zoukankan      html  css  js  c++  java
  • Packing data with Python

    Defining how a sequence of bytes sits in a memory buffer or on disk can be challenging from time to time. Since everything that you’ll work with is a byte, it makes sense that we have an intuitive way to work with this information agnostic of the overlying type restrictions that the language will enforce on us.

    In today’s post, I’m going to run through Python’s byte string packing and unpacking using the struct package.

    Basics

    From the Python documentation:

    This module performs conversions between Python values and C structs represented as Python bytes objects. This can be used in handling binary data stored in files or from network connections, among other sources. It uses Format Strings as compact descriptions of the layout of the C structs and the intended conversion to/from Python values.

    When working with a byte string in Python, you prefix your literals with b.

    >>> b'Hello'
    'Hello'

    The ord function call is used to convert a text character into its character code representation.

    >>> ord(b'H')
    72
    >>> ord(b'e')
    101
    >>> ord(b'l')
    108

    We can use list to convert a whole string of byte literals into an array.

    >>> list(b'Hello')
    [72, 101, 108, 108, 111]

    The compliment to the ord call is chr, which converts the byte-value back into a character.

    Packing

    Using the struct module, we’re offered the pack function call. This function takes in a format of data and then the data itself. The first parameter defines how the data supplied in the second parameter should be laid out. We get started:

    >>> import struct

    If we pack the string 'Hello' as single bytes:

    >>> list(b'Hello')
    [72, 101, 108, 108, 111]
    >>> struct.pack(b'BBBBB', 72, 101, 108, 108, 111)
    b'Hello'

    The format string b'BBBBB' tells pack to pack the values supplied into a string of 5 unsigned values. If we were to use a lower case b in our format string, pack would expect the byte value to be signed.

    >>> struct.pack(b'bbbbb', 72, 101, 108, 108, 111)
    b'Hello'

    This only gets interesting once we send a value that would make the request overflow:

    >>> struct.pack(b'bbbbb', 72, 101, 108, 129, 111)
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    struct.error: byte format requires -128 <= number <= 127

    The following tables have been re-produced from the Python documentation.

    Byte order, size and alignment

    CharacterByte orderSizeAlignment
    @ native native native
    = native standard none
    < little-endian standard none
    > big-endian standard none
    ! network (= big-endian) standard none

    Types

    FormatC TypePython typeStandard sizeNotes
    x pad byte no value    
    c char bytes of length 1 1  
    b signed char integer 1 (1),(3)
    B unsigned char integer 1 (3)
    ? _Bool bool 1 (1)
    h short integer 2 (3)
    H unsigned short integer 2 (3)
    i int integer 4 (3)
    I unsigned int integer 4 (3)
    l long integer 4 (3)
    L unsigned long integer 4 (3)
    q long long integer 8 (2), (3)
    Q unsigned long long integer 8 (2), (3)
    n ssize_t integer   (4)
    N size_t integer   (4)
    f float float 4 (5)
    d double float 8 (5)
    s char[] bytes    
    p char[] bytes    
    P void * integer   (6)

    Unpacking

    The direct reverse process of packing bytes into an array, is unpacking them again into usable variables inside of your python code.

    >>> struct.unpack(b'BBBBB', struct.pack(b'BBBBB', 72, 101, 108, 108, 111))
    (72, 101, 108, 108, 111)
    >>> struct.unpack(b'5s', struct.pack(b'BBBBB', 72, 101, 108, 108, 111))
    (b'Hello',)
  • 相关阅读:
    Elasticsearch 支持拼音自动补全
    laravel自动补全链接
    laravel的服务容器(药箱)、服务提供者(小盒子)、Facades(更方便用药),方便大家透彻理解
    php static静态属性和静态方法
    php面向对象的构造方法与析构方法
    MySQL事务-ROLLBACK,COMMIT用法详解
    php 事务处理transaction
    Python:初步学习Python
    iOS:自己写的一个星级评价的小Demo
    iOS:枚举enum的使用
  • 原文地址:https://www.cnblogs.com/lakeone/p/7205424.html
Copyright © 2011-2022 走看看