py-packed-struct 1.0.1

An implementation of C-like packed structures in Python

py-packed-struct

A Python library for defining and manipulating C-like packed structures with support for bit-fields, nested structures, and various data types.

Overview

py-packed-struct provides an elegant, Pythonic interface for working with binary data structures that are compatible with C's packed structures. Built on top of the bitstruct library, it eliminates the need to manually craft format strings while offering powerful features like nested structures, bit-field manipulation, and automatic serialization/deserialization.

Key Features

• Intuitive API: Define structures using native Python syntax instead of cryptic format strings
• Type Safety: Strongly-typed data fields with validation
• Bit-Level Precision: Full support for bit-fields and non-byte-aligned data
• Nested Structures: Create complex hierarchical data structures
• Endianness Control: Support for big-endian, little-endian, and native byte order
• Array Support: Built-in support for fixed-size arrays
• C Compatibility: Generate structures that are binary-compatible with C's __attribute__((packed)) structs
• Zero Dependencies: Only requires bitstruct package

Installation

Install via pip:

pip install py-packed-struct

Requirements:

• Python >= 3.8
• bitstruct

Quick Start

Here's a comparison between the standard library's struct module and py-packed-struct:

# Using Python's built-in struct module
from struct import pack
one, two, three = 1, 2, 3
data = pack(">bhl", one, two, three)
# Result: b'\x01\x00\x02\x00\x00\x00\x03'

# Using py-packed-struct
from packed_struct import Struct, c_signed_int

s = Struct({
    "one": c_signed_int(8),
    "two": c_signed_int(16),
    "three": c_signed_int(32)
})
s.set_data(one=1, two=2, three=3)
data = s.pack(byte_endianness="big")
# Result: b'\x01\x00\x02\x00\x00\x00\x03'

# Unpack back to dictionary
values = s.unpack(data, byte_endianness="big")
# Result: {'one': 1, 'two': 2, 'three': 3}

API Reference

Data Types

All data types inherit from the base Type class and support bit-level precision:

Integer Types

• c_unsigned_int(bits): Unsigned integer

  age = c_unsigned_int(8)  # 8-bit unsigned int (0-255)
  

• c_signed_int(bits): Signed integer (two's complement)

  temperature = c_signed_int(16)  # 16-bit signed int (-32768 to 32767)
  

Floating Point Types

• c_float(bits): IEEE 754 floating-point number
  • Supported sizes: 16, 32, or 64 bits

  weight = c_float(32)  # 32-bit float
  

Boolean Type

• c_bool(bits): Boolean value

  flag = c_bool(1)  # Single bit boolean
  

Character/Text Types

• c_char(bits): Text string (UTF-8 encoded)
  • Size must be multiple of 8 bits

  name = c_char(80)  # 10-character string (10 * 8 bits)
  

Raw Data Type

• c_raw_bytes(bits): Raw binary data

  buffer = c_raw_bytes(64)  # 8 bytes of raw data
  

Padding

• c_padding(bits): Reserved/padding space (always zero)

  padding = c_padding(16)  # 16 bits of padding
  

Arrays

• c_array(type, type_size_bits, array_size): Fixed-size array
  • type_size_bits must be multiple of 8

  # Array of 5 unsigned 8-bit integers
  values = c_array(c_unsigned_int, 8, 5)
  values.set_value([10, 20, 30, 40, 50])
  

Struct Class

The Struct class represents a collection of typed fields:

person = Struct({
    "name": c_char(10*8),      # 10 characters
    "age": c_unsigned_int(8),   # 1 byte
    "height": c_float(32)       # 4 bytes
})

Methods

• set_data(**kwargs): Set values for fields

  person.set_data(name="Alice", age=30, height=165.5)
  

• pack(byte_endianness="="): Serialize to bytes

  • byte_endianness: "big", "little", or "=" (native)

  binary_data = person.pack(byte_endianness="big")
  

• unpack(byte_string, byte_endianness="=", text_encoding="utf-8", text_errors="strict"): Deserialize from bytes

  values = person.unpack(binary_data, byte_endianness="big")
  

• get_data(): Get dictionary of all fields

  fields = person.get_data()
  

Properties

• size: Total size in bits
• fmt: Format string (bitstruct format)
• value: List of all field values

