To write bytes to a file, use ‘wb’ mode to write bytes to a new file (overwrites if the file exists). For appending bytes to an existing file, use ‘ab’ mode.

For writing bytes, you can always use a bytes object or a bytearray.

The basic syntax for writing a binary file is this:

with open('filename', 'wb') as file:
   file.write(byte_data)

“With statement” helps us write a file, and it automatically closes when the writing is done.

Writing raw bytes to a file

Before writing, we need to create raw bytes, and to do that, we can make a bytes object using the b”…” syntax. Then, use the with open() statement and pass “wb” argument, meaning we are writing bytes to the file.

data = b'Hello, this is a byte string.'


with open('raw_file.bin', 'wb') as file:
    file.write(data)

Writing a Byte Array

You can define a bytearray using bytearray([]) syntax. Then, write that array in the file using “with open()” and pass “wb” as an argument.

data = bytearray([72, 101, 108, 108, 111])  # Corresponds to "Hello"

# Write the bytearray to a file
with open('bytearray_file.bin', 'wb') as file:
    file.write(data)

As shown in the above output screenshot, the file contains the binary representation of the string “Hello”.

Writing an encoded string as bytes

First, we will define a unicode text and then convert the string to a byte sequence using the .encode() method.

Then, we write that binary to a file.

hindi_text = "नमस्ते, दुनिया!"  # Some Unicode text
data = hindi_text.encode('utf-8')

with open('namaste.bin', 'wb') as file:
    file.write(data)

Appending bytes to an existing file

As we previously discussed, we need to pass “ab” mode to append data to the end of the file instead of creating a new file and writing to it.

Let’s append the data to the previously written “namaste.bin” file:

more_data = b'\nThis is appended data.'

with open('namaste.bin', 'ab') as file:  # 'ab' = append binary
    file.write(more_data)

You can see that we appended the binary string at the end of the file.