README.md

Chapter 19 — Concurrency Models in Python

Theme: Concurrency is not Parallelism. Python offers three deeply distinct paradigms (Threads, Processes, Asyncio) to solve the exact same problem: waiting for things to finish.

What You'll Learn

Paradigm	Concurrency	Parallelism	Best For	Caveat
Threads	Preemptive	No (GIL)	Network I/O, File I/O	Race conditions, limited scaling
Processes	Preemptive	Yes	Heavy Math, CPU load	High memory overhead
Asyncio	Cooperative	No (GIL)	Heavy Network I/O	Complex syntax, blocking traps

Key Files

• examples.py — The exact same console Spinner animation built with threading, multiprocessing, and asyncio.
• exercises.py — Proving the GIL does not protect you from race conditions, and why threading.Lock is mandatory.
• mini_project.py — A ThreadPool API scraper proving how to easily parallelize network requests.
• benchmarks.py — Benchmark proving threads speed up sleep() by 20x, but speed up math by 0x.
• notes.md — The exact mechanics of the Global Interpreter Lock (GIL) and its loopholes.
• pitfalls.md — The trap of using threads for CPU work, and the multiprocessing zombie leak.
• interview_questions.md — L3 to L6 interview prep.
• architecture_notes.md — How the CPython OS-level lock actually functions (sys.setswitchinterval).

30-Second Rules

# Rule 1: Use ThreadPoolExecutor for making HTTP requests or reading disks.
with concurrent.futures.ThreadPoolExecutor() as ex:
    results = ex.map(download_url, urls)

# Rule 2: Use ProcessPoolExecutor for doing heavy math or image processing.
with concurrent.futures.ProcessPoolExecutor() as ex:
    results = ex.map(calculate_primes, numbers)

# Rule 3: Use Asyncio when you need to handle 10,000+ simultaneous network connections.

Reference: Fluent Python 2nd ed., Chapter 19 — pages 723–750