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Copy-ready Python snippets by topic and difficulty — short, focused, and runnable in the browser editor.
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…Implement circuit breaker open after failures demo in Python
A minimal CircuitBreaker class that calls a function and automatically 'opens' after a set number of consecutive failures, blocking further calls with a RuntimeError.
import time
from datetime import datetime
class CircuitBreaker:
def __init__(self, threshold=3):
self.threshold = threshold
self.failure_count = 0
self.is_open = False
def call(self, func, *args, **kwargs):
if self.is_open:
raise RuntimeError("Circuit is OPEN")
…Retry an Operation on ConnectionError in Python
Retries an unreliable operation a fixed number of times when it raises a transient ConnectionError, with a small delay between attempts.
import time
import random
def unreliable_operation():
"""Simulates an operation that throws ConnectionError occasionally."""
if random.random() < 0.6:
raise ConnectionError("Transient network failure")
return "Operation succeeded"
def retry_operation(attempts=4, delay=0.2):
"""Retries the o…Try Except ValueError in Python: Handle Conversion Errors
Catch ValueError exceptions when converting strings to integers or performing arithmetic, returning None on failure instead of crashing.
def convert_to_int(value):
try:
return int(value)
except ValueError as error:
print(f"Conversion failed: {error}")
print(f"Problem value was: {repr(value)}")
return None
def divide_numbers(numerator, denominator):
try:
result = numerator / denominator
retur…Build a Personal Work Hours Tracker in Python
A Python class that logs daily work hours to a CSV file and produces a weekly summary of total hours worked.
import csv
from pathlib import Path
from datetime import datetime, date
class WorkHoursTracker:
def __init__(self, file_path="work_hours.csv"):
self.file_path = Path(file_path)
if not self.file_path.exists():
with open(self.file_path, "w", newline="") as f:
writer = csv…Generate Timesheet Reports from Daily Logs in Python
Aggregate daily log entries by project and produce a formatted timesheet report using Python's standard library.
import json
from pathlib import Path
from collections import defaultdict
def generate_timesheet_report(daily_logs: list[dict]) -> str:
"""
Generate a timesheet report from daily log entries.
Args:
daily_logs: List of dicts with 'date', 'project', 'hours', 'task' keys
Returns:
…Generate a Monthly Calendar PDF in Python
Create a Python utility that generates a monthly calendar PDF using ReportLab, with weekday headers and day numbers laid out in a grid.
from calendar import TextCalendar
from datetime import datetime
from reportlab.lib.pagesizes import letter
from reportlab.pdfgen import canvas
import os
def generate_monthly_calendar_pdf(year, month, filename="calendar.pdf"):
cal = TextCalendar()
days = cal.monthdays2calendar(year, month)
month_name …How to Filter Files by Extension and Size in Python
Use pathlib to list files in a directory, filter by extension or minimum size, and return matching names or (name, size) pairs.
from pathlib import Path
def filter_files_by_extension(directory: str, extension: str) -> list:
"""Return a list of file names in directory with the given extension."""
path = Path(directory)
return [f.name for f in path.iterdir() if f.is_file() and f.suffix == extension]
def filter_files_by_size(directo…How to Memory Map Large Files Read-Only in Python
This code demonstrates reading only the tail of a large file using a read-only memory map (mmap) to avoid loading the entire file into memory.
import mmap
import os
def read_tail_with_mmap(filepath, bytes_from_end=64):
"""Read the last bytes of a large file using a read-only mmap."""
file_size = os.path.getsize(filepath)
start = max(0, file_size - bytes_from_end)
with open(filepath, "rb") as f:
with mmap.mmap(f.fileno(), length=0, a…How to Merge Dicts from Two JSON Files Like a Pro
This helper reads two JSON files that contain dicts, merges them with the second file overriding duplicate keys, and saves the result to a new file.
import json
from pathlib import Path
def merge_json_files(file1: str, file2: str, output: str = "merged.json") -> dict:
"""Merge two JSON files containing dicts, with file2 overriding file1."""
