struct.error: required argument is not an integer

[MAKOMO]

Versions

• Python: 3.12
• OS: macOS 15
• Pymodbus: 3.7.3
• Modbus Hardware (if used):

Description

The type of the Client methods write_register (and write_registers) was changed in client/mixin.py (#2309). Now the value to be sent should be of type bytes instead of int as in the previous releases.

Converting the int value to bytes using value.to_bytes(2, 'big') makes mypy happy, but results in

  File "/Library/Frameworks/Python.framework/Versions/3.12/lib/python3.12/site-packages/pymodbus/client/base.py", line 104, in async_execute
    packet = self.ctx.framer.buildPacket(request)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Library/Frameworks/Python.framework/Versions/3.12/lib/python3.12/site-packages/pymodbus/framer/base.py", line 85, in buildPacket
    data = message.function_code.to_bytes(1,'big') + message.encode()
                                                     ^^^^^^^^^^^^^^^^
  File "/Library/Frameworks/Python.framework/Versions/3.12/lib/python3.12/site-packages/pymodbus/pdu/register_write_message.py", line 42, in encode
    packet += struct.pack(">H", self.value)
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
struct.error: required argument is not an integer

Sending the int value as before works, but makes mypy sad.

What to do?

[janiversen]

You should be able to send bytes or int, seems I did a bad review on #2309.

Fix will be available in a moment.

[MAKOMO]

Thanks for your quick reaction. Your fix looks good to me.

[janiversen]

Fix will be part of 3.7.4, which I expect in a couple of weeks, I need time to make more RTU testing with the new framers.

[MAKOMO]

Excellent! I added a type:ignore for now at this point in my code and keep sending ints.

[MAKOMO]

Thanks for providing v3.7.4 of your great lib!

However, I am not completely happy with the new type annotation of the values in mixin:write_registers which is now list[bytes | int].

As list is invariant it is now not type correct to provide this function with a homogeneous list of either bytes or int (which might be the main application).

For example on a test.py with

from typing import Sequence

def f(l:list[int | bytes]) -> None:
    return
def g(l:list[int] | list[bytes]) -> None:
    return
def h(l:Sequence[int | bytes]) -> None:
    return

l:list[int] = [1,2,3]

f(l) # type error
g(l) # type correct
h(l) # type correct

mypy reports

# mypy test.py
test.py:13: error: Argument 1 to "f" has incompatible type "list[int]"; expected "list[int | bytes]"  [arg-type]
test.py:13: note: "List" is invariant -- see https://mypy.readthedocs.io/en/stable/common_issues.html#variance
test.py:13: note: Consider using "Sequence" instead, which is covariant
Found 1 error in 1 file (checked 1 source file)

The simplest solution to this would be to restrict the use to homogenous lists of either type using list[int]|list[bytes] or using Sequence[int | bytes] as suggested by mypy.

[janiversen]

I can live with both solutions, at the end of the day, the call accept a list of int or bytes. It also accepts a list of mixed bytes/int but that is more an accident.

Looking forward to see a PR.