Chapter Introduction and Learning Objectives

Learning Objectives

By the end of this chapter, you should be able to:

1. 1Explain why programs use files and what persistence means.
2. 2Describe how the file system organizes files into folders and identify the parts of a path.
3. 3Distinguish between absolute paths and relative paths and choose the right one for a project.
4. 4Open and close a file using open() and explain the purpose of each file mode.
5. 5Use the with statement to open files safely and explain why it is preferred over manual opening and closing.
6. 6Read file contents using .read(), .readline(), .readlines(), and direct iteration.
7. 7Write text to a file using .write() and .writelines().
8. 8Explain the difference between write mode and append mode and choose the correct one for a task.
9. 9Explain how newline characters work in text files and manage them correctly when reading and writing.
10. 10Identify and explain common file errors, including FileNotFoundError.
11. 11Trace, debug, and write complete programs that read from and write to text files.

Sections at a Glance

• 21.1Why Programs Use FilesPersistence, input, output, logging, configuration, and data exchange — the reasons real programs read and write files
• 21.2Files, Folders, and PathsThe file system as a hierarchy; the root; what a path is; base names and extensions
• 21.3Relative and Absolute PathsAbsolute paths from the root; relative paths from the current working directory; why relative paths suit beginner projects
• 21.4Opening and Closing Filesopen(), file modes, file objects, closing files, and the safer with statement
• 21.5Reading Text Files.read(), .readline(), .readlines(), iterating directly over a file, and a worked line/word counting example
• 21.6Writing Text Files.write(), .writelines(), overwriting behavior, and a worked report-generation example
• 21.7Appending TextAppend mode versus write mode; building a timestamped log file
• 21.8The with StatementContext managers; why with guarantees a file is closed; opening multiple files at once
• 21.9Newlines and Text FormattingThe newline character; stripping newlines on read; adding newlines on write; cross-platform line endings
• 21.10Common File ErrorsFileNotFoundError, PermissionError, IsADirectoryError, silent logic errors, and a preview of try/except

Why Programs Use Files

Files, Folders, and Paths

The File System

What a Path Is

C:\Users\Rosa\Documents\grades.txt       (Windows)
/home/rosa/documents/grades.txt          (macOS / Linux)

What a File Name Is

grades.txt        base name: grades     extension: .txt
notes.md          base name: notes      extension: .md
report.csv        base name: report     extension: .csv

Relative and Absolute Paths

Absolute Paths

C:\Users\Rosa\Documents\grades.txt
/home/rosa/documents/grades.txt

Relative Paths

grades.txt                     means /home/rosa/projects/grades/grades.txt
data/scores.csv                means /home/rosa/projects/grades/data/scores.csv
../notes.txt                   means /home/rosa/projects/notes.txt

Which One to Use?

# If grades.txt is in the same folder as your program:
filename = "grades.txt"

filename = "data/grades.txt"

Opening and Closing Files

The open() Function

file = open("notes.txt", "r")

Closing the File

file = open("notes.txt", "r")
# ... do work with the file ...
file.close()

The with Statement — the Right Way to Open Files

with open("notes.txt", "r") as file:
    # work with file here
    pass
# file is automatically closed here

Reading Text Files

Reading the Entire File at Once with .read()

with open("message.txt", "r") as file:
    content = file.read()

print(content)

Hello, Rosa.
Your report is ready.
Please review it by Friday.

Reading One Line at a Time with .readline()

with open("message.txt", "r") as file:
    line1 = file.readline()
    line2 = file.readline()

print(line1)
print(line2)

line1 = file.readline().rstrip("\n")

line1 = file.readline().strip()

Reading All Lines into a List with .readlines()

with open("message.txt", "r") as file:
    lines = file.readlines()

print(lines)

Iterating Over a File Directly — the Cleanest Approach

with open("message.txt", "r") as file:
    for line in file:
        print(line.strip())

line_count = 0
word_count = 0

with open("message.txt", "r") as file:
    for line in file:
        line = line.strip()
        line_count += 1
        words = line.split()
        word_count += len(words)

print("Lines:", line_count)
print("Words:", word_count)

Writing Text Files

Writing with .write()

with open("output.txt", "w") as file:
    file.write("Line one\n")
    file.write("Line two\n")
    file.write("Line three\n")

Line one
Line two
Line three

Writing a List of Lines with .writelines()

lines = ["apple\n", "banana\n", "cherry\n"]

with open("fruits.txt", "w") as file:
    file.writelines(lines)

apple
banana
cherry

names = ["Aisha", "Ben", "Clara"]
scores = [88, 74, 95]

with open("report.txt", "w") as file:
    file.write("Student Score Report\n")
    file.write("====================\n")
    for i in range(len(names)):
        line = f"{names[i]}: {scores[i]}\n"
        file.write(line)
    file.write("====================\n")
    average = sum(scores) / len(scores)
    file.write(f"Class average: {average:.1f}\n")

print("Report saved to report.txt")

