refactor(api): Make sure command implementations return something com…

…patible with the command's result type (#15051)

api/src/opentrons/protocol_engine/commands/command.py

@@ -13,6 +13,8 @@
     TypeVar,
     Tuple,
     List,
+    Type,
+    Union,
 )
 
 from pydantic import BaseModel, Field
@@ -29,11 +31,11 @@
     from ..state import StateView
 
 
-CommandParamsT = TypeVar("CommandParamsT", bound=BaseModel)
-
-CommandResultT = TypeVar("CommandResultT", bound=BaseModel)
-
-CommandPrivateResultT = TypeVar("CommandPrivateResultT")
+_ParamsT = TypeVar("_ParamsT", bound=BaseModel)
+_ParamsT_contra = TypeVar("_ParamsT_contra", bound=BaseModel, contravariant=True)
+_ResultT = TypeVar("_ResultT", bound=BaseModel)
+_ResultT_co = TypeVar("_ResultT_co", bound=BaseModel, covariant=True)
+_PrivateResultT_co = TypeVar("_PrivateResultT_co", covariant=True)
 
 
 class CommandStatus(str, Enum):
@@ -58,7 +60,7 @@ class CommandIntent(str, Enum):
     FIXIT = "fixit"
 
 
-class BaseCommandCreate(GenericModel, Generic[CommandParamsT]):
+class BaseCommandCreate(GenericModel, Generic[_ParamsT]):
     """Base class for command creation requests.
 
     You shouldn't use this class directly; instead, use or define
@@ -72,7 +74,7 @@ class BaseCommandCreate(GenericModel, Generic[CommandParamsT]):
             "execution behavior"
         ),
     )
-    params: CommandParamsT = Field(..., description="Command execution data payload")
+    params: _ParamsT = Field(..., description="Command execution data payload")
     intent: Optional[CommandIntent] = Field(
         None,
         description=(
@@ -97,7 +99,7 @@ class BaseCommandCreate(GenericModel, Generic[CommandParamsT]):
     )
 
 
-class BaseCommand(GenericModel, Generic[CommandParamsT, CommandResultT]):
+class BaseCommand(GenericModel, Generic[_ParamsT, _ResultT]):
     """Base command model.
 
     You shouldn't use this class directly; instead, use or define
@@ -127,8 +129,8 @@ class BaseCommand(GenericModel, Generic[CommandParamsT, CommandResultT]):
         ),
     )
     status: CommandStatus = Field(..., description="Command execution status")
-    params: CommandParamsT = Field(..., description="Command execution data payload")
-    result: Optional[CommandResultT] = Field(
+    params: _ParamsT = Field(..., description="Command execution data payload")
+    result: Optional[_ResultT] = Field(
         None,
         description="Command execution result data, if succeeded",
     )
@@ -167,10 +169,15 @@ class BaseCommand(GenericModel, Generic[CommandParamsT, CommandResultT]):
         ),
     )
 
+    _ImplementationCls: Union[
+        Type[AbstractCommandImpl[_ParamsT, _ResultT]],
+        Type[AbstractCommandWithPrivateResultImpl[_ParamsT, _ResultT, object]],
+    ]
+
 
 class AbstractCommandImpl(
     ABC,
-    Generic[CommandParamsT, CommandResultT],
+    Generic[_ParamsT_contra, _ResultT_co],
 ):
     """Abstract command creation and execution implementation.
 
@@ -204,14 +211,14 @@ def __init__(
         pass
 
     @abstractmethod
-    async def execute(self, params: CommandParamsT) -> CommandResultT:
+    async def execute(self, params: _ParamsT_contra) -> _ResultT_co:
         """Execute the command, mapping data from execution into a response model."""
         ...
 
 
 class AbstractCommandWithPrivateResultImpl(
     ABC,
-    Generic[CommandParamsT, CommandResultT, CommandPrivateResultT],
+    Generic[_ParamsT_contra, _ResultT_co, _PrivateResultT_co],
 ):
     """Abstract command creation and execution implementation if the command has private results.
 
@@ -247,7 +254,7 @@ def __init__(
 
     @abstractmethod
     async def execute(
-        self, params: CommandParamsT
-    ) -> Tuple[CommandResultT, CommandPrivateResultT]:
+        self, params: _ParamsT_contra
+    ) -> Tuple[_ResultT_co, _PrivateResultT_co]:
         """Execute the command, mapping data from execution into a response model."""
         ...

api/tests/opentrons/protocol_engine/execution/test_command_executor.py

@@ -242,9 +242,7 @@ class _TestCommand(BaseCommand[_TestCommandParams, _TestCommandResult]):
         params: _TestCommandParams
         result: Optional[_TestCommandResult]
 
-        @property
-        def _ImplementationCls(self) -> Type[_TestCommandImpl]:
-            return TestCommandImplCls
+        _ImplementationCls: Type[_TestCommandImpl] = TestCommandImplCls
 
     command_params = _TestCommandParams()
     command_result = _TestCommandResult()
@@ -407,9 +405,7 @@ class _TestCommand(BaseCommand[_TestCommandParams, _TestCommandResult]):
         params: _TestCommandParams
         result: Optional[_TestCommandResult]
 
-        @property
-        def _ImplementationCls(self) -> Type[_TestCommandImpl]:
-            return TestCommandImplCls
+        _ImplementationCls: Type[_TestCommandImpl] = TestCommandImplCls
 
     command_params = _TestCommandParams()