Code Explainers

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python 
from fastapi import Depends, FastAPI, HTTPException
from sqlalchemy import create_engine
from sqlalchemy.orm import Session, sessionmaker
 

Wiring SQLAlchemy sessions into FastAPI

dependency-injection orm sessions
Intermediate 7 steps
java 
package com.example.billing;
 
import java.math.BigDecimal;
import java.math.RoundingMode;

Locale-aware currency formatting in Java

internationalization rounding immutability
Intermediate 6 steps
ruby 
class EmailValidator < ActiveModel::EachValidator
  EMAIL_FORMAT = /\A[a-z0-9._%+-]+@[a-z0-9.-]+\.[a-z]{2,}\z/i
 
  def validate_each(record, attribute, value)

Writing a custom email validator in Rails

validation regex custom validators
Intermediate 7 steps
php 
<?php
 
namespace App\Http\Controllers;
 

Streaming a large CSV export in Laravel

streaming csv-export memory-efficiency
Intermediate 9 steps
typescript 
import { Injectable } from '@angular/core';
import { HttpClient } from '@angular/common/http';
import {
  AsyncValidator,

How an async username validator works in Angular

async-validation reactive-forms rxjs
Intermediate 6 steps
rust 
use std::borrow::Cow;
 
fn sanitize_filename(input: &str) -> Cow<'_, str> {
    if input.chars().all(|c| c.is_alphanumeric() || matches!(c, '.' | '-' | '_')) {

Avoiding allocations with Cow in Rust

clone-on-write borrowing zero-copy
Intermediate 7 steps
javascript 
const { validationResult, matchedData } = require('express-validator');
 
function validate(schema) {
  const runners = schema.map((rule) => rule.run.bind(rule));

A reusable validation middleware in Express

middleware validation higher-order-functions
Intermediate 8 steps
python 
import re
 
_UNIT_SECONDS = {
    "w": 604800,

Parsing duration strings like 1h30m in Python

regex parsing validation
Intermediate 8 steps
php 
public function importUsers(array $users): int
{
    $inserted = 0;
 

Bulk-inserting users with batched PDO upserts

batching bulk-insert prepared-statements
Intermediate 6 steps
python 
import csv
from collections import defaultdict
from dataclasses import dataclass, field
from decimal import Decimal, InvalidOperation

Aggregating CSV sales by category in Python

dataclasses csv-parsing defaultdict
Intermediate 8 steps
rust 
use axum::{
    http::StatusCode,
    response::IntoResponse,
    routing::get,

Serving an SPA and API with Axum

routing static-files spa-fallback
Intermediate 7 steps
java 
@Service
public class ProductCatalogService {
 
    private final ProductRepository productRepository;

How Spring cache annotations keep data fresh

caching cache-invalidation dependency-injection
Intermediate 7 steps
go 
func RecoveryHandler(logger *zap.Logger) gin.HandlerFunc {
	return gin.CustomRecovery(func(c *gin.Context, recovered any) {
		var brokenPipe bool
		if ne, ok := recovered.(*net.OpError); ok {

A panic-recovery middleware in Gin

middleware panic-recovery structured-logging
Intermediate 6 steps
ruby 
class ConfigFlattener
  def initialize(config)
    @config = config
  end

Flattening nested config into dotted keys

recursion hashes tree-traversal
Intermediate 8 steps
php 
<?php
 
namespace App\Events;
 

A priority event dispatcher in PHP

observer-pattern callbacks priority-ordering
Intermediate 8 steps
typescript 
import {
  CanActivate,
  ExecutionContext,
  Injectable,

A rate-limiting guard in NestJS

rate-limiting guards metadata
Intermediate 7 steps
javascript 
const STORAGE_KEY = "app_state";
const CURRENT_VERSION = 3;
 
const migrations = {

Versioned state migrations in localStorage

migrations persistence versioning
Intermediate 9 steps
python 
from flask import Blueprint, jsonify, request, abort
 
from .models import Article, db
from .schemas import article_schema, articles_schema

Building a REST articles API with Flask Blueprints

rest-api blueprints serialization
Intermediate 7 steps