Phase 2: Programming Fundamentals II · ~35 minutes · Python · uv

Reading and Writing Files

A file you wrote to but haven't closed can look completely empty to anything else that checks it — even though your program "already wrote" the data.

Hiring signal: Always uses with for file handling and can explain exactly why an unflushed write looks empty

What you will learn

Open, read, and write files correctly using with blocks
Explain why with matters: automatic, guaranteed closing and flushing, even if an error happens
Reproduce and explain the surprising 'file looks empty before close()' buffering behavior
Read and write CSV files using the csv module

Introduction

Type: Learn Languages: Python Prerequisites: Lesson 03 (Strings and Text Processing) Time: ~35 minutes

Objective

Learning objectives

Open, read, and write files correctly using with blocks
Explain why with matters: automatic, guaranteed closing and flushing, even if an error happens
Reproduce and explain the surprising "file looks empty before close()" buffering behavior
Read and write CSV files using the csv module

What you're building

A script (transform_products.py) that:

Writes a real CSV file (at least 5 rows) with columns name and price, using csv.writer
Reads it back with csv.DictReader, applies a transformation to the price column (e.g., a 10% price increase), and writes the result to a new CSV file
Uses with for every file operation — no manual .close() calls
Contains a comment explaining, in your own words, why checking a file's contents before its writer has closed it can show stale or missing data

A script opens a file with open(path, "w") (no with, no explicit .close()), writes to it, and the script then crashes on an unrelated line before reaching the end. What's the most accurate prediction about the file's contents?

This is the direct real-world consequence of this lesson's buffering example. Without with (which guarantees a flush on exit, even during an exception) or an explicit, successfully-reached .close() call, there's no guarantee that buffered writes have actually reached disk by the time a crash happens elsewhere in the script. The data might be there, might be partially there, or might be missing — genuinely unreliable, which is exactly the case for using with by default rather than as an optional nicety.

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