Python's Common module (struct) goes

To be precise, Python does not have a data type that specifically handles bytes. But because it is both a string and a str byte, the byte array is =str. In the C language, we can easily use structs, unions to handle bytes, and the conversion of bytes and int,float.

In Python, for example, to turn a 32-bit unsigned integer into a byte, or 4-length str, you have to do this with the bitwise operator:

>>> n = 10240099>>> b1 = chr((n & 0xff000000) >> 24)>>> b2 = chr((n & 0xff0000) >> 16)>>> b3 = chr((n & 0xff00) >> 8)>>> b4 = chr(n & 0xff)>>> s = b1 + b2 + b3 + b4>>> s‘\x00\[email protected]‘

Very troublesome. If you replace it with a floating point, you can do nothing.

Fortunately, Python provides a struct module to resolve str conversions to and from other binary data types.

structpackfunction to turn any data type into a string:

>>> import struct>>> struct.pack(‘>I‘, 10240099)‘\x00\[email protected]‘

packThe first parameter is the processing instruction, ‘>I‘ which means:

>Indicates that the byte order is Big-endian, which is the network order, which I represents a 4-byte unsigned integer.

The number of parameters to be followed is the same as the processing instructions.

unpackTurn it str into the appropriate data type:

>>> struct.unpack(‘>IH‘, ‘\xf0\xf0\xf0\xf0\x80\x80‘)(4042322160, 32896)

Depending on >IH the description, the str following changes to I : 4-byte unsigned integer and H : 2-byte unsigned integer.

So, although Python is not a good place to write code for the underlying operation of the byte stream, it is much easier to use in places where performance requirements are low struct .

structThe data types defined by the module can refer to the official Python documentation:

Https://docs.python.org/2/library/struct.html#format-characters

The bitmap file (. bmp) of Windows is a very simple file format that we use to struct analyze.

First find a BMP file, no words with "paint" to draw one.

Read in the first 30 bytes to parse:

>>> s = ‘\x42\x4d\x38\x8c\x0a\x00\x00\x00\x00\x00\x36\x00\x00\x00\x28\x00\x00\x00\x80\x02\x00\x00\x68\x01\x00\x00\x01\x00\x18\x00‘

BMP format uses a small-end way to store data, the structure of the file header in order as follows:

Two bytes: ‘BM‘ represents a Windows Bitmap, ‘BA‘ represents a OS/2 bitmap, a 4-byte integer: Represents the bitmap size, a 4-byte integer: Reserved bit, always 0, and a 4-byte integer: The actual image offset; a 4-byte integer: The number of bytes in the header; A 4-byte integer: Image width; a 4-byte integer: Image height; a 2-byte integer: Always 1; a 2-byte integer: The number of colors.

So, combine them with unpack read:

>>> struct.unpack(‘<ccIIIIIIHH‘, s)(‘B‘, ‘M‘, 691256, 0, 54, 40, 640, 360, 1, 24)

The results are displayed, the ‘B‘ ‘M‘ description is a Windows bitmap, the bitmap size is 640x360, and the number of colors is 24.

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Python's Common module (struct) goes