Python JAM Codec

Description

Python implementation of the JAM codec as specified in the graypaper.

Installation

pip install jamcodec

Code examples

from jamcodec.types import JamBytes, Bool, String, U32, U8, U16, Struct, Vec, VarInt64, Tuple, Enum

# encode a Vec<u16>
obj = Vec(U16).new()
value = [1, 2]
data = obj.encode(value)

# Define and decode a Struct
scale_obj = Struct(test=U8, test2=Tuple(U8, U8)).new()
value = scale_obj.decode(JamBytes("0x020105"))

# Define and encode an Enum
scale_obj = Enum(
    Bool=Bool(),
    Number=U32,
    Complex=Struct(data=String(), version=VarInt64),
    None_=None
).new()
value = {'Bool': True}

data = scale_obj.encode(value)

Examples of different types

Type	Description	Example SCALE decoding value	SCALE encoded value
bool	Boolean values are encoded using the least significant bit of a single byte.	True	0x01
u16	Basic integers are encoded using a fixed-width little-endian (LE) format.	42	0x2a00
VarInt64	xxxxxxx	0	0x00
		1	0x04
		42	0xa8
		69	0x1501
		100000000000000	0x0b00407a10f35a
Vec	A collection of same-typed values is encoded, prefixed with a compact encoding of the number of items, followed by each item's encoding concatenated in turn.	[4, 8, 15, 16, 23, 42]	0x18040008000f00100017002a00
BitVec	A sequence of bools, represented in a more space efficient bit format	0b00000010_01111101	0x287d02
str,Bytes, String	Strings are Vectors of bytes (Vec<u8>) containing a valid UTF8 sequence.	"Test"	0x1054657374
		b"Test"	0x1054657374
		[84, 101, 115, 116]	0x1054657374
[u8; 4]	Fixed sized array of in this case an u8	b"babe"	0x62616265
		"0x62616265"	0x62616265
		[98, 97, 98, 101]	0x62616265
Enum Example: enum IntOrBool { Int(u8), Bool(bool),}	A fixed number of variants, each mutually exclusive and potentially implying a further value or series of values. Encoded as the first byte identifying the index of the variant that the value is. Any further bytes are used to encode any data that the variant implies. Thus, no more than 256 variants are supported.	{'Int': 8}	0x002a
		{'Bool': True}	0x0101
Struct Example: struct Motion { pub votes: Vec<AccountId>, pub id: u32 }	For structures, the values are named, but that is irrelevant for the encoding (names are ignored - only order matters). All containers store elements consecutively. The order of the elements is not fixed, depends on the container, and cannot be relied on at decoding. This implicitly means that decoding some byte-array into a specified structure that enforces an order and then re-encoding it could result in a different byte array than the original that was decoded.	{"votes": ["5GDyPHLVHcQYPTWfygtPYeo gQjyZy7J9fsi4brPhgEFq4pcv"], "id": 4}	0x04b80269ec500e458a630846b99105c397ee57 4125823d6f4388e9c7572e115c0504000000