在 Python 中进行位字段操作的最佳方法是什么？

发布于 2024-07-04 05:56:20 字数 163 浏览 7 评论 0原文

我正在阅读一些基于 UDP 的 MPEG 传输流协议，其中有一些时髦的位字段（例如长度 13）。我正在使用“struct”库进行广泛的解包，但是有没有一种简单的方法可以说“获取接下来的 13 位”，而不必手动调整位操作？我想要类似 C 处理位字段的方式（无需恢复到 C）。

建议？

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情丝乱 2024-07-11 05:56:20

bitstring 模块旨在解决这个问题。它将让您使用位作为基本构建块来读取、修改和构造数据。最新版本适用于 Python 2.6 或更高版本（包括 Python 3），但 1.0 版本也支持 Python 2.4 和 2.5。

一个与您相关的示例可能是这样的，它从传输流中去除所有空数据包（并且很可能使用您的 13 位字段？）：

from bitstring import Bits, BitStream  

# Opening from a file means that it won't be all read into memory
s = Bits(filename='test.ts')
outfile = open('test_nonull.ts', 'wb')

# Cut the stream into 188 byte packets
for packet in s.cut(188*8):
    # Take a 13 bit slice and interpret as an unsigned integer
    PID = packet[11:24].uint
    # Write out the packet if the PID doesn't indicate a 'null' packet
    if PID != 8191:
        # The 'bytes' property converts back to a string.
        outfile.write(packet.bytes)

这是另一个示例，包括从比特流中读取：

# You can create from hex, binary, integers, strings, floats, files...
# This has a hex code followed by two 12 bit integers
s = BitStream('0x000001b3, uint:12=352, uint:12=288')
# Append some other bits
s += '0b11001, 0xff, int:5=-3'
# read back as 32 bits of hex, then two 12 bit unsigned integers
start_code, width, height = s.readlist('hex:32, 2*uint:12')
# Skip some bits then peek at next bit value
s.pos += 4
if s.peek(1):
    flags = s.read(9)

您可以使用标准切片符号来切片、删除位级别的反转、覆盖等，还有位级别的查找、替换、分割等功能。还支持不同的字节序。

# Replace every '1' bit by 3 bits
s.replace('0b1', '0b001')
# Find all occurrences of a bit sequence
bitposlist = list(s.findall('0b01000'))
# Reverse bits in place
s.reverse()

完整文档位于此处。

The bitstring module is designed to address just this problem. It will let you read, modify and construct data using bits as the basic building blocks. The latest versions are for Python 2.6 or later (including Python 3) but version 1.0 supported Python 2.4 and 2.5 as well.

A relevant example for you might be this, which strips out all the null packets from a transport stream (and quite possibly uses your 13 bit field?):

from bitstring import Bits, BitStream  

# Opening from a file means that it won't be all read into memory
s = Bits(filename='test.ts')
outfile = open('test_nonull.ts', 'wb')

# Cut the stream into 188 byte packets
for packet in s.cut(188*8):
    # Take a 13 bit slice and interpret as an unsigned integer
    PID = packet[11:24].uint
    # Write out the packet if the PID doesn't indicate a 'null' packet
    if PID != 8191:
        # The 'bytes' property converts back to a string.
        outfile.write(packet.bytes)

Here's another example including reading from bitstreams:

# You can create from hex, binary, integers, strings, floats, files...
# This has a hex code followed by two 12 bit integers
s = BitStream('0x000001b3, uint:12=352, uint:12=288')
# Append some other bits
s += '0b11001, 0xff, int:5=-3'
# read back as 32 bits of hex, then two 12 bit unsigned integers
start_code, width, height = s.readlist('hex:32, 2*uint:12')
# Skip some bits then peek at next bit value
s.pos += 4
if s.peek(1):
    flags = s.read(9)

You can use standard slice notation to slice, delete, reverse, overwrite, etc. at the bit level, and there are bit level find, replace, split etc. functions. Different endiannesses are also supported.

# Replace every '1' bit by 3 bits
s.replace('0b1', '0b001')
# Find all occurrences of a bit sequence
bitposlist = list(s.findall('0b01000'))
# Reverse bits in place
s.reverse()

The full documentation is here.

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