"Given value is not a list" while the value is U256

[ales-tsurko]

Hi!

I'm trying to compose a call for my frontier-based node:

    call = interface.compose_call(
        call_module="EVM",
        call_function="call",
        call_params={
            "source": "0x" + keypair.public_key.hex(),
            "target": contract_addr,
            "input": input.hex(),
            "value": 0,
            "gas_limit": 10000000,
            "max_fee_per_gas": 10000000,
            "max_priority_fee_per_gas": None,
            "nonce": None,
            "access_list": [],
        },
    )

But I'm getting an error:

  File "erc20_balances.py", line 85, in call_contract
    call = interface.compose_call(
           ^^^^^^^^^^^^^^^^^^^^^^^
  File ".venv/lib/python3.11/site-packages/substrateinterface/base.py", line 1380, in compose_call
    call.encode({
  File ".venv/lib/python3.11/site-packages/scalecodec/base.py", line 925, in encode
    self.data = self.process_encode(self.value_serialized)
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File ".venv/lib/python3.11/site-packages/scalecodec/types.py", line 1687, in process_encode
    data += arg_obj.encode(param_value)
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File ".venv/lib/python3.11/site-packages/scalecodec/base.py", line 925, in encode
    self.data = self.process_encode(self.value_serialized)
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File ".venv/lib/python3.11/site-packages/scalecodec/types.py", line 672, in process_encode
    data += element_obj.encode(value[idx])
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File ".venv/lib/python3.11/site-packages/scalecodec/base.py", line 925, in encode
    self.data = self.process_encode(self.value_serialized)
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File ".venv/lib/python3.11/site-packages/scalecodec/types.py", line 1961, in process_encode
    raise ValueError('Given value is not a list')
ValueError: Given value is not a list

I don't know wether it's ok to expect a list in the place of U256, but here is the metadata for the function:

{'name': 'call', 'fields': [{'name': 'source', 'type': 43, 'typeName': 'H160', 'docs': []}, {'name': 'target', 'type': 43, 'typeName': 'H160', 'docs': []}, {'name': 'input', 'type': 13, 'typeName': 'Vec
<u8>', 'docs': []}, {'name': 'value', 'type': 56, 'typeName': 'U256', 'docs': []}, {'name': 'gas_limit', 'type': 10, 'typeName': 'u64', 'docs': []}, {'name': 'max_fee_per_gas', 'type': 56, 'typeName': '
U256', 'docs': []}, {'name': 'max_priority_fee_per_gas', 'type': 132, 'typeName': 'Option<U256>', 'docs': []}, {'name': 'nonce', 'type': 132, 'typeName': 'Option<U256>', 'docs': []}, {'name': 'access_li
st', 'type': 133, 'typeName': 'Vec<(H160, Vec<H256>)>', 'docs': []}], 'index': 1, 'docs': ['Issue an EVM call operation. This is similar to a message call transaction in Ethereum.']}

Could you tell me how can I properly specify the params? Or is it a bug?

[arjanz]

I did some debugging and I found out that there are some unexpected SCALE encoding issues with the U256 types in this call like the value and max_fee_per_gas attributes.

In the metadata of the runtime, this type is defined as followed:

{'path': ['primitive_types', 'U256'], 'params': [], 'def': {'composite': {'fields': [{'name': None, 'type': 114, 'typeName': '[u64; 4]', 'docs': []}]}}, 'docs': []}

Somehow the metadata dictates that this U256 should be interpreted as a [u64; 4] and the library does accordingly, but this seems unwanted behaviour. So I'm not sure if this is a bug on the library end, or I'm totally missing something here.

Until this is figured out, as a workaround you can overwrite this type with:

substrate = SubstrateInterface(
    url=url,
    type_registry={"types": {"primitive_types::U256": "U256"}}
)

[arjanz]

Path override included in new release of scalecodec, if you update to https://github.com/polkascan/py-substrate-interface/releases/tag/v1.7.3 this shouldn't be an issue anymore

[ales-tsurko]

Thanks! I checked the workaround yesterday and I had the same error. But it might be related to Option<U256> instead of U256.