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How to Truncate a List to Max Length in Python (Keep Head)
This code returns a new list containing only the first max_length items from the original list, using Python's slice syntax.
from typing import List
def truncate_head(lst: List[object], max_length: int) -> List[object]:
"""Return a new list with at most max_length items from the head."""
if max_length < 0:
raise ValueError("max_length must be non-negative")
return lst[:max_length]
if __name__ == "__main__":
# Examp…Export List of Dicts to CSV in Python
Write a list of dictionaries (dataframe-like) to a CSV file with headers using the standard library csv module and verify by reading it back.
import csv
def export_to_csv(data, filename):
"""Export a list of dicts to a CSV file."""
if not data:
print("No data to export")
return
# Get column names from the keys of the first dict
fieldnames = list(data[0].keys())
with open(filename, 'w', newline='', encoding='utf…Export SQLite Query Results to CSV in Python
Connects to a SQLite database, runs a query, and writes the result rows and column headers to a CSV file using the standard library.
import sqlite3
import csv
def export_query_to_csv(db_path, query, csv_path):
conn = sqlite3.connect(db_path)
cursor = conn.cursor()
cursor.execute(query)
rows = cursor.fetchall()
column_names = [description[0] for description in cursor.description]
with open(csv_path, 'w', newline='', encodi…How to Convert CSV Column Types While Reading in Python
Read a CSV file and automatically convert column values to int, float, str, or bool based on type suffixes in the header names.
import csv
from pathlib import Path
from typing import Any
def read_csv_with_types(filepath: str) -> list[dict[str, Any]]:
"""Read CSV and convert column types based on header suffixes."""
converters = {
"int": int,
"float": float,
"str": str,
"bool": lambda v: v.strip().lower(…How to Read Binary File Bytes and Inspect the Header in Python
Read the first bytes of a binary file with pathlib and display them as a hex dump plus an ASCII view to inspect file headers.
import pathlib
def inspect_binary_header(filepath: str, num_bytes: int = 16) -> None:
"""Read the first bytes of a binary file and display them as hex and ASCII."""
path = pathlib.Path(filepath)
data = path.read_bytes()[:num_bytes]
hex_str = ' '.join(f"{byte:02x}" for byte in data)
ascii_str …Normalize CSV Column Names to snake_case in Python
Convert CSV header names to snake_case using a regular expression and write the updated file in place.
import csv
import re
import sys
def to_snake_case(header):
header = re.sub(r"(?<=[a-z0-9])(?=[A-Z])", "_", header)
header = re.sub(r"[^a-zA-Z0-9]+", "_", header).strip("_").lower()
return header
def normalize_csv_headers(input_path, output_path=None):
with open(input_path, newline="", encoding="utf…Split CSV Files into Smaller Chunks in Python
Splits a large CSV file into multiple smaller chunk files, preserving the header row in each chunk.
import csv
import os
def split_csv(input_file, chunk_size=1000, output_prefix="chunk"):
"""Split a large CSV file into smaller chunks."""
with open(input_file, 'r', newline='') as infile:
reader = csv.reader(infile)
header = next(reader)
file_count = 1
row_count = 0
…Write CSV file with csv DictWriter in Python
Write a list of dictionaries to a CSV file using Python's csv.DictWriter, including a header row.
import csv
from pathlib import Path
fieldnames = ["name", "city", "age"]
rows = [
{"name": "Alice", "city": "New York", "age": 30},
{"name": "Bob", "city": "Los Angeles", "age": 25},
{"name": "Charlie", "city": "Chicago", "age": 35},
]
path = Path("people.csv")
with path.open("w", newline="") as csvfile:…Slots Class: How to Reduce Memory Usage in Python
Use __slots__ to prevent dynamic attribute creation and reduce per-instance memory overhead, while keeping methods intact.
class SlotsDemo:
__slots__ = ("name", "age", "email")
def __init__(self, name, age, email):
self.name = name
self.age = age
self.email = email
def describe(self):
return f"{self.name}, {self.age}, {self.email}"
if __name__ == "__main__":
instance = SlotsDemo("Alice", …How to Generate Fibonacci Numbers in Python Without Recursion
Build an efficient infinite Fibonacci sequence using a generator function with O(1) memory and no recursion overhead.
def fib(n):
a, b = 0, 1
for _ in range(n):
yield a
a, b = b, a + b
if __name__ == "__main__":
count = 10
result = list(fib(count))
print(result)How to Create a Mock Headless Browser Screenshot Stub in Python
This code provides a deterministic stub that simulates capturing webpage screenshots with a headless browser, returning formatted output without real browser dependencies.
import subprocess
import sys
def mock_screenshot_webpage(url: str, width: int = 1280, height: int = 800) -> str:
"""Stub that simulates taking a screenshot of a webpage using headless browser."""