Accessing Field Values

# Using dictionary syntax
name = person["name"]

# Using attribute syntax
age = person.age

Nested Structures

Structures can be nested to create complex hierarchical data:

address = Struct({
    "street": c_char(20*8),
    "number": c_unsigned_int(16)
})

person = Struct({
    "name": c_char(10*8),
    "age": c_unsigned_int(8),
    "address": address  # Nested structure
})

person.set_data(name="Bob", age=25)
person["address"].set_data(street="Main St", number=123)

Examples

Bit-Fields

Working with individual bits and bit-fields:

from packed_struct import Struct, c_unsigned_int, c_bool

# Define a status register with bit-fields
status = Struct({
    "enabled": c_bool(1),      # 1 bit
    "ready": c_bool(1),        # 1 bit
    "error": c_bool(1),        # 1 bit
    "reserved": c_unsigned_int(5),  # 5 bits padding
    "code": c_unsigned_int(8)  # 8 bits
})

status.set_data(enabled=True, ready=False, error=False, reserved=0, code=42)
packed = status.pack()

Nested Structures Example

A complete example demonstrating nested structures (from examples/mqtt):

from packed_struct import Struct, c_char, c_unsigned_int, c_float

# Define nested shoe structure
shoes = Struct({
    "number": c_unsigned_int(8),
    "brand": c_char(10*8)
})

# Define clothing structure with nested shoes
clothes = Struct({
    "tshirt": c_char(10*8),
    "shorts": c_char(10*8),
    "shoes": shoes  # Nested structure
})

# Define person structure with nested clothes
person = Struct({
    "name": c_char(10*8),
    "age": c_unsigned_int(8),
    "weight": c_float(32),
    "dresses": clothes  # Nested structure
})

# Set values
person.set_data(name="Luca", age=29, weight=76.9)
person["dresses"].set_data(tshirt="foo", shorts="boo")
person["dresses"]["shoes"].set_data(number=42, brand="bar")

# Serialize
binary_data = person.pack()

This Python structure is binary-compatible with the following C structure:

typedef struct __attribute__((packed)) {
    uint8_t number;
    char brand[10];
} shoes_t;

typedef struct __attribute__((packed)) {
    char tshirt[10];
    char shorts[10];
    shoes_t shoes;
} clothes_t;

typedef struct __attribute__((packed)) {
    char name[10];
    uint8_t age;
    float weight;
    clothes_t clothes;
} person_t;

Array Example

from packed_struct import Struct, c_array, c_unsigned_int

# Array of integers
data = Struct({
    "header": c_unsigned_int(16),
    "values": c_array(c_unsigned_int, 8, 10)  # 10 bytes
})

data.set_data(header=0xFFFF)
data["values"].set_value([1, 2, 3, 4, 5, 6, 7, 8, 9, 10])

# Iterate over array elements
for i, element in enumerate(data["values"]):
    print(f"values[{i}] = {element.value}")

Use Cases

• Embedded Systems: Communicate with hardware devices using binary protocols
• Network Protocols: Implement custom network packet formats
• File Formats: Read/write binary file formats
• IoT Applications: Exchange data between Python and C/C++ applications
• Data Serialization: Efficient binary serialization for size-constrained environments

Supported Features

Feature	Status
C-like structures	✅ Supported
Bit-fields	✅ Supported
Nested structures	✅ Supported
Arrays	✅ Supported
Byte endianness	✅ Supported (big, little, native)
Bit endianness	🚧 Planned
Dynamic arrays	🚧 Planned

Contributing

Contributions are welcome! Please feel free to submit a Pull Request. For major changes, please open an issue first to discuss what you would like to change.

Development Setup

# Clone the repository
git clone https://github.com/lu-maca/py-packed-struct.git
cd py-packed-struct

# Install in development mode
pip install -e .

# Run tests
python tests/tests.py

Running Tests

The project includes comprehensive unit tests covering all data types and functionality:

python tests/tests.py

License

This project is licensed under the MIT License - see the LICENSE file for details.

Links

• PyPI: https://pypi.org/project/py-packed-struct/
• GitHub: https://github.com/lu-maca/py-packed-struct
• Documentation: https://github.com/lu-maca/py-packed-struct/tree/main/examples
• Issue Tracker: https://github.com/lu-maca/py-packed-struct/issues

Acknowledgments

Built on top of the excellent bitstruct library by Erik Moqvist.

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Author: Luca Macavero