4. The Build → Improve → Secure Cycle

Every software project evolves through three stages:

1. Build – Get it working.
2. Improve – Make it efficient and maintainable.
3. Secure – Fortify it against real-world misuse.

Example: Evolving a Simple Reader

# 4. Build
def read_file(path):
    return open(path).read()

# 4. Improve
def read_file_clean(path):
    with open(path, "r", encoding="utf-8") as f:
        return f.read()

# 4. Secure
import os
def read_file_secure(path):
    if not os.path.exists(path):
        raise FileNotFoundError("Missing file!")
    if ".." in path or path.startswith("/"):
        raise ValueError("Invalid path.")
    with open(path, "r", encoding="utf-8") as f:
        return f.read()

✅ Lesson: The final step isn’t just polish — it’s protection.