Python Basics — Writing Files

Writing Files

Writing data to files lets you save program output, create reports, store configuration, and persist user data.

Write Mode ("w")

Write mode creates a new file or overwrites an existing one:

with open("output.txt", "w") as f:
    f.write("Hello, World!\n")
    f.write("This is line 2.\n")

The file now contains:

Hello, World!
This is line 2.

Warning: "w" mode erases all existing content. Be careful not to lose data!

Append Mode ("a")

Append mode adds content to the end of an existing file (or creates it if it doesn’t exist):

with open("log.txt", "a") as f:
    f.write("2025-03-03 10:00 — Server started\n")

# Later...
with open("log.txt", "a") as f:
    f.write("2025-03-03 10:05 — User logged in\n")

The file accumulates entries:

2025-03-03 10:00 — Server started
2025-03-03 10:05 — User logged in

write() and writelines()

write() — Write a Single String

with open("output.txt", "w") as f:
    f.write("Line 1\n")
    f.write("Line 2\n")

writelines() — Write a List of Strings

lines = ["Line 1\n", "Line 2\n", "Line 3\n"]

with open("output.txt", "w") as f:
    f.writelines(lines)

Note: writelines() does NOT add newlines automatically — you must include \n in each string.

Creating New Files

Exclusive Create Mode ("x")

Creates a file only if it doesn’t already exist:

try:
    with open("newfile.txt", "x") as f:
        f.write("This is a brand new file!\n")
    print("File created successfully!")
except FileExistsError:
    print("File already exists — not overwriting.")

Binary Mode ("wb")

For non-text files (images, PDFs, etc.):

# Copy a binary file
with open("photo.jpg", "rb") as source:
    data = source.read()

with open("photo_copy.jpg", "wb") as dest:
    dest.write(data)

print() to a File

You can use print() with the file parameter:

with open("output.txt", "w") as f:
    print("Name: Alice", file=f)
    print("Age:", 30, file=f)
    print(f"Score: {95.5:.1f}", file=f)

Contents:

Name: Alice
Age: 30
Score: 95.5

This is handy because print() automatically adds newlines.

Checking if File Exists Before Writing

from pathlib import Path

file_path = Path("important_data.txt")

if file_path.exists():
    print("File exists — using append mode")
    mode = "a"
else:
    print("File doesn't exist — creating new")
    mode = "w"

with open(file_path, mode) as f:
    f.write("New data entry\n")

Writing with pathlib

from pathlib import Path

# Write text
Path("output.txt").write_text("Hello from pathlib!\n", encoding="utf-8")

# Read it back
content = Path("output.txt").read_text(encoding="utf-8")
print(content)  # Hello from pathlib!

write_text() always overwrites. There’s no append mode with this method.

Practical Example: Writing a Report

students = [
    {"name": "Alice", "score": 85},
    {"name": "Bob", "score": 92},
    {"name": "Charlie", "score": 78},
    {"name": "Diana", "score": 95},
]

with open("report.txt", "w", encoding="utf-8") as f:
    f.write("=" * 30 + "\n")
    f.write("  Student Report Card\n")
    f.write("=" * 30 + "\n\n")

    for student in students:
        grade = "A" if student["score"] >= 90 else ("B" if student["score"] >= 80 else "C")
        f.write(f"  {student['name']:<10} {student['score']}  ({grade})\n")

    avg = sum(s["score"] for s in students) / len(students)
    f.write(f"\n  Average: {avg:.1f}\n")
    f.write("=" * 30 + "\n")

# Verify
with open("report.txt", "r") as f:
    print(f.read())

Output:

==============================
  Student Report Card
==============================

  Alice      85  (B)
  Bob        92  (A)
  Charlie    78  (C)
  Diana      95  (A)

  Average: 87.5
==============================

Summary

Mode	Action	Existing File	No File
"w"	Write (overwrite)	Erases content	Creates new
"a"	Append	Adds to end	Creates new
"x"	Exclusive create	FileExistsError	Creates new
"wb"	Write binary	Erases content	Creates new

• write() writes a string; writelines() writes a list of strings
• Neither adds newlines automatically — include \n yourself
• Use print(..., file=f) for convenient formatted output
• Use pathlib.Path.write_text() for quick one-line writes
• Always specify encoding="utf-8" for text files

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