Unit 11.4A · Term 4

Writing Files

Writing files allows your program to save data permanently — results, reports, logs, and user data that persist after the program ends. Master both write mode (overwrite) and append mode (add to end).

Learning Objectives

11.4.1.2 Write data to text files

Lesson Presentation

11.4A-lesson-22-file-write.pdf · Slides for classroom use

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Conceptual Anchor

Writing vs Appending

Write mode ("w") is like getting a fresh notebook — it erases everything and starts from scratch. Append mode ("a") is like opening your notebook to the last page and continuing where you left off. Choose wisely!

Rules & Theory

Writing to Files

# Write mode — creates new file or OVERWRITES existing!
with open("output.txt", "w") as file:
    file.write("Hello, World!\n")
    file.write("This is line 2.\n")
    file.write(f"Score: {95}\n")

# Append mode — adds to end of existing file
with open("log.txt", "a") as file:
    file.write("New entry added\n")

# writelines() — write a list of strings
lines = ["Line 1\n", "Line 2\n", "Line 3\n"]
with open("output.txt", "w") as file:
    file.writelines(lines)

Write vs Append

Feature	Write ("w")	Append ("a")
Existing file	Overwrites (deletes old content!)	Adds to end
New file	Creates it	Creates it
Use for	Fresh output, reports	Logs, adding records
Danger	Data loss if not careful!	File grows over time

Writing Different Data Types

# write() only accepts strings!
score = 95
name = "Ali"

with open("result.txt", "w") as file:
    # Must convert to string
    file.write(str(score) + "\n")    # Explicit conversion
    file.write(f"{name},{score}\n")  # f-string (cleaner!)

    # Writing from a list
    scores = [85, 90, 78, 92]
    for s in scores:
        file.write(f"{s}\n")

    # Using print() with file parameter
    print("This goes to file!", file=file)
    print(score, file=file)

Remember: Newlines Are Manual

Unlike print(), file.write() does NOT add a newline automatically. You must include \n yourself at the end of each line.

Worked Examples

1 Saving Student Data as CSV

students = [
    ("Ali", 16, 95),
    ("Dana", 15, 88),
    ("Marat", 16, 72)
]

with open("students.csv", "w") as file:
    file.write("Name,Age,Score\n")  # Header row
    for name, age, score in students:
        file.write(f"{name},{age},{score}\n")

print("Data saved to students.csv!")

2 Read → Process → Write

# Read scores, calculate grade, write results
with open("scores.txt", "r") as infile:
    scores = [int(line.strip()) for line in infile]

def get_grade(s):
    if s >= 90: return "A"
    if s >= 80: return "B"
    if s >= 70: return "C"
    if s >= 60: return "D"
    return "F"

with open("results.txt", "w") as outfile:
    outfile.write("Score,Grade\n")
    for score in scores:
        grade = get_grade(score)
        outfile.write(f"{score},{grade}\n")
    outfile.write(f"\nAverage: {sum(scores)/len(scores):.1f}\n")

3 Activity Logger

from datetime import datetime

def log_activity(activity):
    """Append a timestamped entry to the log file."""
    timestamp = datetime.now().strftime("%Y-%m-%d %H:%M:%S")
    with open("activity_log.txt", "a") as file:
        file.write(f"[{timestamp}] {activity}\n")

# Each call ADDS to the file (doesn't overwrite)
log_activity("User logged in")
log_activity("Viewed dashboard")
log_activity("Updated profile")

Pitfalls & Common Errors

"w" Mode Destroys Data!

Opening a file in "w" mode immediately erases all content, even if you don't write anything. Always double-check before using write mode on important files.

write() Doesn't Add Newlines

file.write("Hello") followed by file.write("World") produces HelloWorld — all on one line. Include \n yourself.

write() Only Accepts Strings

file.write(42) raises TypeError. Convert first: file.write(str(42)) or use f"{42}".

Pro-Tips for Exams

File Writing Tips

print(data, file=outfile) — automatically adds \n, easier than write()
CSV pattern: file.write(",".join([str(x) for x in row]) + "\n")
Always use "a" for logs and records that should accumulate
Use "w" only for creating fresh output files

Graded Tasks

Remember

What is the difference between "w" and "a" mode?

Understand

Why does file.write(42) cause an error? What is the fix?

Apply

Write a program that asks for 5 names and saves them to a file, one per line.

Apply

Read numbers from input.txt, double each one, and write results to output.txt.

Analyze

What happens if you open a file with "w", write nothing, and close it? What about with "a"?

Create

Build a diary program: users enter text entries that are appended to a file with timestamps. Include a "read all entries" option.

Self-Check Quiz

1. What mode opens a file for writing (overwriting)?

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2. Does file.write() add a newline automatically?

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3. How do you add data to an existing file without erasing it?

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4. Can write() accept an integer?

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5. How do you write a list to a file, one item per line?

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