struct module in Python

This module performs conversions between python values and C structs represented as Python bytes objects. Format strings are the mechanism used to specify the expected layout when packing and unpacking data. Module struct is available in Python 3.x and not on 2.x, thus these codes will run on Python3 interpreter.

Struct Functions struct.pack() Syntax:
struct.pack(format, v1, v2, ...)

Return a string containing the values v1, v2, … , that are packed according to the given format (Format strings are the mechanism used to specify the expected layout when packing and unpacking data).The values followed by the format must be as per the format only, else struct.error is raised.

import struct
# Format: h is short in C type
# Format: l is long in C type
# Format 'hhl' stands for 'short short long'
var = struct.pack('hhl',1,2,3)
print(var)
# Format: i is int in C type
# Format 'iii' stands for 'int int int'
var = struct.pack('iii',1,2,3)
print(var)

Output:

b'\x01\x00\x02\x00\x00\x00\x00\x00\x03\x00\x00\x00\x00\x00\x00\x00'
b'\x01\x00\x00\x00\x02\x00\x00\x00\x03\x00\x00\x00' struct.unpack() Syntax:
struct.unpack(fmt, string)

Return the values v1, v2, … , that are unpacked according to the given format(1st argument). Values returned by this function are returned as tuples of size that is equal to the number of values passed through struct.pack() during packing.

import struct
# '?' -> _BOOL , 'h' -> short, 'i' -> int and 'l' -> long
var = struct.pack('?hil', True, 2, 5, 445)
print(var)
# struct.unpack() return a tuples
# Variables V1, V2, V3,.. are returned as elements of tuple
tup = struct.unpack('?hil', var)
print(tup)
# q -> long long int and f -> float
var = struct.pack('qf', 5, 2.3)
print(var)
tup = struct.unpack('qf', var)
print(tup)

Output:

b'\x01\x00\x02\x00\x05\x00\x00\x00\xbd\x01\x00\x00\x00\x00\x00\x00'
(True, 2, 5, 445)
b'\x05\x00\x00\x00\x00\x00\x00\x0033\x13@'
(5, 2.299999952316284)

Note: ‘b’ in the Output stands for binary.

struct.calcsize() Syntax:
struct.calcsize(fmt)
fmt: format

Return the size of the struct (and hence of the string) corresponding to the given format. calcsize() is important function, and is required for function such as struct.pack_into() and struct.unpack_from(), which require offset value and buffer as well.

import struct
var = struct.pack('?hil', True, 2, 5, 445)
print(var)
# Returns the size of the structure
print(struct.calcsize('?hil'))
print(struct.calcsize('qf'))

Output:

b'\x01\x00\x02\x00\x05\x00\x00\x00\xbd\x01\x00\x00\x00\x00\x00\x00'
16
12 import struct
var = struct.pack('bi', 56, 0x12131415)
print(var)
print(struct.calcsize('bi'))
var = struct.pack('ib', 0x12131415, 56)
print(var)
print(struct.calcsize('ib'))

Output:

b'8\x00\x00\x00\x15\x14\x13\x12'
8
b'\x15\x14\x13\x128'
5

Note: The ordering of format characters may have an impact on size.

Exception struct.error

Exception struct.error describes what is wrong at passing arguments, when a wrong argument is passed struct.error is raised.

from struct import error
print(error)

Note: This is piece of code is not useful, anywhere other than exception handling, and is used to show that ‘error’ upon interpreted shows about the class.

struct.pack_into() Syntax:
struct.pack_into(fmt, buffer, offset, v1, v2, ...)
fmt: data type format
buffer: writable buffer which starts at offset (optional)
v1,v2.. : values struct.unpack_from() Syntax:
struct.unpack_from(fmt, buffer[,offset = 0])fmt: data type format
buffer: writable buffer which starts at offset (optional)

Returns a tuple, similar to struct.unpack()

import struct
# ctypes in imported to create string buffer
import ctypes
# SIZE of the format is calculated using calcsize()
siz = struct.calcsize('hhl')
print(siz)
# Buffer 'buff' is created
buff = ctypes.create_string_buffer(siz)
# struct.pack() returns packed data
# struct.unpack() returns unpacked data
x = struct.pack('hhl', 2, 2, 3)
print(x)
print(struct.unpack('hhl', x))
# struct.pack_into() packs data into buff, doesn't return any value
# struct.unpack_from() unpacks data from buff, returns a tuple of values
struct.pack_into('hhl', buff, 0, 2, 2, 3)
print(struct.unpack_from('hhl', buff, 0))

Output:

16
b'\x02\x00\x02\x00\x00\x00\x00\x00\x03\x00\x00\x00\x00\x00\x00\x00'
(2, 2, 3)
(2, 2, 3)

Reference https://docs.python.org/2/library/struct.html

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