Mypy – Incompatible types in assignment

April 20, 2023

I am wondering why I am getting Incompatible types in assignment here?

from typing import cast

import pyvisa
from pyvisa.constants import InterfaceType
from pyvisa.resources import GPIBInstrument, TCPIPInstrument


class Instrument:
    resource_manager = pyvisa.ResourceManager()

    def __init__(self, resource: str):
        self.resource = self.resource_manager.open_resource(resource_name=resource)
        if self.resource.interface_type == InterfaceType.tcpip:
            self.instance: TCPIPInstrument = cast(TCPIPInstrument, resource)
        elif self.resource.interface_type == InterfaceType.gpib:
            self.instance: GPIBInstrument = cast(GPIBInstrument, resource)
        else:
            raise TypeError(f"Unsupported resource interface type: {self.resource.interface_type}")

Gives Incompatible types in assignment (expression has type "GPIBInstrument", variable has type "TCPIPInstrument")

self.instance gets correctly the type instance: TCPIPInstrument | GPIBInstrument in vscode.

I am using python 3.11.3 and mypy 1.2.0.

Link to a gist with the same issue, but with slightly different code since I could not get pyvisa to install in the playground.

Edit 1:
Found some issues in the code from the comments, here is the more correct code, but still with the same issue.

from typing import cast

import pyvisa
from pyvisa.constants import InterfaceType
from pyvisa.resources import GPIBInstrument, TCPIPInstrument


class Instrument:
    resource_manager = pyvisa.ResourceManager()

    def __init__(self, resource_name: str):
        self.resource = self.resource_manager.open_resource(resource_name=resource_name)
        if self.resource.interface_type == InterfaceType.tcpip:
            self.instance = cast(TCPIPInstrument, self.resource)
        elif self.resource.interface_type == InterfaceType.gpib:
            self.instance = cast(GPIBInstrument, self.resource)
        else:
            raise TypeError(f"Unsupported resource interface type: {self.resource.interface_type}")

>Solution :

self.instance can only have one type. If you want it to be a union, you need to specify that when it’s first declared:

        if self.resource.interface_type == InterfaceType.tcpip:
            self.instance: GPIBInstrument | TCPIPInstrument = cast(TCPIPInstrument, resource)
        elif self.resource.interface_type == InterfaceType.gpib:
            self.instance = cast(GPIBInstrument, resource)
        else:
            raise TypeError(f"Unsupported resource interface type: {self.resource.interface_type}")