Appending Text

with open("log.txt", "a") as file:
    file.write("Program started at 9:00 AM\n")

import datetime

def log_event(message):
    timestamp = datetime.datetime.now().strftime("%Y-%m-%d %H:%M:%S")
    entry = f"[{timestamp}] {message}\n"
    with open("events.log", "a") as file:
        file.write(entry)

log_event("Program started")
log_event("User entered a name")
log_event("Program finished")

The with Statement

with open("data.txt", "r") as file:
    content = file.read()

file = open("data.txt", "r")
content = file.read()
file.close()       # If an error happens above, this never runs

with open("data.txt", "r") as file:
    content = file.read()
# file is always closed here

Opening Multiple Files at Once

with open("input.txt", "r") as infile, open("output.txt", "w") as outfile:
    for line in infile:
        outfile.write(line.upper())

Newlines and Text Formatting

What Newlines Look Like in Python

with open("items.txt", "r") as file:
    line = file.readline()
    print(repr(line))

Stripping Newlines

with open("items.txt", "r") as file:
    for line in file:
        item = line.strip()
        print(item)

Adding Newlines When Writing

with open("test.txt", "w") as file:
    file.write("first")
    file.write("second")
    file.write("third")

with open("test.txt", "w") as file:
    file.write("first\n")
    file.write("second\n")
    file.write("third\n")

Cross-Platform Newlines

Common File Errors

FileNotFoundError

with open("missing.txt", "r") as file:
    content = file.read()

import os
path = "missing.txt"
print("File exists:", os.path.exists(path))

PermissionError

IsADirectoryError

with open("my_folder", "r") as file:
    pass

Writing to a File Fails Silently

with open("output.txt", "r") as file:
    file.write("hello\n")    # io.UnsupportedOperation: not writable

Overwriting Instead of Appending

# Bug: this erases the file each time instead of adding to it
with open("log.txt", "w") as file:
    file.write("New entry\n")

Handling File Errors with try/except

filename = "notes.txt"

try:
    with open(filename, "r") as file:
        content = file.read()
    print(content)
except FileNotFoundError:
    print(f"Error: the file '{filename}' was not found.")
    print("Please check the file name and location.")

Chapter Summary

• Section 21.1Files let programs persist data between runs. Programs use files for persistence, input, output, logging, configuration, and data exchange.
• Section 21.2The file system organizes files in a hierarchy of folders starting from a root. A path identifies a file's location; a file name has a base name and an extension.
• Section 21.3Absolute paths describe a location from the root and work anywhere, but break if a project moves. Relative paths are interpreted from the current working directory and are usually the better choice for beginner projects.
• Section 21.4open() creates a file object using a path and a mode: "r" for reading, "w" for writing (overwriting), "a" for appending. Files should always be closed; the with statement does this automatically and safely.
• Section 21.5.read() returns the whole file as one string, .readline() reads one line at a time, and .readlines() returns a list of lines. Iterating directly over a file object with a for loop is the cleanest, most memory-efficient way to process a file line by line.
• Section 21.6.write() writes one string, and .writelines() writes a list of strings. Neither adds newlines automatically. Opening an existing file in "w" mode erases its previous contents.
• Section 21.7Append mode ("a") adds new content to the end of a file without destroying what is already there, which makes it the right choice for log files and other accumulating records.
• Section 21.8The with statement is a context manager that opens a file, runs a block of code, and guarantees the file is closed afterward — even if an error occurs. It can open more than one file at a time using commas.
• Section 21.9Lines in a text file end with the newline character \n. Reading methods preserve this character, so it is usually stripped with .strip(). Writing methods never add it, so it must be included explicitly.
• Section 21.10FileNotFoundError is the most common file error, typically caused by a wrong path or misspelled name. PermissionError and IsADirectoryError occur less often. Logic errors — like using "w" when you meant "a" — fail silently and are caught only by careful checking. Chapter 23 introduces try/except to handle these errors gracefully.

Key Terms

Read each program carefully. Answer the questions without running the code.

with open("greeting.txt", "w") as file:
    file.write("Hello\n")
    file.write("World\n")

with open("data.txt", "r") as file:
    for line in file:
        print(line.strip())

apple
banana
cherry

total = 0
count = 0

with open("scores.txt", "r") as file:
    for line in file:
        score = int(line.strip())
        total += score
        count += 1

if count > 0:
    print(f"Average: {total / count:.1f}")
else:
    print("No scores found.")