data1 = json.loads(Path(file1).read_text())
data2 = json.loads(Path(file2).read_text())
merged = {**data1,…How to Read a File with Retry on Temporary IOError in Python
Read a file with automatic retries on temporary IOError/OSError failures, using the pathlib module with configurable attempts and delay.
import time
from pathlib import Path
def read_file_with_retry(filepath: str | Path, max_attempts: int = 3, delay: float = 0.5) -> str:
"""Read a file with retries on temporary IO errors."""
path = Path(filepath)
last_error = None
for attempt in range(max_attempts):
try:
return pat…How to Validate JSON Schema Shape in Python
Validate JSON data against a schema using manual checks for required fields, types, and constraints.
import json
from typing import Any, Dict
def validate_person_schema(data: Dict[str, Any]) -> bool:
"""Validate a person object against expected schema shape."""
if not isinstance(data, dict):
return False
# Required fields check
required_fields = {"name", "age", "email"}
if not requir…Tail last N lines of growing log file in Python
Prints the last n lines of a log file and follows new content appended to it, polling for size changes.
import time
from pathlib import Path
def tail_log(file_path, n=10, poll_interval=1.0, timeout=10):
"""
Print the last n lines and follow new lines appended to a growing log file.
"""
path = Path(file_path)
# Read the last n lines from the current file
with path.open("r", encoding="utf-8") as f…Check Invertible Mapping for Duplicate Values in Python
Detect duplicate values among (key, value) pairs to ensure the mapping is invertible, using a dictionary for O(1) lookups.
def invertible_after_dedup(pairs):
"""
Check whether a set of (key, value) pairs is invertible,
i.e., no duplicate values exist for different keys.
"""
seen = {}
for key, value in pairs:
if value in seen and seen[value] != key:
return False, f"Duplicate value '{value}' for k…Convert Lists and Dictionaries to Sets in Python
Convert lists of pairs into dictionaries and lists or dictionaries into sets using simple helper functions.
def convert_to_dict(data):
"""Convert list of tuples or lists into a dictionary."""
return dict(data)
def convert_to_set(data):
"""Convert list or dictionary into a set of its keys/values."""
if isinstance(data, dict):
return set(data.keys())
return set(data)
def convert_collection(data…Count word frequency in Python with dict and Counter
Count how often each word appears in a string using Counter, converted to a plain dict, and print results alphabetically.
from collections import Counter
import re
def count_word_frequency(text):
words = re.findall(r'\b\w+\b', text.lower())
return dict(Counter(words))
if __name__ == "__main__":
sample_text = "The quick brown fox jumps over the lazy dog. The dog barks, and the fox runs."
frequency = count_word_frequency(…Filter Dictionary Keys by Prefix in Python
Use a dict comprehension to build a new dictionary containing only keys that start with a given prefix.
def filter_dict_keys(data, prefix="temp_"):
"""
Filter a dictionary by keeping only keys that start with a given prefix.
Uses a dict comprehension to build a new dictionary.
"""
if not isinstance(data, dict):
raise ValueError("data must be a dictionary")
return {key: value for key, valu…How to Check if a Set is a Subset in Python
Check whether one set contains all elements of another set using the issubset method.
def is_subset(allowed_set, check_set):
"""
Check if check_set is a subset of allowed_set.
Returns True if all elements of check_set are in allowed_set, otherwise False.
"""
return check_set.issubset(allowed_set)
if __name__ == "__main__":
# Example usage
allowed = {1, 2, 3, 4, 5}
valid…How to Convert a Counter to a Plain Dict with Sorted Items in Python
This code converts a collections.Counter into a regular dictionary with items sorted by key, useful for stable, readable output.
from collections import Counter
def counter_to_sorted_dict(counter):
"""Convert a Counter to a plain dict with sorted items."""
return dict(sorted(counter.items()))
if __name__ == "__main__":
# Example usage
data = Counter(['apple', 'banana', 'apple', 'cherry', 'banana', 'date', 'apple'])
print("…Browse by section
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