03 · Paths & Files

Programs that remember things need to read and write files.

What you'll learn

• Paths — where a file lives on disk. Absolute vs relative. pathlib.Path (the modern, cross-platform way).
• Reading and writing text — open(), the with statement (ensures files close even if errors happen).
• Encodings — why utf-8 matters (hint: it's the difference between niño and ni�o).
• Structured formats:
• CSV — tabular data like spreadsheets (csv module).
• JSON — nested data structures, web-friendly (json module).
• Pickle — Python objects straight to disk (use with care; not safe for untrusted sources).

Plain-language analogy

A file path is like a postal address: the operating system needs the full address (C:\Users\you\notes.txt) to find your document. A relative path is "two houses down from where you're standing right now".

Common pitfalls

• Forgetting to close files → use with open(...) as f: and never think about it again.
• Mixing path separators between OS → use pathlib.Path, never string concatenation.
• Encoding mismatches → always specify encoding="utf-8" unless there's a reason not to.

Source code

courses/python-course/03 - Paths and Files/