Python TypeError: write() argument must be str, not bytes

Updated: March 1, 2024 By: Guest Contributor Post a comment

Understanding the Error

Encountering a TypeError: write() argument must be str, not bytes in Python can be a frustrating experience. This often occurs when dealing with file operations, especially when you’re trying to write binary data to a file opened in text mode instead of binary mode. Understanding the root of the problem and knowing how to fix it can save you precious debugging time.

What is the Point?

The key to resolving this error lies in understanding the differences between strings and bytes in Python. Strings are sequences of characters, whereas bytes are sequences of bytes. Python 3 made a clear distinction between these two, which is why attempting to write bytes to a file opened in text mode causes a TypeError.

Solution 1: Converting Bytes to String

If your data can be safely converted to a string format using character encoding, this is often the simplest solution.

  1. Encode your bytes object using a character encoding scheme (usually utf-8).
  2. Write the encoded string to the file.

Code Example:

with open('myfile.txt', 'w') as f:
    my_bytes = b'Hello, world!'
    my_string = my_bytes.decode('utf-8')

Notes: While this solution is simple and effective for data that can be converted, it’s not suitable for binary data not meant to be read as text (e.g., images, executable files).

Solution 2: Opening File in Binary Mode

This solution involves opening the file in a mode that accepts bytes directly, avoiding the need for conversion.

  1. Open the file in binary mode by using ‘wb’ instead of ‘w’.
  2. Write bytes directly to the file.

Code Example:

with open('myfile.dat', 'wb') as f:
    my_bytes = b'Hello, world!'

Notes: This is the preferable solution for writing binary data. However, if you’re not dealing with binary data, make sure that your data doesn’t need to be in a string format for readability or processing reasons.

Solution 3: Using Buffering with io.BytesIO

For complex scenarios where you need to first manipulate the bytes in memory before writing them, using a buffer can be handy.

  1. Import io module.
  2. Create a buffer using io.BytesIO().
  3. Write the bytes to the buffer.
  4. Write the buffer’s contents to a file in binary mode.

Code Example:

import io

buff = io.BytesIO()
buff.write(b'Hello, Buffer World!')

with open('myfile_buffered.dat', 'wb') as f:  # move back to the beginning of the buffer

Notes: This solution is especially useful when working with binary data that needs to manipulated or generated in memory before being persisted to disk. It’s a bit more complex but offers greater flexibility.


Fixing the TypeError: write() argument must be str, not bytes involves understanding the nature of your data and how you intend to use it. Whether it’s converting your bytes to a string, writing in binary mode, or using a buffer, the right solution depends on your specific case. By understanding these techniques, you can avoid common pitfalls and work more efficiently with file I/O in Python.