Struct Module

Introduction

The struct module converts between Python values and C structs packed into bytes. It's used for binary file formats and network protocols.

Pack and Unpack

import struct

# Pack values into bytes
data = struct.pack("i", 42)  # int
print(data)

data = struct.pack("3i", 1, 2, 3)  # 3 ints
print(data)

data = struct.pack("ii?", 10, 20, True)
print(data)

# Unpack from bytes
unpacked = struct.unpack("i", data)
print(unpacked)

unpacked = struct.unpack("3i", data)
print(unpacked)

Format Strings

import struct

# Byte order: @ (native), = (native, standard), < (little endian), > (big endian), ! (network)
# Types: b (signed char), B (unsigned char), h (short), H (unsigned short), 
#        i (int), I (unsigned int), q (long long), f (float), d (double), s (char[])

# Little endian int
data = struct.pack("<i", 42)

# Big endian int
data = struct.pack(">i", 42)

# With size prefixes
data = struct.pack("=i", 42)

Struct Objects

import struct

# Create Struct object for efficiency
Header = struct.Struct("III")  # 3 unsigned ints

data = b"\x01\x00\x00\x00\x02\x00\x00\x00\x03\x00\x00\x00"

# Unpack
unpacked = Header.unpack(data)
print(unpacked)  # (1, 2, 3)

# Pack
packed = Header.pack(1, 2, 3)
print(packed)

# Size
print(Header.size)  # 12

Mixed Types

import struct

# Header + data pattern
format = "HHI"  # 2 shorts, 1 int (10 bytes total)
header_size = struct.calcsize(format)

# Pack header
header = struct.pack(format, 1, 2, 100)
data = struct.pack(f"{header_size}s", b"Hello")

# Unpack
msg_type, flags, length = struct.unpack(format, header)
message = struct.unpack(f"{length}s", data[header_size:])[0]

Binary File Format

import struct

class WAVReader:
    def __init__(self, filename):
        with open(filename, "rb") as f:
            # RIFF header
            self.riff = f.read(4)
            self.size = struct.unpack("I", f.read(4))[0]
            self.wave = f.read(4)
            
            # fmt chunk
            self.fmt = f.read(4)
            self.fmt_size = struct.unpack("I", f.read(4))[0]
            self.audio_format = struct.unpack("H", f.read(2))[0]
            self.num_channels = struct.unpack("H", f.read(2))[0]
            self.sample_rate = struct.unpack("I", f.read(4))[0]

Practice Problems

1. Pack and unpack integers
2. Use different byte orders
3. Create Struct objects for reuse
4. Read binary file format
5. Convert between Python and C structs