Reading coils fails when server returns a tuple from getValues

[snorfalorpagus]

Problem

The function signature for the ModbusBaseSlaveContext.getValues method is as follows:

def getValues(self, fc_as_hex: int, address: int, count: int = 1) -> Sequence[int | bool]:

However, I've found that if I return a tuple for this method instead of a list it causes a failure when trying to read coils but not holding registers. The workaround is easy (just return a list instead) but I've not found the underlying cause.

More information

• python 3.13.3
• pymodbus 3.9.2

Code

server.py

import asyncio
from typing import Sequence

from pymodbus.datastore import ModbusServerContext, ModbusBaseSlaveContext
from pymodbus.server import ModbusTcpServer


class ModbusSlaveContext(ModbusBaseSlaveContext):
    def getValues(self, fc_as_hex: int, address: int, count: int = 1) -> Sequence[int | bool]:
        print(f"requested ({fc_as_hex=}, {address=}, {count=})")
        return list([1] * count)  # <-- HERE using tuple instead of list fails


slave_context = ModbusSlaveContext()

context = ModbusServerContext(slaves=slave_context, single=True)


async def main() -> None:
    server = ModbusTcpServer(
        context=context,
        address=("127.0.0.1", 5022),
    )

    await server.serve_forever()


if __name__ == "__main__":
    asyncio.run(main())

client.py

import asyncio

from pymodbus.client import AsyncModbusTcpClient


async def main() -> None:
    client = AsyncModbusTcpClient(
        host="127.0.0.1",
        port=5022,
    )
    await client.connect()
    result = await client.read_coils(address=500, count=2, slave=1)
    print(result)


if __name__ == "__main__":
    asyncio.run(main())

[janiversen]

A list is also a sequence so that is ok.

This is really an internal method, so it follows the internal guidelines and not like the API a signature that must be 100% true. Declaring it as a list gave a lot of problems since a list is immutable.

If is correct you can create your own datastore as you did, but that must fit into the internal structure.

Pull requests are welcome.

[janiversen]

When dealing with coils, please remember the difference between shared memory (single coil cannot be adressed or returned) vs blocked memory (single must be addressed and returned).

The return value is always a multiple of 16bits.