80
90
70
100

with open("notes.txt", "a") as file:
    file.write("Meeting at 3pm\n")

with open("notes.txt", "a") as file:
    file.write("Call Rosa\n")

Trace each program by tracking variables after each significant step.

cat
dog
fish

result = []

with open("words.txt", "r") as file:
    for line in file:
        word = line.strip()
        result.append(word.upper())

print(result)

lines_written = 0

items = ["pen", "notebook", "eraser"]

with open("supplies.txt", "w") as file:
    for item in items:
        file.write(item + "\n")
        lines_written += 1

print(f"Wrote {lines_written} lines.")

with open("log.txt", "w") as file:
    file.write("Start\n")

with open("log.txt", "a") as file:
    file.write("Middle\n")

with open("log.txt", "a") as file:
    file.write("End\n")

with open("log.txt", "r") as file:
    content = file.read()

print(content)

For each program, predict exactly what is printed to the screen. If the program causes an error, identify the error and explain why.

with open("hello.txt", "r") as file:
    text = file.read()

print(len(text))
print(text.upper())

with open("numbers.txt", "w") as file:
    for n in range(1, 5):
        file.write(str(n))

with open("numbers.txt", "r") as file:
    print(file.read())

with open("ghost.txt", "r") as file:
    print(file.read())

lines = ["one\n", "two\n", "three\n"]

with open("count.txt", "w") as file:
    file.writelines(lines)

with open("count.txt", "r") as file:
    data = file.readlines()

print(len(data))
print(data[1])

Each program below contains a bug or design problem. Identify the issue and write a corrected version.

file = open("output.txt", "w")
file.write("Hello\n")
file.write("World\n")

with open("journal.txt", "w") as file:
    file.write("Day 1: Arrived safely.\n")

with open("journal.txt", "w") as file:
    file.write("Day 2: Explored the city.\n")

with open("journal.txt", "r") as file:
    print(file.read())

with open("prices.txt", "r") as file:
    for line in file:
        price = float(line)
        print(f"${price:.2f}")

3.99
12.50
7.00

with open("report.txt", "w") as file:
    file.write("Name: Aisha")
    file.write("Score: 95")
    file.write("Grade: A")

filename = "data.txt"
with open(filename, "r") as file:
    lines = file.readlines()

with open(filename, "w") as file:
    for line in lines:
        file.write(line)

Write a complete Python program for each task. Test your program before submitting it.

1. Word Saver (21-S1). Write a program that asks the user to enter five words, one at a time, and saves each word on its own line in a file called words.txt. After saving, open the file and print all the words back.
2. Line Counter (21-S2). Write a function called count_lines(filename) that takes a file name as a parameter and returns the number of lines in the file. If the file does not exist, the function should return -1. Test your function with at least two different files.
3. File Reverser (21-S3). Write a program that reads all the lines from a file, reverses their order, and writes them to a new file. For example, if original.txt contains first, second, third on separate lines, then reversed.txt should contain third, second, first.
4. Append Log (21-S4). Write a function called log(message) that appends a line to a file called program.log. The line should contain the message followed by a newline. Call the function at least four times with different messages, then read and print the entire log file.
5. File Statistics (21-S5). Write a program that reads a text file and prints the number of lines, the number of non-empty lines, the total number of words, and the total number of characters (not counting newlines). Test it with a file you create.
6. Number File Processor (21-S6). Write a program that reads a file called numbers.txt where each line contains one integer. The program should compute and print the minimum, maximum, sum, and average of all the numbers. Handle the case where the file is empty.

Core Features

=== Notes Program ===
1. View all notes
2. Add a note
3. Clear all notes
4. Quit

Requirements

=== Notes Program ===
1. View all notes
2. Add a note
3. Clear all notes
4. Quit
> 2
Enter your note: Buy milk and eggs.
Note saved.

=== Notes Program ===
1. View all notes
2. Add a note
3. Clear all notes
4. Quit
> 1
--- Your Notes ---
Buy milk and eggs.
------------------

=== Notes Program ===
...
> 4
Goodbye.

These questions draw on earlier chapters. Keep your earlier skills sharp.

1. (21-MR1) Write a function called words_in_list(filename) that reads a text file (one word per line) and returns a list of all the words, stripped of whitespace. If the file does not exist, return an empty list.
2. (21-MR2) Write a program that generates a multiplication table for numbers 1 through 5 and saves it to times_table.txt. Each row should look like 1 x 1 = 1 through 5 x 5 = 25.
3. (21-MR3) A file called inventory.txt has entries in this format, one per line: pencil,50. Write a program that reads the file and prints each item and its quantity, then prints the total number of items across all categories.
4. (21-MR4) Write a function called save_list(items, filename) that takes a list of strings and writes each item on its own line to the given file. Write a second function called load_list(filename) that reads the file and returns a list of strings (stripped). Demonstrate that saving a list and then loading it gives back the same list.