How to Write Binary File in Python: Simple Guide

To write a binary file in Python, open the file with open('filename', 'wb') mode and use the write() method to save bytes data. This mode ensures data is written exactly as bytes without encoding.

Syntax

Use open(filename, 'wb') to open a file for writing in binary mode. The write() method writes bytes data to the file. Always close the file or use with to handle it automatically.

• filename: Name of the file to write.
• 'wb': Write mode in binary.
• write(bytes_data): Writes bytes to the file.

python

with open('example.bin', 'wb') as file:
    file.write(b'Hello binary world')

Example

This example writes a sequence of bytes to a binary file named data.bin. It demonstrates opening the file in binary write mode and writing bytes data.

python

data = bytes([120, 3, 255, 0, 100])
with open('data.bin', 'wb') as f:
    f.write(data)

print('Binary file written successfully.')

Output

Binary file written successfully.

Common Pitfalls

Common mistakes include:

• Opening the file in text mode ('w') instead of binary mode ('wb'), which causes errors or data corruption.
• Trying to write strings directly without converting to bytes.
• Not closing the file, risking incomplete writes.

Always use with to manage file closing automatically.

python

wrong = "Hello"
# Wrong: writing string in binary mode without encoding
with open('wrong.bin', 'wb') as f:
    # f.write(wrong)  # This causes TypeError
    f.write(wrong.encode('utf-8'))  # Correct way: encode string to bytes

# Correct way:
with open('correct.bin', 'wb') as f:
    f.write(b'Hello')

Quick Reference

Summary tips for writing binary files in Python:

• Use 'wb' mode to write binary files.
• Write only bytes objects, not strings.
• Use with statement to handle files safely.
• Convert strings to bytes with encode() before writing.

Key Takeaways

Open files with 'wb' mode to write binary data safely.

Write bytes objects, not plain strings, to binary files.

Use the with statement to automatically close files after writing.

Convert strings to bytes using encode() before writing if needed.

Avoid mixing text and binary modes to prevent errors.