deconstruct 0.3

Pythonic C-style structs

deconstruct

deconstruct is a tiny (125 lines of code) package that provides a Pythonic analogue of C's struct, primarily for the purpose of interpreting (i.e. deconstructing) contiguous binary data.

Internally, deconstruct uses Python's struct module and can be considered an abstraction of sorts. struct (the module) can be frustrating to use: its format strings appear arcane and furthermore separate the description of the data from its representation, a definite strength of C's struct.

In contrast, deconstruct allows structs to be defined and used using a syntax that is Pythonic while maintaining close correspondence to C.

Usage

With deconstruct, the struct definition (adapted from input.h):

#include <stdint.h>

struct input_event {  
    uint64_t time[2];
    int16_t type;
    int16_t code;
    int32_t value;
};

can be represented as:

import deconstruct as c

class InputEvent(c.Struct):
    time: c.uint64[2]
    type: c.int16
    code: c.int16
    value: c.int32

This definition can be used to interpret and then access binary data:

>>> buffer = b'Some  arbitrary  buffer!'
>>> event = InputEvent(buffer)
>>> event.type
25120
>>> event.code
26229
>>> event.time[0]
8241904116577431379
>>> event.time[1]
2340027244253309282
>>> event.value
561145190

Of course, in reality the buffer passed in is more likely to come from something more useful, like a file. Notice that fixed-size arrays can be specified using the syntax type[length], a further improvement on Python's struct.

Installation

deconstruct is now on PyPI:

pip install deconstruct

Alternatively you can install straight from this repository:

pip install https://github.com/biqqles/deconstruct/archive/master.zip

Built wheels are also available under Releases, as is a changelog.

deconstruct has no dependencies but requires Python >= 3.6 as it makes use of the class annotations added in that release (see PEP 526).

API listing

Struct(buffer: bytes)

Subclass this to define your own structs. Subclasses should only contain field definitions of C types defined in this package (more on this below).

When you instantiate your Struct with a bytes object, deconstruct creates a format string and uses it to unpack that buffer. In the instance, deconstruct types will be replaced with their equivalent Python types for use (e.g. bytes for char, int for schar and float for double).

Attributes

Name	Type	Description
__byte_order__	ByteOrder	Set this in your subclass definition to define the byte order used when unpacking the struct. One of: ByteOrder.NATIVE (default value) ByteOrder.BIG_ENDIAN ByteOrder.LITTLE_ENDIAN
__type_width__	TypeWidth	Set this in your subclass definition to define the type width and padding used for the struct. One of: TypeWidth.NATIVE TypeWidth.STANDARD (default value) When TypeWidth.NATIVE is set, the struct will use native type widths and alignment. When TypeWidth.STANDARD is used, the struct will use Python's struct's "standard" widths1 and no padding.

Note that TypeWidth.NATIVE can only be used with ByteOrder.NATIVE. This is a limitation of Python's struct.

Properties

Name	Type	Description
format_string	str	The struct.py-compatible format string for this struct
sizeof	int	The total size in bytes of the struct. Equivalent to C's sizeof

Methods

Signature	Return type	Description
to_bytes()	bytes	Returns the in-memory (packed) representation of this struct instance

You can also print Struct instances for easier debugging.

C types

deconstruct defines the following special types for use in Struct field definitions:²

deconstruct type	C99 type	Python format character	"Standard" width (bytes)1
char	char	c	1
schar	signed char	b	1
uchar	unsigned char	B	1
short	short	h	2
ushort	unsigned short	H	2
int	int	i	2
uint	unsigned int	I	2
long	long	l	4
ulong	unsigned long	L	4
longlong	long long	q	8
ulonglong	unsigned long long	Q	8
bool	bool (_Bool)	?	1
float	float	f	4
double	double	d	8
int8	int8_t	b*	1
uint8	uint8_t	B*	1
int16	int16_t	h*	2
uint16	uint16_t	H*	2
int32	int32_t	l*	4
uint32	uint32_t	L*	4
int64	int64_t	q*	8
uint64	uint64_t	Q*	8
ptr	void*/intptr_t/uintptr_t	P	N/A**
size	size_t	n	N/A**
ssize	ssize_t	N	N/A**

^{* format character with `__type_width__ = TypeWidth.STANDARD` - platform specific otherwise.
** only available with `__type_width__ = TypeWidth.NATIVE`.}

As mentioned earlier, these types support a type[length] syntax to define fixed-size, n-dimensional arrays. When a Struct is used to unpack a buffer, these types will resolve to a Python tuple of their equivalent types. You can also use this to define padding sequences.

---

1. Python's struct has the concept of "standard" type sizes. This is somewhat confusing coming from C as its standards go to some length not to define a standard ABI. However, as this terminology is so fundamental to the documentation of Python's struct it is replicated here for simplicity's sake. These sizes correspond with the minimum sizes implied for C's types.

2. Because some of these conflict with Python's primitives, it is not recommended to import * from deconstruct as this will severely pollute your namespace (in fact this is a bad idea in general). I like to import deconstruct as c as shown above.