5 Best Ways to Write a List of Bytes to a File in Python

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💡 Problem Formulation: In many programming scenarios, there’s a need to write binary data, such as a list of bytes, to a file. This could be for purposes such as persisting serialized data or storing multimedia content. For instance, we might want to convert a list of byte values [112, 121, 116, 104, 111, 110] into a binary file where these bytes are written exactly as-is. The solution should create a file containing the bytes in the specified order without any additional formatting.

Method 1: Using write() with Byte Objects

This method involves opening a file in binary write mode and using the file object’s write() function to write a bytes object, which is created by converting the list of bytes. It is simple and straightforward for writing raw byte data to a file.

Here’s an example:

byte_list = [112, 121, 116, 104, 111, 110]
byte_data = bytes(byte_list)
with open('output.bin', 'wb') as file:
    file.write(byte_data)

Output: A file named ‘output.bin’ is created with the corresponding byte sequence “python”.

This code snippet first creates a list of bytes. It then converts the list into a bytes object using the built-in bytes() function. Finally, it opens a file named ‘output.bin’ in binary write (‘wb’) mode and writes the byte data into it. The file is automatically closed after the with block.

Method 2: Using bytes() and open() Context Manager

This approach is very similar to Method 1, with the difference that it uses the bytes() constructor directly in the write() call, which can make the code more concise and direct. It’s also efficient in handling file closure.

Here’s an example:

with open('output.bin', 'wb') as file:
    file.write(bytes([112, 121, 116, 104, 111, 110]))

Output: An ‘output.bin’ file is created with the byte sequence “python”.

This example compactly demonstrates opening a file and writing to it in one go. The byte list is directly passed to the bytes() constructor within the write() method call, which writes the byte data to the opened file.

Method 3: Using bytearray()

By using bytearray(), we can create a mutable sequence of bytes before writing it into the file. This is useful when we might need to manipulate the data before writing it to the file.

Here’s an example:

byte_list = [112, 121, 116, 104, 111, 110]
byte_array = bytearray(byte_list)
# imagine we want to change the first byte
byte_array[0] = 80  # 'P' in ASCII
with open('output.bin', 'wb') as file:
    file.write(byte_array)

Output: ‘output.bin’ contains the byte sequence “Python”.

This method takes our list of byte values and converts it into a mutable bytearray(), which allows us to modify the bytes before they’re written to the file. The first byte is modified as an example of what you might want to do before writing. Then, the bytearray is written to the file in a binary write mode.

Method 4: Using struct.pack() for Structured Binary Data

If you’re dealing with structured binary data (e.g., numbers that need to be represented in a specific format), the struct.pack() function is invaluable. It packs data into a binary representation based on the format specified.

Here’s an example:

import struct
# using struct to ensure specific binary format (e.g., little-endian)
byte_list = [112, 121, 116, 104, 111, 110]
bin_data = struct.pack('<6B', *byte_list)
with open('output.bin', 'wb') as file:
    file.write(bin_data)

Output: ‘output.bin’ contains the byte sequence “python”, packed according to little-endian order.

The code snippet demonstrates the use of struct.pack() with a format string (‘<6B’) that indicates six unsigned bytes are to be packed in little-endian order. The byte list is unpacked into the function call, and the resulting binary data is then written to the file.

Bonus One-Liner Method 5: File Object’s writelines() Method

The file object’s writelines() method can write a list of byte strings to a file without needing to join them into a single bytes object first. Note that this is different from writing a list of bytes and typically used for byte strings, not single byte values.

Here’s an example:

byte_strings = [b'python', b'rocks']
with open('output.bin', 'wb') as file:
    file.writelines(byte_strings)

Output: ‘output.bin’ contains the concatenated byte strings “pythonrocks”.

In this compact example, we’re taking a list of byte strings and writing them consecutively into the file. There is no need to manually combine the byte strings first; writelines() handles it for us.

Summary/Discussion

  • Method 1: Using write() with Byte Objects. Simple and easy to understand. It’s best for writing straightforward byte streams to a file.
  • Method 2: Using bytes() and open() Context Manager. More concise than Method 1, but with the same strengths and weaknesses.
  • Method 3: Using bytearray(). Offers flexibility by allowing changes to the byte data before writing. However, it requires an extra step if no changes are necessary.
  • Method 4: Using struct.pack() for Structured Binary Data. Ideal for complex binary structures that need specific formatting, but overkill for simple byte lists.
  • Method 5: File Object’s writelines() Method. Efficient for writing a sequence of byte strings without additional processing, but not suitable for lists of individual bytes.