After finishing this lesson, you will be able to confidently do the following three things.

• ▸✅ Explain why Python became the standard language for AI/data
• ▸✅ Set up venv + requirements.txt for Python 3.x
• ▸✅ Use the 4 built-in functions: print / input / type / dir

Keep these learning goals as a checklist, and close the lesson once you can answer all of them.

pytest = Python's standard testing framework (used by Django, FastAPI, numpy, and more). Any function whose name starts with test_ is automatically discovered.

1. Installation + First Test

$ pip install pytest

# test_math.py
def add(a, b):
    return a + b

def test_add_normal():
    assert add(2, 3) == 5

def test_add_negative():
    assert add(-1, -1) == -2

$ pytest test_math.py
# ============= 2 passed in 0.05s =============

2. Detailed Error on Failure

def test_fail():
    assert add(2, 3) == 6        # ❌ 5 != 6

FAILED test_math.py::test_fail
assert 5 == 6
 +  where 5 = add(2, 3)

3. Exception Assertion

import pytest

def divide(a, b):
    if b == 0:
        raise ValueError("Cannot divide by 0")
    return a / b

def test_raises_exception_on_zero_division():
    with pytest.raises(ValueError, match="Cannot divide by 0"):
        divide(10, 0)

4. Parameterization — Multiple Cases at Once

import pytest

@pytest.mark.parametrize("a, b, expected", [
    (2, 3, 5),
    (-1, -1, -2),
    (0, 0, 0),
    (100, 200, 300),
])
def test_add(a, b, expected):
    assert add(a, b) == expected

# 4 tests run automatically

5. Fixture — Shared Setup

import pytest

@pytest.fixture
def user():
    return {"name": "Hong Gil-dong", "age": 28}

def test_name(user):                       # fixture automatically injected
    assert user["name"] == "Hong Gil-dong"

def test_age(user):
    assert user["age"] >= 18

One-line Summary

def test_X(): assert ... + parametrize + fixture — these three cover 95% of cases.

1. File names: test_.py or _test.py
2. Function names: test_ prefix
3. Verify simply with a single assert statement

Python is used across many fields thanks to its concise and readable syntax. As an interpreted language, it can be executed immediately in a REPL environment. Follow the PEP 8 coding style guide and autoformat with Black/autopep8. Use type hints to improve code readability and IDE support. Manage packages with pip and set up virtual environments with venv/conda.

Knowing the concepts in this lesson lets you give specific instructions to AI. Instead of a vague 'fix this,' make vocabulary-driven requests — that is the starting point for saving tokens.

• ▸"Add pytest unit tests + parametrize to this function"
• ▸"Extract this fixture into conftest.py"

Why This Saves Tokens

Without knowing the concepts, you receive an AI response and still have to ask "What does that mean?" again. That follow-up question is what eats your tokens. Learn the concept once and the conversation ends in one round.