Lesson 04

Files, errors, and testing

Work with files safely, handle failure gracefully, and get fast feedback with tests.

File I/O

# Read a whole file
from pathlib import Path
p = Path("data.txt")
text = p.read_text(encoding="utf-8")
print(len(text))

# Write safely with context manager
with open("out.txt", "w", encoding="utf-8") as f:
    f.write("hello\n")

# CSV and JSON
import csv, json
rows = [("SIG", 12.34), ("EURUSD", 1.074)]
with open("prices.csv", "w", newline="", encoding="utf-8") as f:
    writer = csv.writer(f)
    writer.writerow(["symbol","price"]) 
    writer.writerows(rows)

print(json.dumps({"ok": True, "count": len(rows)}))

Errors and exceptions

def parse_price(s: str) -> float:
    try:
        return float(s)
    except ValueError as e:
        raise ValueError(f"invalid price: {s}") from e
    finally:
        pass  # close resources if you opened anything

# Using try/except/else
value = "12.5"
try:
    price = parse_price(value)
except ValueError as e:
    print(e)
else:
    print("parsed:", price)

Testing with pytest

# test_app.py
from app import parse_price


def test_parse_price_ok():
    assert parse_price("12.5") == 12.5


def test_parse_price_bad():
    import pytest
    with pytest.raises(ValueError):
        parse_price("N/A")

Run tests with pytest -q. Keep tests small and focused.

Next steps

Next up: OOP, async, and HTTP in Lesson 05.