Intermediate 45 min

Module 16 — File I/O

Reading and writing files safely — text, and structured data with CSV.

Prerequisite: Module 15 — Error & Exception Handling

After this lesson, you will be able to

Open, read, and write text files safely using a with block
Explain the common file modes: r, w, a
Read and write CSV files with the csv module

Concept & Syntax

So far every program's data has disappeared the moment it stops running. File I/O (input/ output) is how a program persists data — saving it to disk, and reading it back later. Python opens files with open(), but the safe, idiomatic way to do it is inside a with block, which automatically closes the file for you, even if an error happens partway through.

Syntax

with open("notes.txt", "r", encoding="utf-8") as f:
    content = f.read()
 
with open("notes.txt", "w", encoding="utf-8") as f:
    f.write("New content")

Examples

Writing a text file Very Easy

Writing a text file

with open("notes.txt", "w", encoding="utf-8") as f:
    f.write("First line\n")
    f.write("Second line\n")

Output(creates notes.txt on disk — no console output)

Mode "w" creates the file if it doesn't exist, or overwrites it completely if it does. The with block closes the file automatically when it's done.

Reading a whole file Easy

Reading a whole file

with open("notes.txt", "r", encoding="utf-8") as f:
    content = f.read()
print(content)

OutputFirst line Second line

Mode "r" (the default) opens for reading. read() returns the entire file's contents as one string.

Reading line by line Intermediate

Reading line by line

with open("notes.txt", "r", encoding="utf-8") as f:
    for line in f:
        print("Line:", line.strip())

OutputLine: First line Line: Second line

Looping directly over an open file gives you one line at a time — memory-efficient for large files. strip() removes the trailing newline.

Appending instead of overwriting Real World

Appending instead of overwriting

with open("notes.txt", "a", encoding="utf-8") as f:
    f.write("Third line\n")

Output(adds a new line to the end of the existing file)

Mode "a" (append) adds to the end of the file instead of erasing what's already there.

Reading and writing CSV Challenge

Reading and writing CSV

import csv
 
with open("scores.csv", "w", newline="", encoding="utf-8") as f:
    writer = csv.writer(f)
    writer.writerow(["name", "score"])
    writer.writerow(["Ada", 92])
 
with open("scores.csv", "r", encoding="utf-8") as f:
    reader = csv.DictReader(f)
    for row in reader:
        print(row["name"], row["score"])

OutputAda 92

csv.writer writes rows as comma-separated values; csv.DictReader reads each row back as a dictionary, keyed by the header row.

Visual Explanation

A with block guarantees the file gets closed, freeing the resource safely.

💡 Why does this work?

Read the concept explanation above again slowly, line by line — every keyword there maps directly onto a line of code in the examples. If it still doesn't click, re-run the traced example (if this lesson has one) one step at a time.

🔍 What happens internally? (advanced)

Under the hood, Python compiles your source into bytecode and runs it on a virtual machine (the CPython interpreter). Variables are names in a namespace dictionary pointing at objects in memory — which is exactly what the memory panel in the tracer above is showing you.

Common Mistakes

✗ Common mistake 1

Opening a file without a with block (or forgetting to close it) can leave data unwritten or the file locked by your program.

✗ Avoid this

This raises not always an immediate crash, but data can be lost or the file left locked

f = open("notes.txt", "w")
f.write("Hello")
# forgot f.close() — data may not be saved yet

✓ Better approach

Fixed

with open("notes.txt", "w") as f:
    f.write("Hello")
# file is guaranteed to be closed here

✓ Best practices

Always use a with block to open files — it closes them automatically, even on error.
Specify encoding="utf-8" explicitly, since default encodings vary across operating systems.
Use mode "a" only when you genuinely want to add to existing content — "w" erases everything first.
Prefer the csv module over manually splitting lines on commas — it correctly handles quoted fields.

Exercise & Challenge

Exercise 1 — Predict the output

Given this code

with open("notes.txt", "a") as f:
    f.write("World")

If notes.txt already contains 'Hello', what will it contain after this code runs?

🏆 Challenge

Write a program that writes 5 numbers (1 through 5) to a file, one per line, then reads them back and prints their sum.

Write with mode "w"
Read the file back and convert each line to int()
Print the total sum

💡 Need a hint?

int(line.strip()) removes the trailing newline before converting.

🔍 Show a sample solution

Python

with open("numbers.txt", "w") as f:
    for n in range(1, 6):
        f.write(f"{n}\n")
 
total = 0
with open("numbers.txt", "r") as f:
    for line in f:
        total += int(line.strip())
print(total)

Quiz

Answer every question, then submit to see your score and explanations.

1. What does opening a file in mode "w" do if the file already exists?

Appends to it

Raises an error

Overwrites it completely

Opens it read-only

"w" mode erases existing content before writing.

2. Why is a with block preferred when working with files?

It's required syntax

It automatically closes the file, even if an error occurs

It makes the file bigger

It only works with CSV files

with guarantees cleanup (closing the file) regardless of errors.

3. What does csv.DictReader give you for each row?

A plain string

A list of strings

A dictionary keyed by the header row

A tuple

DictReader uses the first row as keys for every subsequent row.

Summary

open(filename, mode) opens a file; always wrap it in a with block.
Mode "r" reads, "w" overwrites, "a" appends.
Looping over an open file yields one line at a time.
The csv module handles reading and writing tabular data correctly, including quoted fields.

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