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.
python - Sample - python code :
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)python tutorial - 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'
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.
python - Sample - python code :
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)python tutorial - 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.
python - Sample - python code :
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'))python tutorial - Output :
b'\x01\x00\x02\x00\x05\x00\x00\x00\xbd\x01\x00\x00\x00\x00\x00\x00' 16 12
python - Sample - python code :
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'))python tutorial - 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.
python - Sample - python code :
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
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()
python - Sample - python code :
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))python tutorial - 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)