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CodeSparring

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Learn the fundamentals, then prove you can use them

CodeSparring's courses are free to read and run entirely in your browser. Every lesson takes one concept, explains it properly, and then makes you write it yourself while automated checks grade the result. There is nothing to install and no sample project to clone.

Reading is open to everyone. Sign in when you want to run the exercises and keep your progress.

PythonSQLSystem DesignEvery lesson

How a lesson works

Every lesson in every course follows the same three steps, so once you have done one you know how to do all of them.

  1. Step 1

    Read

    One idea, explained in full, with a runnable example you can edit and re-run. No video, no local setup, nothing to install.

  2. Step 2

    Apply

    A guided exercise on the concept you just read. Hints are there if you stall, and automated checks tell you the moment your answer is right.

  3. Step 3

    Practice

    The same idea on a harder, interview-shaped problem with the guidance removed. This is the rep that shows whether it actually stuck.

System Design works the same way with one substitution: instead of running code you write your own design answer, save it, and then reveal a model answer to compare against.

Who it is for

  • Students preparing for internship and new-grad software or data engineering interviews.
  • Career switchers who can follow a tutorial but freeze in front of an empty editor.
  • Working engineers who write one language all day and need SQL or system design back in working memory.
  • Anyone who wants to read the material first and decide later whether the interview practice is worth paying for.

The courses

Three tracks, each broken into levels you can start from wherever you already are. No level is locked, so if you only need window functions you can go straight there.

Python

The Python that interviews actually test. 5 levels, 79 lessons.

Not a language tour. Each lesson takes one behaviour that trips people up under pressure, such as mutable default arguments, integer division, truthiness, or the GIL, explains why it works that way, and then makes you write it. Every exercise runs in your browser and is graded against real test cases.

  1. Level 1: FoundationsReference-style basics: variables, types, loops, and functions.21 lessons, about 3 hours
  2. Level 2: IdiomsComprehensions, generators, classes, dataclasses, decorators, and the standard-library idioms for dates, text, files, and APIs.21 lessons, about 4 hours
  3. Level 3: PatternsThe production-shaped syntax: modules, imports, and working across real files.14 lessons, about 4 hours
  4. Level 4: EngineeringAdvanced OOP, decorators, concurrency, async, profiling, and packaging.10 lessons, about 4 hours
  5. Level 5: Judging Code You Did Not WriteRead it, break it, test it, repair it. The review skills that decide what ships.13 lessons, about 5 hours

SQL

A real database, running in the tab. 6 levels, 77 lessons.

You write queries against seeded tables and see the rows come back, from your first SELECT through joins, aggregation, and window functions. Because the database is real, a wrong query fails the way it would in production rather than the way a quiz says it should.

  1. Level 1: SQL Foundations (Reading Source Data)SELECT, WHERE, ORDER BY, types: querying raw source tables the way a DE does on day one.11 lessons, about 3 hours
  2. Level 2: Aggregation & Joins (Combining Source Data)Aggregates, GROUP BY/HAVING, every join flavor, subqueries, CTEs: building metrics.11 lessons, about 5 hours
  3. Level 3: Data Modeling & Schema DesignDDL, keys, constraints, and normalization: designing schemas the database enforces.12 lessons, about 6 hours
  4. Level 4: Data Engineering with SQLWindow functions, recursive CTEs, SCD, idempotent merge, data-quality: warehouse transforms.15 lessons, about 8 hours
  5. Level 5: Advanced & Company-Specific SQL for DE InterviewsRehearse the four rounds of a Data Engineering loop as graded SQL: the power screen, the modeling round, the pipeline round, and the system-design round.14 lessons, about 8 hours
  6. Level 6: Cloud & Data Engineering FoundationsLearn the platform your SQL runs on: object storage, columnar files, partitioning, and distributed pipelines, the way a DE interview asks about them. It is beginner-friendly and can be started right after Level 2, since its graded queries lean on GROUP BY and joins.14 lessons, about 6 hours

System Design

Write the answer, then compare it. 12 levels, 208 lessons.

System design is not multiple choice, so these lessons are free response. You read the concept, write your own design answer, and only then reveal a model answer to compare against. The comparison is where the learning is, and it is the closest thing to a design round you can do alone.

  1. Level 0: Interview & Communication MethodRun a system-design round like a senior: scope, estimate, structure the walkthrough, and drive tradeoffs.15 lessons, about 6 hours
  2. Level 1: Foundations & Mental ModelsThe networking, API-contract, and performance fundamentals every later design assumes.21 lessons, about 9 hours
  3. Level 2: Data Storage & ModelingRelational vs NoSQL, storage engines, indexing, and modeling for access patterns.17 lessons, about 8 hours
  4. Level 3: Scaling the Data TierReplication, sharding, caching, CDN/search, and keeping derived data in sync at scale.16 lessons, about 8 hours
  5. Level 4: Scaling Compute & TrafficStateless scale-out, load balancing, gateways, rate limiting, autoscaling, and overload protection.14 lessons, about 7 hours
  6. Level 5: Distributed Systems CoreCAP/PACELC, consistency, clocks, consensus, distributed transactions, and failure handling.18 lessons, about 9 hours
  7. Level 6: Asynchronous & Event-Driven SystemsMessaging models, Kafka and the log, delivery guarantees, stream processing, and schema governance.15 lessons, about 7 hours
  8. Level 7: Reliability, Resilience & OperationsSLOs and error budgets, observability, resilience patterns, redundancy and multi-region DR, and safe deploys and chaos.17 lessons, about 8 hours
  9. Level 8: Security, Privacy & Multi-tenancyAuthentication and identity, authorization and tenancy, encryption and secrets, abuse and perimeter defense, and privacy, compliance, and audit.16 lessons, about 8 hours
  10. Level 9: Modern Architecture & DeliveryService architecture, containers and orchestration, serverless and edge, delivery and FinOps, and data-intensive analytics.16 lessons, about 8 hours
  11. Level 10: Applied Case StudiesThe full-length 'design X' interviews: foundational building blocks, social and feed, geo and media, storage and infra systems, and commerce, money, and analytics.28 lessons, about 16 hours
  12. Level 11: Specialized & Frontier SystemsThe frontier: ML systems, LLM and GenAI infrastructure, real-time analytics and globally consistent data, and IoT, edge, and time-series.15 lessons, about 8 hours

Prefer to see everything at once?

The lesson index lists all 364 lessons, grouped by course and level, on a single page. It is the fastest way to find the one concept you came for.

Browse the full lesson index