scalecodec 1.1.2

Python SCALE Codec Library

Python SCALE Codec

Description

Substrate uses a lightweight and efficient encoding and decoding program to optimize how data is sent and received over the network. The program used to serialize and deserialize data is called the SCALE codec, with SCALE being an acronym for Simple Concatenated Aggregate Little-Endian.

Installation

pip install scalecodec

API reference documentation

https://polkascan.github.io/py-scale-codec/

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
Compact	A "compact" or general integer encoding is sufficient for encoding large integers (up to 2**536) and is more efficient at encoding most values than the fixed-width version. (Though for single-byte values, the fixed-width integer is never worse.)	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
AccountId	An SS58 formatted representation of an account. See also the SS58 util functions	"5GDyPHLVHcQYPTWfygtPY eogQjyZy7J9fsi4brPhgEFq4pcv"	0xb80269ec500e458a630846b99105c397 ee574125823d6f4388e9c7572e115c05
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

Code Examples

Encode a Call

runtime_config = RuntimeConfigurationObject()
# This types are all hardcoded types needed to decode metadata types
runtime_config.update_type_registry(load_type_registry_preset(name="core"))

# Decode retrieved metadata from the RPC
metadata = runtime_config.create_scale_object(
    'MetadataVersioned', data=ScaleBytes(response.get('result'))
)
metadata.decode()

# Add the embedded type registry to the runtime config
runtime_config.add_portable_registry(metadata)

call = runtime_config.create_scale_object(
    "Call", metadata=metadata
)
call.encode({
    "call_module": "Balances",
    "call_function": "transfer",
    "call_args": {"dest": "5GNJqTPyNqANBkUVMN1LPPrxXnFouWXoe2wNSmmEoLctxiZY", "value": 3},
})

Decode the result of a state_getStorageAt RPC call

event_data = "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"

system_pallet = metadata.get_metadata_pallet("System")
event_storage_function = system_pallet.get_storage_function("Events")

event = runtime_config.create_scale_object(
    event_storage_function.get_value_type_string(), metadata=metadata
)
print(event.decode(ScaleBytes(event_data)))

Generate type decomposition information

The function generate_type_decomposition can be used when more information is needed how to encode a certain SCALE type:

Example 1

scale_obj = runtime_config.create_scale_object("RawBabePreDigest")

type_info = scale_obj.generate_type_decomposition()

print(type_info)
# {
#   'Phantom': None, 
#   'Primary': {'authority_index': 'u32', 'slot_number': 'u64', 'vrf_output': '[u8; 32]', 'vrf_proof': '[u8; 64]'}, 
#   'SecondaryPlain': {'authority_index': 'u32', 'slot_number': 'u64'}, 
#   'SecondaryVRF': {'authority_index': 'u32', 'slot_number': 'u64', 'vrf_output': '[u8; 32]', 'vrf_proof': '[u8; 64]'}
# }

Example 2

pallet = metadata.get_metadata_pallet("Tokens")
storage_function = pallet.get_storage_function("TotalIssuance")

param_type_string = storage_function.get_params_type_string()
param_type_obj = runtime_config.create_scale_object(param_type_string[0])

type_info = param_type_obj.generate_type_decomposition()

print(type_info)
# [{
#   'Token': ('ACA', 'AUSD', 'DOT', 'LDOT', 'RENBTC', 'CASH', 'KAR', 'KUSD', 'KSM', 'LKSM', 'TAI', 'BNC', 'VSKSM', 'PHA', 'KINT', 'KBTC'), 
#   'DexShare': ({'Token': ('ACA', 'AUSD', 'DOT', 'LDOT', 'RENBTC', 'CASH', 'KAR', 'KUSD', 'KSM', 'LKSM', 'TAI', 'BNC', 'VSKSM', 'PHA', 'KINT', 'KBTC'), 'Erc20': '[u8; 20]', 'LiquidCrowdloan': 'u32', 'ForeignAsset': 'u16'}, {'Token': ('ACA', 'AUSD', 'DOT', 'LDOT', 'RENBTC', 'CASH', 'KAR', 'KUSD', 'KSM', 'LKSM', 'TAI', 'BNC', 'VSKSM', 'PHA', 'KINT', 'KBTC'), 'Erc20': '[u8; 20]', 'LiquidCrowdloan': 'u32', 'ForeignAsset': 'u16'}), 
#   'Erc20': '[u8; 20]', 
#   'StableAssetPoolToken': 'u32', 
#   'LiquidCrowdloan': 'u32', 
#   'ForeignAsset': 'u16'
# }]

In the above examples are simple value Enums representation as a tuple of possible values and complex Enums as a dict.

Examples (prior to MetadataV14)

Decode a SCALE-encoded Compact<Balance>

RuntimeConfiguration().update_type_registry(load_type_registry_preset("legacy"))
RuntimeConfiguration().update_type_registry(load_type_registry_preset("kusama"))
obj = RuntimeConfiguration().create_scale_object('Compact<Balance>', data=ScaleBytes("0x130080cd103d71bc22"))
obj.decode()
print(obj.value)

Encode to Compact<Balance>

RuntimeConfiguration().update_type_registry(load_type_registry_preset("legacy"))
obj = RuntimeConfiguration().create_scale_object('Compact<Balance>')
scale_data = obj.encode(2503000000000000000)
print(scale_data)

Encode to Vec<Bytes>

RuntimeConfiguration().update_type_registry(load_type_registry_preset("legacy"))
value = ['test', 'vec']
obj = RuntimeConfiguration().create_scale_object('Vec<Bytes>')
scale_data = obj.encode(value)
print(scale_data)

Add custom types to type registry

RuntimeConfiguration().update_type_registry(load_type_registry_preset("legacy"))

custom_types = {
    "types": {
        "MyCustomType": "u32",
        "CustomNextAuthority": {
          "type": "struct",
          "type_mapping": [
             ["AuthorityId", "AuthorityId"],
             ["weight", "AuthorityWeight"]
          ]
        }
    }   
}

RuntimeConfiguration().update_type_registry(custom_types)

Or from a custom JSON file

RuntimeConfiguration().update_type_registry(load_type_registry_preset("legacy"))
RuntimeConfiguration().update_type_registry(load_type_registry_file("/path/to/type_registry.json"))

Multiple runtime configurations

By default a singleton is used to maintain the configuration, for multiple instances:

# Kusama runtime config
runtime_config_kusama = RuntimeConfigurationObject()
runtime_config_kusama.update_type_registry(load_type_registry_preset("legacy"))
runtime_config_kusama.update_type_registry(load_type_registry_preset("kusama"))


# Polkadot runtime config
runtime_config_polkadot = RuntimeConfigurationObject()
runtime_config_polkadot.update_type_registry(load_type_registry_preset("legacy"))
runtime_config_polkadot.update_type_registry(load_type_registry_preset("polkadot"))

# Decode extrinsic using Kusama runtime configuration
extrinsic = runtime_config_kusama.create_scale_object(
    type_string='Extrinsic', 
    metadata=metadata_decoder
)
extrinsic.decode(ScaleBytes(extrinsic_data))

License

https://github.com/polkascan/py-scale-codec/blob/master/LICENSE