Introduction to Python

Python is the friendliest programming language to learn first — readable enough to mistake for English, powerful enough to run NASA missions, Instagram, and most AI on the planet. If you can write a shopping list, you can write Python.

What Python Actually Is

Python is an interpreted, dynamically-typed, multi-paradigm language. In plain English:

• Interpreted — you don't compile to a binary; the python interpreter reads your code and runs it.
• Dynamically typed — you don't declare types; Python figures them out at runtime.
• Multi-paradigm — procedural, object-oriented, or functional, your call.

Your First Program

print("Hello, world!")

One line. No main, no semicolons, no curly braces. The interpreter reads top-to-bottom and runs each statement.

How Python Actually Runs Your Code

.py source  →  bytecode (.pyc)  →  Python Virtual Machine  →  output

First run, Python compiles your file to bytecode (cached in __pycache__). The PVM executes that bytecode. You don't think about this day-to-day, but it's why Python is portable: the same .py file runs anywhere with a Python interpreter.

Setting Up Without Pain

1. Install Python 3.12+ from python.org or via the Microsoft Store / Homebrew. 2. Verify: python --version (or python3). 3. Use VS Code with the Python extension. It picks up your interpreter automatically. 4. Always work inside a virtual environment: python -m venv .venv then .venv\Scripts\activate (Windows) or source .venv/bin/activate (mac/linux). Keeps project deps isolated.

The REPL — Your Sandbox

Type python in a terminal and you get an interactive prompt. Type 2 + 2, hit Enter, see 4. Perfect for tiny experiments. Press Ctrl+D / Ctrl+Z to exit.

For anything bigger, use Jupyter (pip install notebook) — same idea, but cells you can re-run.

Indentation Is Syntax

Python uses whitespace to mark blocks. There are no { }. Four spaces, every time:

if age >= 18:
    print("adult")    # 4 spaces in
else:
    print("minor")

Mix tabs and spaces and you'll get IndentationError. Configure your editor to insert 4 spaces on Tab.

Beginner Mistakes to Skip

1. print x instead of print(x). Python 3 made print a function. Always use parentheses. 2. Mixing tabs and spaces. Pick spaces, configure once, forget about it. 3. Forgetting the colon at the end of if / for / def / class lines. 4. Running with the system Python. Always use a virtualenv — prevents "it works on my laptop" bugs. 5. Naming files random.py or json.py. Shadows the standard library, breaks imports. 6. Ignoring the error message. Python tracebacks are descriptive — read the *last* line first; that's the actual error.

Intermediate: Comments, Docstrings & PEP 8

# single-line comment
"""
Multi-line string. When placed as the first statement of a module/function/class
it becomes a docstring — readable via help() and tooling.
"""

PEP 8 is Python's style guide:

• 4-space indentation, no tabs.
• snake_case for variables and functions; PascalCase for classes; UPPER_SNAKE for constants.
• Lines ≤ 88 chars (modern teams use [Black](https://black.readthedocs.io/) which auto-formats).
• Blank line between functions; two between top-level definitions.

Learn it once; tools like Black + Ruff enforce it for you.

Intermediate: Reading Errors

A traceback ends with the actual exception. Train yourself to read bottom-up:

Traceback (most recent call last):
  File "main.py", line 3, in <module>
    print(items[5])
IndexError: list index out of range

Last line = the error class + message. Line above = where it happened. 90% of debugging is reading this carefully.

Intermediate: if __name__ == '__main__'

def main():
    print("running directly")
if __name__ == '__main__':
    main()

This lets a file work both as a script (python file.py) and as an importable module (from file import main). Standard pattern in every Python project.

Advanced: Python 2 vs 3 (and Why We Ignore 2)

Python 2 reached end-of-life in January 2020. Don't write new Python 2 code. Differences you'll occasionally see in old Stack Overflow answers:

• print x (statement, Py2) vs print(x) (function, Py3).
• 5 / 2 returns 2 in Py2, 2.5 in Py3.
• Strings are bytes in Py2, Unicode in Py3.

Advanced: CPython, PyPy & Other Implementations

• CPython — the reference implementation in C. What python usually means.
• PyPy — a JIT-compiled Python; sometimes 5–10× faster on pure-Python loops.
• MicroPython — stripped-down Python for microcontrollers.
• Mojo / Cython / Numba — paths to native-speed Python.

For learning and 99% of work, CPython is what you want. Performance issues? Profile first; reach for NumPy / Cython / PyPy only when justified.

Advanced: The Python Ecosystem in One Map

• Web — Django, Flask, FastAPI.
• Data — NumPy, Pandas, Polars, DuckDB.
• ML / AI — scikit-learn, PyTorch, TensorFlow, transformers.
• Automation / scripting — requests, BeautifulSoup, playwright, click/typer.
• Tooling — pip, venv, uv, poetry, pytest, black, ruff, mypy.

Every library has a one-line install: pip install . Once you've used it, you're plugged into the largest open-source ecosystem alive.

Practice Path

1. Install Python 3.12, verify with python --version, create a virtualenv, activate it. 2. Write a one-file program that prints Hello, , takes input with input(), and runs from the command line. 3. Force an error on purpose (int("abc")); read the traceback bottom-up and explain it. 4. Add an if __name__ == '__main__': block and import the file from a second script to confirm it doesn't auto-run.