Lesson 13 - File I/O Operations

Learning Objective

Read from and write to text files using with open(...), and understand the difference between read, write, and append modes.

Introduction

Every program you’ve written so far stores its data only in memory.

When the program stops running, that data disappears.

Files solve this problem by allowing programs to save information to disk and read it back later.

Examples include:

• Saving student records
• Storing robot logs
• Reading configuration settings
• Loading datasets
• Writing reports

Python makes file handling straightforward through the built-in open() function.

In this lesson you’ll learn the three file modes you’ll use most often:

Opening a File

The modern Python approach is:

with open("example.txt", "r") as f:
    content = f.read()

This creates a file object named f.

Python automatically closes the file when the with block finishes.

Writing a File

Use "w" mode to create or overwrite a file.

with open("notes.txt", "w") as f:
    f.write("Hello, Python!\n")

If notes.txt already exists:

Its previous contents are erased.

Appending to a File

Use "a" mode when you want to keep existing data.

with open("notes.txt", "a") as f:
    f.write("Another line\n")

The new text is added to the end of the file.

Existing content remains untouched.

Reading a File

Use "r" mode to read data.

with open("notes.txt", "r") as f:
    text = f.read()
 
print(text)

Output:

Hello, Python!
Another line

Code

# file_basics.py
 
names = ["Aditi", "Rahul", "Meera"]
 
with open("students.txt", "w") as f:
    for name in names:
        f.write(name + "\n")
 
with open("students.txt", "r") as f:
    content = f.read()
    print(content)
 
with open("students.txt", "r") as f:
    for line in f:
        print(f"Student: {line.strip()}")
 
with open("students.txt", "a") as f:
    f.write("Kabir\n")
 
with open("students.txt", "r") as f:
    all_lines = f.readlines()
    print(all_lines)

Expected Output

Aditi
Rahul
Meera

Student: Aditi
Student: Rahul
Student: Meera

['Aditi\n', 'Rahul\n', 'Meera\n', 'Kabir\n']

Three Common Reading Approaches

Read Entire File

with open("students.txt", "r") as f:
    content = f.read()

Returns:

"Aditi\nRahul\nMeera\n"

Useful for small files.

Read Line by Line

with open("students.txt", "r") as f:
    for line in f:
        print(line)

Useful for large files because it doesn’t load everything into memory at once.

Read All Lines into a List

with open("students.txt", "r") as f:
    lines = f.readlines()

Returns:

[
    "Aditi\n",
    "Rahul\n",
    "Meera\n"
]

Each item is one line.

Why Use .strip()?

When reading text files, each line usually ends with:

"\n"

Example:

line = "Aditi\n"

Using:

line.strip()

returns:

"Aditi"

without the newline character.

This is especially useful when printing or parsing file contents.

Common File Modes

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

open("file.txt", "w")

open("file.txt", "a")

Example Workflow

Step 1:

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

File contents:

Start

Step 2:

with open("log.txt", "a") as f:
    f.write("Finished\n")

File contents:

Start
Finished

Step 3:

with open("log.txt", "r") as f:
    print(f.read())

Output:

Start
Finished

🔧 Under the Hood

f = open("file.txt", "w")

f.close()

f = open("file.txt", "w")
 
do_something()
 
f.close()

do_something()

f.close()

with open("file.txt", "w") as f:
    do_something()

with open(...)

with open("students.txt") as f:
    lines = f.readlines()

[
    "Aditi\n",
    "Rahul\n",
    "Meera\n"
]

line.strip()

Student Challenge

Challenge 1 — Python Only

Write a small grade report to a file, then read it back and calculate the average score.

• Write student records ("Aditi,82", "Rahul,91", "Meera,76") to a text file grades.txt.
• Read grades.txt line by line, parse each student’s name and score, print each formatted line (e.g. Aditi: 82), and calculate the average score.

line.strip()

.split(",")

# grade_report.py
 
records = [
    "Aditi,82",
    "Rahul,91",
    "Meera,76"
]
 
with open("grades.txt", "w") as f:
    for record in records:
        f.write(record + "\n")
 
total = 0
count = 0
 
with open("grades.txt", "r") as f:
    for line in f:
        name, score_text = line.strip().split(",")
 
        score = int(score_text)
 
        total += score
        count += 1
 
        print(f"{name}: {score}")
 
print(f"Average score: {total / count:.1f}")

Aditi: 82
Rahul: 91
Meera: 76
Average score: 83.0

Challenge 2 — Robot

Log your BonicBot’s battery and position during a short patrol, then read the log back.

• Open patrol_log.txt in write mode and log battery level, position, and obstacle distance (bot.get_min_obstacle_distance()) during a 3-step patrol.
• Open patrol_log.txt in read mode and print the logged content.

Additional sensor methods are available in the Python SDK reference.

f"battery={battery}, "
f"position={position}, "
f"obstacle={distance}"

# challenge_flight_log.py
 
from bonicbot_bridge import BonicBot
 
with BonicBot(host='192.168.0.188') as bot:
    bot.wait_for_data(timeout=3.0)
 
    with open("patrol_log.txt", "w") as log_file:
        for step in range(3):
            bot.drive_distance(0.3, speed=0.2)
 
            battery = bot.get_battery()
            position = bot.get_position()
            distance = bot.get_min_obstacle_distance()
 
            log_file.write(
                f"Step {step}: "
                f"battery={battery}%, "
                f"position={position}, "
                f"obstacle={distance}\n"
            )
 
            print(f"Logged step {step}")
 
    bot.stop()
 
with open("patrol_log.txt", "r") as log_file:
    print(log_file.read())

Real-World Uses of File Handling

Common examples include:

• Saving robot telemetry
• Recording experiment results
• Reading CSV datasets
• Writing application logs
• Loading configuration files
• Storing user preferences

File handling is one of the most practical Python skills you’ll learn because nearly every real application needs persistent storage.

Reflection Question

The patrol log is written inside the with BonicBot(...) block while the robot is actively moving. If an error happened mid-loop — for example, the robot lost connection during step 2 — what would with open(...) guarantee about the log file that manually calling open() and close() would not?