🐍 100 Days of Python

A structured, day-by-day Python learning journal documenting progress through ~80+ concepts — from bare-bones syntax to multithreading, async I/O, OOP design patterns, web scraping, and CLI tooling.

This repository tracks hands-on practice following the CodeWithHarry 100 Days of Python curriculum, with each day as a self-contained, runnable script.

---

📌 Why This Exists

Learning by doing. Every script in this repo was written from scratch — the goal was to understand why each construct works, not just what it does. Topics escalate deliberately: the same building blocks that appear in Day 10 (loops, conditionals) resurface in Day 39 (quiz game), Day 47 (encoder/decoder), and Day 98 (concurrent downloads) — showing how fundamentals compose into real programs.

---

🗂️ Repository Structure

Python-main/
│
├── Day 1/          # Hello World, first variable
├── Day 2–9/        # Core syntax: types, operators, typecasting, input
├── Day 10–15/      # Strings, slicing, built-in methods, time module
├── Day 16–19/      # Control flow: match/case, for/while, break/continue
├── Day 20–30/      # Functions: args, kwargs, recursion, scope
├── Day 31–35/      # Data structures: lists, tuples, sets
├── Day 36–38/      # Exception handling: try/except/finally, raise
├── Day 39–44/      # Projects + modules: quiz game, encoder, imports
├── Day 47–54/      # File I/O, lambdas, map/filter/reduce
├── Day 56–74/      # OOP: classes, inheritance, polymorphism, magic methods
├── Day 77–86/      # Advanced OOP, CLI tooling, operator overloading
├── Day 87–98/      # Concurrency: threading, asyncio, web scraping
│
├── myfile.txt      # Sample data used in file I/O exercises
├── sample.txt      # Sample data used in file seek/truncate exercises
└── requirements.txt

---

🧠 Concepts Covered

🔰 Foundations

Day	Concept
1–4	Variables, print(), basic data types (int, float, str, bool, complex)
5	Escape sequences, print() separators and end characters
6	Lists, tuples, dictionaries — first look
7–8	Arithmetic and assignment operators
9	Typecasting — explicit (int(), float()) and implicit
10	input(), stdin handling, type conversion from user input

📝 Strings

Day	Concept
11	String indexing, iteration, concatenation
12	Slicing with positive and negative indices
13	15+ string methods: upper, lower, split, replace, find, index, count, startswith, endswith, isalpha, isalnum, isspace, swapcase, title, center
28	str.format() and f-strings with format specifiers (e.g. :.2f)

🔁 Control Flow

Day	Concept
14	if, elif, else — including nested conditions
15	time.strftime for time-based branching
16	match/case (Python 3.10+ structural pattern matching)
17	for loops — strings, lists, range() with step
18	while loop with else clause
19	break and continue

⚙️ Functions

Day	Concept
20	Defining functions, return values, pass
21	Default args, *args (variadic positional), **kwargs (variadic keyword)
29	Docstrings — __doc__ attribute
30	Recursion — factorial, Fibonacci (conceptual)
48	Variable scope — local vs global, global keyword
52	Lambda functions
53	Higher-order functions: map(), filter(), reduce()
59	Decorators — wrapping functions, preserving args with *args/**kwargs
92	lru_cache — memoization for expensive repeated calls

🗄️ Data Structures

Day	Concept
22–23	Lists — comprehensions, sort, reverse, count, copy, insert, extend
24–25	Tuples — immutability, count(), index() with start/end bounds
31–32	Sets — creation, union, intersection, difference, symmetric_difference, issubset, issuperset, isdisjoint
33–34	Dictionaries — keys(), values(), items(), pop(), popitem(), update()

📂 File I/O

Day	Concept
49	Reading and writing files — open(), modes (r, w, a), with statement
50	Parsing CSV-like text files line by line
51	seek(), tell(), truncate() for byte-level file control

🚨 Error Handling

Day	Concept
36	try/except — catching ValueError, IndexError
37	finally block — guaranteed execution regardless of outcome
38	raise — manually throwing exceptions with custom messages

📦 Modules & Imports

Day	Concept
43–44	import, from x import y, import as, dir(), math module
3, 43	Third-party imports: pandas, hashlib