# In real implementation, you would use playwright/selenium/headless chrome
result = {
"url": url,
…How to List Changed Files in the Last Git Commit with Python
Runs `git diff --name-only HEAD~1 HEAD` via subprocess to list the names of files changed in the most recent commit.
import subprocess
def list_changed_files():
result = subprocess.run(
["git", "diff", "--name-only", "HEAD~1", "HEAD"],
capture_output=True,
text=True,
check=True
)
files = result.stdout.strip().splitlines()
return files
if __name__ == "__main__":
changed = list_cha…How to Revert a Commit and Create a New Revert Commit in Python
Demonstrates a mock Git repository that creates a new revert commit on top of the current head when reverting an existing commit.
class GitCommit:
"""Minimal mock of a git commit for demonstrating revert behavior."""
def __init__(self, sha, message):
self.sha = sha
self.message = message
self.parent = None
class GitRepository:
"""Mock repository tracking a simple commit chain."""
def __init__(self):
…Generate an Idempotency-Key header mock with UUID in Python
This code provides a mock idempotency service that generates a UUID-based Idempotency-Key header token and validates it, useful for simulating production API behavior in tests.
import uuid
class MockIdempotencyService:
def __init__(self):
self._tokens = {}
def get_token(self, header_name="Idempotency-Key"):
token = str(uuid.uuid4())
self._tokens[header_name] = token
return token
def validate(self, header_name="Idempotency-Key"):
return s…How to plan reserved capacity from a CSV in Python
Read a CSV of workloads with csv.DictReader and compute a mock reserved capacity plan with headroom per service.
import csv
import io
def plan_reserved_capacity(workloads_csv: str) -> list[dict]:
"""Read a CSV of workloads and return a plan for reserved capacity per service."""
reader = csv.DictReader(io.StringIO(workloads_csv))
plan = []
for row in reader:
service = row["service"]
avg_load = fl…Build a Textual TUI App Skeleton in Python
Create a minimal Textual terminal UI app with a header, label, button, and footer, ready for interactive mock demonstrations.
from textual.app import App, ComposeResult
from textual.widgets import Header, Footer, Button, Label
class MockApp(App):
"""A minimal Textual TUI app skeleton."""
BINDINGS = [("q", "quit", "Quit")]
def compose(self) -> ComposeResult:
"""Create child widgets."""
yield Header()
yie…Create a Data Helper Class in Python
A reusable DataHelper class that saves and loads JSON and CSV files from a configurable base directory, with automatic header detection for CSV.
import json
import csv
from pathlib import Path
class DataHelper:
def __init__(self, base_path="."):
self.base_path = Path(base_path)
self.base_path.mkdir(exist_ok=True)
def save_json(self, data, filename):
path = self.base_path / filename
with open(path, "w") as f:
…How to Add a Correlation ID Tracing Header in Python
A mock middleware generates or preserves a correlation ID header and logs structured JSON messages with it for API request tracing.
import uuid
import json
from dataclasses import dataclass, field
from typing import Optional
@dataclass
class Request:
headers: dict = field(default_factory=dict)
def get(self, key, default=None):
return self.headers.get(key, default)
class CorrelationIdMiddleware:
def __init__(self, header_name…How to Decode Basic Auth Credentials in Python
Decode username and password from a Basic Auth header string using base64 and standard string operations.
import base64
def decode_basic_auth(header_value):
"""
Decode credentials from a Basic Auth header value.
Expected format: "Basic base64encoded(username:password)"
Returns a tuple (username, password).
"""
if not header_value.startswith("Basic "):
raise ValueError("Invalid Basic A…How to Mock Content-Disposition and Extract Filename in Python
Parse and mock Content-Disposition headers in Python to extract filenames, handling both plain and RFC 5987 encoded values.
import os
from pathlib import Path
import re
from unittest.mock import patch
def get_filename_from_content_disposition(header_value):
"""
Extract filename from a Content-Disposition header value.
Supports both filename and filename* parameters (RFC 5987).
"""
if not header_value:
return No…How to Mock an API Key Header Authentication Server in Python
A minimal HTTP server that validates requests using an X-API-Key header and returns JSON responses for authenticated and unauthenticated calls.
import json
from http.server import BaseHTTPRequestHandler, ThreadingHTTPServer
API_KEYS = {"test-user": "secret-key-123"}
class AuthHandler(BaseHTTPRequestHandler):
def do_GET(self):
auth = self.headers.get("X-API-Key")
if not auth or auth not in API_KEYS.values():
self.send_response…How to handle CORS preflight OPTIONS requests in Python
Create a mock HTTP server with a CORS preflight OPTIONS handler that returns the correct headers for browser-based API requests.
from http.server import BaseHTTPRequestHandler, HTTPServer
class CORSRequestHandler(BaseHTTPRequestHandler):
def _send_cors_headers(self):
self.send_header("Access-Control-Allow-Origin", "*")
self.send_header("Access-Control-Allow-Methods", "GET, POST, PUT, DELETE, OPTIONS")
self.send_head…How to Generate and Propagate W3C Trace Context Headers in Python
Generate and propagate W3C traceparent and tracestate headers for distributed tracing in Python, with mock service headers.
import uuid
def generate_w3c_traceparent(trace_id=None, parent_id=None, flags="01"):
if trace_id is None:
trace_id = uuid.uuid4().hex[:32]
if parent_id is None:
parent_id = uuid.uuid4().hex[:16]
return f"00-{trace_id}-{parent_id}-{flags}"
def create_mock_headers(service_name, trace_id=N…How to Simulate Trace Sampling Head in Python
Simulate head-based probabilistic trace sampling on mock trace data with a configurable sample rate and optional seed for reproducibility.
import random
def trace_sampling_head(mock_traces, sample_rate=0.5, seed=None):
"""Simulate probabilistic trace sampling (head-based) on mock data.
Args:
mock_traces: list of trace dictionaries with a unique 'trace_id'
sample_rate: float 0.0-1.0, probability of keeping a trace
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