# dataclasses-struct
[](https://pypi.org/project/dataclasses-struct/)
[](https://pypi.org/project/dataclasses-struct/)
[]()
[](https://app.codecov.io/gh/harrymander/dataclasses-struct)
[](https://github.com/harrymander/dataclasses-struct/blob/main/LICENSE)
A simple Python package that combines
[`dataclasses`](https://docs.python.org/3/library/dataclasses.html) with
[`struct`](https://docs.python.org/3/library/struct.html) for packing and
unpacking Python dataclasses to fixed-length `bytes` representations.
```python
from typing import Annotated
import dataclasses_struct as dcs
@dcs.dataclass()
class Test:
x: int # or dcs.UInt64
y: dcs.Float32
z: dcs.Uint8
s: Annotated[bytes, 10]
```
```
>>> t = Test(100, -0.25, 0xff, b'12345')
>>> t
Test(x=100, y=-0.25, z=255, s=b'12345')
>>> packed = t.pack()
>>> packed
b'd\x00\x00\x00\x00\x00\x00\x00\x00\x00\x80\xbe\xff12345\x00\x00\x00\x00\x00'
>>> Test.from_packed(packed)
Test(x=100, y=-0.25, z=255, s=b'12345\x00\x00\x00\x00\x00')
```
## Installation
This package is available on pypi:
```
pip install dataclasses-struct
```
To work correctly with [`mypy`](https://www.mypy-lang.org/), an extension is
required; add to your `mypy.ini`:
```ini
[mypy]
plugins = dataclasses_struct.ext.mypy_plugin
```
## Usage
```python
from typing import Annotated # use typing_extensions on Python <3.9
import dataclasses_struct as dcs
endians = (
dcs.NATIVE_ENDIAN_ALIGNED, # uses system endianness and alignment
dcs.NATIVE_ENDIAN, # system endianness, packed representation
dcs.LITTLE_ENDIAN,
dcs.BIG_ENDIAN,
dcs.NETWORK_ENDIAN,
)
@dcs.dataclass(endians[0]) # if no endian provided, defaults to NATIVE_ENDIAN_ALIGNED
class Test:
# Single char type (must be bytes)
single_char: dcs.Char
single_char_alias: bytes # alias for Char
# Boolean
bool_1: dcs.Bool
bool_2: bool # alias for Bool
# Integers
int8: dcs.Int8
uint8: dcs.Uint8
int16: dcs.Int16
uint16: dcs.Uint16
int32: dcs.Int32
uint32: dcs.Uint32
uint64: dcs.Uint64
int64: dcs.Int64
int64_alias: int # alias for Int64
# Only supported with NATIVE_ENDIAN_ALIGNED
unsigned_size: dcs.Size
signed_size: dcs.SSize
pointer: dcs.Pointer
# Floating point types
single_precision: dcs.Float # or Float32
double_precision: dcs.Double # or Float64
double_precision_alias: float # alias for Float64
# Byte arrays: values shorter than size will be padded with b'\x00'
array: Annotated[bytes, 100] # an array of length 100
# Pad bytes can be added before and after fields: a b'\x00' will be
# inserted for each pad byte.
pad_before: Annotated[int, dcs.PadBefore(4)]
pad_after: Annotated[int, dcs.PadAfter(2)]
pad_before_and_after: Annotated[int, dcs.PadBefore(3), dcs.PadAfter(2)]
```
Decorated classes are transformed to a standard Python
[dataclass](https://docs.python.org/3/library/dataclasses.html) with boilerplate
`__init__`, `__repr__`, `__eq__` etc. auto-generated. Additionally, two methods
are added to the class: `pack`, a method for packing an instance of the class to
`bytes`, and `from_packed`, a class method that returns a new instance of the
class from its packed `bytes` representation.
An additional class attribute, `__dataclass_struct__`, of type
[`struct.Struct`](https://docs.python.org/3/library/struct.html#struct.Struct)
is added. The [`struct` format
string](https://docs.python.org/3/library/struct.html#format-strings) and packed
size can be accessed like so:
```
>>> Test.__dataclass_struct__.format
'@cc??bBhHiIQqqNnPfdd100s'
>>> Test.__dataclass_struct__.size
196
```
Default attribute values will be validated against their expected type and
allowable value range. For example,
```python3
import dataclasses_struct as dcs
@dcs.dataclass()
class Test:
x: dcs.Uint8 = -1
```
will raise a `ValueError`. This can be disabled by passing `validate=False` to
the `dataclasses_struct.dataclass` decorator.