Скачать или смотреть Extracting Bits from Bytes in Python

Learn how to extract specific bits from byte data using simple Python functions. This guide provides a clear methodology without external libraries.
---
This video is based on the question https://stackoverflow.com/q/64424508/ asked by the user 'ice-wind' ( https://stackoverflow.com/u/10981696/ ) and on the answer https://stackoverflow.com/a/64425422/ provided by the user 'Niklas Mertsch' ( https://stackoverflow.com/u/9568847/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: Extracting bits from bytes

Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/l...
The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license.

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Extracting Bits from Bytes in Python: A Step-by-Step Guide

When handling compressed data formats, especially those that involve look-up tables and multiple long integers, you may find yourself needing to extract specific bits from byte data. This task can seem daunting, particularly if you have to deal with varying bit lengths that do not neatly fit into typical data structures. Fortunately, you don't need to start from scratch; Python provides a straightforward way to manage this, even within the standard library.

In this guide, we will walk you through how to extract bits from bytes in a clear and effective manner.

The Challenge: Extracting Bits from Long Integers

Imagine you are working with a long integer value, say 4503672641818897, which needs to be manipulated bit-wise based on certain conditions derived from a lookup table. Here’s a breakdown of the example problem we are trying to solve:

Long Integer: For example, 4503672641818897.

Bit Representation: Convert this long integer into its binary representation.

Lookup Table: Determine the bit length of values from a lookup table (e.g., 5 bits).

Extract Bit Range: From this binary representation, you might need to extract specific bits - say bits 25 to 29.

The goal is to extract the bits in the specified range and convert them into an integer value, which should result in 4 for the example provided.

The Solution: A Python Function

Here's how we can address this problem with a custom Python function without relying on external libraries like NumPy or cumbersome string manipulations:

Step 1: The Function Definition

We will create a function named get_bits. This function will take in three parameters: the long integer, the starting bit position, and the ending bit position. It also includes an optional parameter to handle the bit length.

[[See Video to Reveal this Text or Code Snippet]]

Step 2: Break Down the Function

Mask Calculation: The first step inside the function calculates the mask to identify the bits we want to extract. For a range defined by end-start, it generates a binary number with N ones.

[[See Video to Reveal this Text or Code Snippet]]

Shift Calculation: This value determines how many bits we will shift our mask to the left, as well as how many bits we need to shift the final result to the right to isolate our desired bits.

[[See Video to Reveal this Text or Code Snippet]]

Final Calculation: Using bitwise operations, we construct the final result by applying the mask and shifting bits accordingly:

[[See Video to Reveal this Text or Code Snippet]]

Step 3: Testing the Function

You can test the get_bits function with various examples to see how it performs:

[[See Video to Reveal this Text or Code Snippet]]

Conclusion

Extracting bits from bytes can be straightforward with the right approach. Using the get_bits function outlined above, you can effectively manage and extract the necessary bit ranges from long integers without heavy dependencies or complex string techniques. Whether you're working with compressed data or any binary representation, understanding how to manipulate bits is a valuable skill in Python programming.

Happy coding!