PyPath
Unit 7 • Lesson 4

Writing Data to Files

Overview

Python makes it simple to store data by writing to files. You'll use write() and writelines() to output information and learn how overwriting, appending, and formatting affect stored results, creating persistent data storage solutions.

Beginner 20–25 min
Next: Working with File Paths All Units

What You Will Learn in This Lesson

By the end of this lesson, you will know:

  • Writing to files: Use write() to save data to files.
  • Writing multiple lines: Use writelines() to write lists of strings.
  • Write vs append: Understand the difference between overwriting and adding to files.
  • Formatting output: Learn how to format data before writing it to files.
  • Common patterns: Master the most common ways to save data.

The write() Method

The write() method writes a string to a file. You can only write to files opened in write ('w') or append ('a') mode.

Basic Writing
file = open("output.txt", "w")
file.write("Hello, World!")
file.close()

Important: write() only accepts strings. If you want to write numbers or other types, convert them to strings first using str().

Writing Different Data Types
file = open("data.txt", "w")

file.write("Text: Hello\n")
file.write("Number: " + str(42) + "\n")
file.write("Float: " + str(3.14) + "\n")

file.close()

Write Mode vs Append Mode

Understanding the difference between 'w' and 'a' modes is crucial for file writing:

Write Mode ('w')

Overwrites the entire file. If the file exists, all its content is deleted first.

file = open("log.txt", "w")
file.write("New content")
file.close()
# File now contains ONLY "New content"

Append Mode ('a')

Adds to the end of the file. Existing content stays intact.

file = open("log.txt", "a")
file.write("More content")
file.close()
# File now contains old content + "More content"

When to Use Each

  • Use 'w': When you want to create a new file or replace all existing content
  • Use 'a': When you want to add to an existing file (like logging or keeping a history)

Writing Multiple Lines

To write multiple lines, you need to include newline characters (\n) in your strings.

Writing Multiple Lines
file = open("lines.txt", "w")

file.write("Line 1\n")
file.write("Line 2\n")
file.write("Line 3\n")

file.close()

Remember the Newline

Without \n, everything would be on one line. Always add \n at the end of each line you write.

Using f-strings for Formatting
name = "Alice"
score = 95

file = open("scores.txt", "w")
file.write(f"{name}: {score}\n")
file.close()

The writelines() Method

The writelines() method writes a list of strings to a file. It's useful when you have multiple lines stored in a list.

Using writelines()
lines = ["Line 1\n", "Line 2\n", "Line 3\n"]

file = open("output.txt", "w")
file.writelines(lines)
file.close()

Important: writelines() doesn't add newlines automatically. You must include \n in each string if you want separate lines.

Writing a List of Data
names = ["Alice", "Bob", "Charlie"]

file = open("names.txt", "w")
for name in names:
    file.write(name + "\n")
file.close()

# Or using writelines:
lines = [name + "\n" for name in names]
file = open("names.txt", "w")
file.writelines(lines)
file.close()

Practice: Writing to Files

Try It Yourself

Try writing different types of data to a file:

Click "Run Code" to see your output here

What happened? You simulated writing multiple lines to a file. In a real program, you would use file.write() for each line.

Common Writing Patterns

Here are the most common patterns for writing to files:

1

Writing Formatted Data

Use f-strings or string formatting to create readable output.

with open("report.txt", "w") as file:
    file.write(f"Total: {total}\n")
    file.write(f"Average: {average:.2f}\n")
    file.write(f"Count: {count}\n")
2

Writing from a Loop

Process data and write results line by line.

with open("output.txt", "w") as file:
    for item in data:
        file.write(f"{item}\n")
3

Appending Logs

Use append mode to keep a history of events.

from datetime import datetime
with open("log.txt", "a") as file:
    timestamp = datetime.now().strftime("%Y-%m-%d %H:%M:%S")
    file.write(f"{timestamp}: {event}\n")
4

Writing Structured Data

Write data in a structured format (like CSV).

students = [("Alice", 95), ("Bob", 87)]
with open("grades.txt", "w") as file:
    file.write("Name,Score\n")
    for name, score in students:
        file.write(f"{name},{score}\n")

Important Notes About Writing

Keep these important points in mind when writing to files:

Writing Considerations

  • Newlines: Always add \n at the end of each line you write
  • String conversion: write() only accepts strings - convert numbers using str()
  • Mode matters: 'w' overwrites, 'a' appends - choose carefully!
  • Buffering: Data might not be written immediately - closing the file ensures it's saved
  • File existence: 'w' and 'a' modes create the file if it doesn't exist
Example: Writing Different Data Types
name = "Alice"
age = 25
score = 95.5
is_active = True

with open("data.txt", "w") as file:
    file.write(f"Name: {name}\n")
    file.write(f"Age: {str(age)}\n")  # Convert to string
    file.write(f"Score: {str(score)}\n")
    file.write(f"Active: {str(is_active)}\n")

# Or use f-strings (they convert automatically)
with open("data2.txt", "w") as file:
    file.write(f"Name: {name}\n")
    file.write(f"Age: {age}\n")  # f-string converts automatically
    file.write(f"Score: {score}\n")
    file.write(f"Active: {is_active}\n")

Summary

In this lesson, you learned:

  • write(): Writes a string to a file - remember to add \n for new lines
  • writelines(): Writes a list of strings to a file
  • Write mode ('w'): Overwrites existing content - use to create new files
  • Append mode ('a'): Adds to end of file - use for logs and history
  • Formatting: Use f-strings to format data before writing

Remember

Always close files after writing to ensure your data is saved! And don't forget the \n for newlines.

End-of-Lesson Exercises

Think about these questions to reinforce what you've learned:

Exercise 1: Write vs Append

When would you use write mode vs append mode? Give a real-world example for each.

Exercise 2: Writing Data

Write pseudocode for saving a list of student names and scores to a file, with each student on a separate line.

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