Files, CSV & Errors

Read and write files safely, work with paths and handle predictable failures.

By the end of this lesson

You will be able to

Use pathlib for file paths.
Use a context manager to open files.
Read structured CSV data.
Catch specific exceptions and report useful errors.

Build paths from the project location

Avoid hard-coded machine-specific absolute paths. Use pathlib.Path to construct paths relative to the project and check whether expected files exist before reading them.

from pathlib import Path

PROJECT_DIR = Path(__file__).resolve().parent
DATA_DIR = PROJECT_DIR / "data"
CSV_PATH = DATA_DIR / "hour.csv"

if not CSV_PATH.exists():
    raise FileNotFoundError(f"Dataset not found: {CSV_PATH}")

Let the file close reliably

The with statement manages the file lifecycle. When reading CSV manually, use the standard csv module; when the next lessons begin, pandas will handle tabular loading more directly.

import csv

with CSV_PATH.open(newline="", encoding="utf-8") as file:
    reader = csv.DictReader(file)
    first_row = next(reader)

print(first_row)

Catch only errors you can handle

Catch the specific exception you expect, add context and either recover or stop. Avoid except Exception: pass; it hides the cause and can make a bad report look successful.

try:
    hour = int("not-an-hour")
except ValueError as error:
    print(f"Invalid hour value: {error}")

Apply what you learned

Exercise

Create a reliable dataset loader

Write a loader that reports clear errors for a missing or unreadable CSV file.

Accept a path as a function argument.
Check that the path exists and is a file.
Open the file using a context manager.
Catch file and parsing errors at the user-facing boundary.
Return parsed rows or raise a clear, specific error.

Done when

A missing path and a malformed file produce understandable messages; a valid file loads successfully.

Free references

Study resources

3 links

01
Reading and writing files Article
Read the with statement and text-file examples.

Done
02
Errors and exceptions Article
Focus on handling exceptions and raising your own errors.

Done
03
Try/Except Blocks for Error Handling Video
Watch for the control flow of try, except, else and finally.

Done

Check your understanding

Self-test

2 questions

Question 1

Why is Path(__file__).resolve().parent useful?
Question 2

Why should exceptions be specific?

Completion check

Move on when

The loader handles paths, closes files reliably and reports expected failures clearly.

Mark passed and continue Back to module