lesson.md

Lesson 01: Setup & Hello World

1. Install the toolchain

Java has sdkman + Maven/Gradle. The modern Python equivalent is uv: one fast tool that installs Python versions, creates virtual environments, and manages dependencies.

curl -LsSf https://astral.sh/uv/install.sh | sh   # official installer (or: brew install uv on Apple Silicon)
uv python install 3.13        # installs Python 3.13 (doesn't touch macOS's /usr/bin/python3)
uv init --name python-for-java-developers --python 3.13 .   # creates pyproject.toml (≈ pom.xml); already done in this repo
uv run python --version       # runs inside the project's virtual env (.venv/)

In VS Code, install the Python extension (Microsoft). Then run Python: Select Interpreter and pick .venv/bin/python.

Virtual env ≈ a per-project classpath. Every project gets its own isolated .venv/ folder with its own packages. Never pip install into the system Python.

2. Hello World

public class Hello {
    public static void main(String[] args) {
        System.out.println("Hello, World!");
    }
}

print("Hello, World!")

There is no class, no main, no semicolons, and no compile step. A .py file is a script that runs top to bottom.

3. The 10 biggest mindset shifts from Java

Java	Python
{ } blocks	indentation is syntax (4 spaces)
static types, checked at compile time	dynamic types; optional type hints checked by tools
String name = "Reza";	name = "Reza" (or name: str = "Reza")
"Hi " + name + "!"	f"Hi {name}!" (f-strings)
true / false / null	True / False / None
&& || !	and or not
== is identity for objects	== is equality (calls __eq__); is is identity
for (int i=0; i<n; i++)	for i in range(n):
int overflows at 2^31	int is arbitrary precision: 2**1000 works
public static void main	if __name__ == "__main__":

4. The REPL (like jshell, but you'll actually use it)

uv run python
>>> 2 ** 100
>>> type(3.14)
>>> help(str.split)
>>> dir("hello")      # list every method on a string

Use the REPL constantly for quick experiments.

5. Run the examples

uv run python lessons/01_hello/hello.py
uv run python lessons/01_hello/basics.py

Read basics.py top to bottom. Every block has a Java comparison in its comments.

6. Exercise

Open exercise.py, fill in the TODOs, and run it. Tell me when you're done and I'll review it.