🏗️ Object-Oriented Programming

Day	Concept
56–57	Classes, objects, class variables vs instance variables
58	__init__ constructor
60	@property and .setter for encapsulated attribute access
61–62	Inheritance — Programmer(Employee), protected members (_name)
65	@staticmethod — utility methods without self or cls
66	Class variables, noOfEmployees counter pattern
69–70	@classmethod — alternate constructors, class-level state mutation
71	__dict__ introspection, help()
72	super() — calling parent __init__ cleanly
73	Magic/dunder methods: __str__, __repr__, __len__, __call__
74	Method overriding + super() in multi-level inheritance
77	Operator overloading — __add__ for a custom Vector class
78–79	Polymorphism (method overriding), multiple inheritance
80	MRO (Method Resolution Order) — GoldenRetriever.mro()
81	Hybrid and hierarchical inheritance patterns

⚡ Advanced Python

Day	Concept
41	Ternary operator
42	enumerate() with custom start index
54	None — identity check (is) vs equality (==)
84	time.localtime(), time.strftime(), performance timing
85	argparse — building a real CLI tool with positional + optional args
86	Walrus operator (:=, Python 3.8+) — assignment inside expressions
87	shutil + os — file/directory copy, move, remove, rmtree
91	Generators — yield, lazy evaluation, memory efficiency
95	Regular expressions — re.search, re.finditer, character classes, anchors

🔄 Concurrency & Networking

Day	Concept
89	Web scraping — requests + BeautifulSoup, parsing HTML, extracting tags
96	asyncio — async def, await, asyncio.gather() for concurrent HTTP
97	Threading — threading.Thread, ThreadPoolExecutor, executor.map()
98	Multiprocessing — ProcessPoolExecutor, concurrent file downloading

---

🚀 Notable Programs

🎮 KBC Quiz Game — Day 39

A terminal recreation of Kaun Banega Crorepati (Who Wants to Be a Millionaire). Manages a question bank, prize ladder, quit-and-bank logic, and lifeline milestones. Demonstrates list-of-lists data modelling, loop control, f-string formatting, and stateful game logic — entirely in the standard library.

🔐 Secret Code Encoder/Decoder — Day 47

A two-way string cipher. Short words are reversed; longer words have their first character rotated to the end with fixed random pads appended. Decoding reverses the exact transformation. Shows string slicing, join/split patterns, and conditional branching on string length.

📥 CLI File Downloader — Day 85

A proper command-line tool built with argparse. Accepts a URL as a positional argument and an optional -o/--output filename flag. Streams large files in chunks using requests to avoid loading the full response into memory — a real-world pattern used in production download utilities.

python main.py https://example.com/image.jpg -o output.jpg

🕸️ Web Scraper — Day 89

Fetches a live webpage, parses the full HTML tree with BeautifulSoup, and extracts all <h2> headings. Includes a commented-out POST request example against a JSON API. Demonstrates real HTTP interaction against a live server, not mocked data.

⚡ Concurrent Image Downloader — Day 98

Downloads 60 images in parallel using ProcessPoolExecutor. Handles directory creation, binary file writing, and exception isolation per worker — a realistic pattern for batch data-fetching pipelines.

---

🛠️ Setup & Usage

Requirements: Python 3.8+ (Days 86+), Python 3.10+ for match/case (Day 16)

Install dependencies:

pip install requests beautifulsoup4 pandas

Run any day's script:

cd "Day 39"
python main.py

CLI tool (Day 85):

cd Day_85
python main.py https://picsum.photos/800/600 -o photo.jpg

---

📈 Learning Progression

Days  1–20  ▓▓▓▓▓░░░░░░░░░░░░░░░  Syntax, types, control flow, functions
Days 21–40  ░░░░▓▓▓▓▓░░░░░░░░░░░  Data structures, recursion, first projects
Days 41–60  ░░░░░░░░▓▓▓▓▓░░░░░░░  File I/O, modules, lambdas, decorators
Days 61–80  ░░░░░░░░░░░░▓▓▓▓▓░░░  Full OOP — inheritance, polymorphism, dunder methods
Days 81–98  ░░░░░░░░░░░░░░░░▓▓▓▓  Concurrency, CLI, async, regex, web scraping

---

🔧 Tech Stack

---