.Path("config_backup.json")
try:
with primary.open("r") as f:
return f.read()
except FileNotFoundError:
try:
with fallback.open("r") as f:
return f.read()
…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 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.
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")
#…How to Wrap a Low Level Error in a Higher Level Exception in Python
Wrap low-level exceptions in a higher-level exception while preserving the original cause with the `from` keyword.
class LowLevelError(Exception):
pass
class HighLevelError(Exception):
pass
def low_level_operation():
raise LowLevelError("storage drive failed to respond")
def high_level_operation():
try:
low_level_operation()
except LowLevelError as e:
raise HighLevelError(f"database operation…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.
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__ == "__…How to define an exception hierarchy for domain errors in Python
Create a custom exception hierarchy with a base DomainError class and specific subclasses to handle validation, not-found, permission, and concurrency errors cleanly in Python apps.
class DomainError(Exception):
"""Base class for all domain errors."""
pass
class ValidationError(DomainError):
"""Raised when input data fails validation rules."""
pass
class NotFoundError(DomainError):
"""Raised when a requested entity does not exist."""
pass
class PermissionDeniedError(Dom…Implement a Context Manager That Suppresses Exceptions in Python
Shows how to write a custom context manager that catches specified exceptions and optionally re-raises others, plus the stdlib contextlib.suppress alternative.
import contextlib
class SuppressExceptions:
def __init__(self, *exceptions):
self.exceptions = exceptions
def __enter__(self):
return self
def __exit__(self, exc_type, exc_val, exc_tb):
if exc_type is None:
return False
if not self.exceptions or exc_type in se…Browse by section
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