added a callback to Attribute enabling backend to change values

For example, updating an attribute to have an `Int` with different units will make the epics backend update `EGU` of the corresponding record.

pyproject.toml

@@ -16,7 +16,8 @@ dependencies = [
     "pydantic",
     "pvi~=0.10.0",
     "pytango",
-    "softioc",
+    # This is needed for softioc.device.set_field
+    "softioc>=4.5.0",
 ]
 dynamic = ["version"]
 license.file = "LICENSE"
@@ -42,7 +43,7 @@ dev = [
     "tox-direct",
     "types-mock",
     "aioca",
-    "p4p",
+    "p4p>=4.2.0",
 ]
 
 [project.scripts]
@@ -61,7 +62,7 @@ version_file = "src/fastcs/_version.py"
 
 [tool.pyright]
 typeCheckingMode = "standard"
-reportMissingImports = false # Ignore missing stubs in imported modules
+reportMissingImports = false  # Ignore missing stubs in imported modules
 
 [tool.pytest.ini_options]
 # Run pytest with all our checkers, and don't spam us with massive tracebacks on error

src/fastcs/attributes.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+from collections.abc import Callable
 from enum import Enum
 from typing import Any, Generic, Protocol, runtime_checkable
 
@@ -65,6 +66,10 @@ def __init__(
         self._allowed_values: list[T] | None = allowed_values
         self.description = description
 
+        # A callback to use when setting the datatype to a different value, for example
+        # changing the units on an int. This should be implemented in the backend.
+        self._update_datatype_callback: Callable[[DataType[T]], None] | None = None
+
     @property
     def datatype(self) -> DataType[T]:
         return self._datatype
@@ -85,6 +90,20 @@ def group(self) -> str | None:
     def allowed_values(self) -> list[T] | None:
         return self._allowed_values
 
+    def set_update_datatype_callback(
+        self, callback: Callable[[DataType[T]], None] | None
+    ) -> None:
+        self._update_datatype_callback = callback
+
+    def update_datatype(self, datatype: DataType[T]) -> None:
+        if not isinstance(self._datatype, type(datatype)):
+            raise ValueError(
+                f"Attribute datatype must be of type {type(self._datatype)}"
+            )
+        self._datatype = datatype
+        if self._update_datatype_callback is not None:
+            self._update_datatype_callback(datatype)
+
 
 class AttrR(Attribute[T]):
     """A read-only ``Attribute``."""

src/fastcs/backends/epics/ioc.py

@@ -1,5 +1,5 @@
 from collections.abc import Callable
-from dataclasses import dataclass
+from dataclasses import asdict, dataclass
 from types import MethodType
 from typing import Any, Literal
 
@@ -15,7 +15,7 @@
     enum_value_to_index,
 )
 from fastcs.controller import BaseController
-from fastcs.datatypes import Bool, Float, Int, String, T
+from fastcs.datatypes import Bool, DataType, Float, Int, String, T
 from fastcs.exceptions import FastCSException
 from fastcs.mapping import Mapping
 
@@ -27,6 +27,25 @@ class EpicsIOCOptions:
     terminal: bool = True
 
 
+DATATYPE_NAME_TO_RECORD_FIELD = {
+    "prec": "PREC",
+    "units": "EGU",
+    "min": "DRVL",
+    "max": "DRVH",
+    "min_alarm": "LOPR",
+    "max_alarm": "HOPR",
+    "znam": "ZNAM",
+    "onam": "ONAM",
+}
+
+
+def datatype_to_epics_fields(datatype: DataType) -> dict[str, Any]:
+    return {
+        DATATYPE_NAME_TO_RECORD_FIELD[field]: value
+        for field, value in asdict(datatype).items()
+    }
+
+
 class EpicsIOC:
     def __init__(self, pv_prefix: str, mapping: Mapping):
         _add_pvi_info(f"{pv_prefix}:PVI")
@@ -184,36 +203,38 @@ def _get_input_record(pv: str, attribute: AttrR) -> RecordWrapper:
         return builder.mbbIn(pv, **state_keys, **attribute_fields)
 
     match attribute.datatype:
-        case Bool(znam, onam):
-            return builder.boolIn(pv, ZNAM=znam, ONAM=onam, **attribute_fields)
-        case Int(units, min, max, min_alarm, max_alarm):
-            return builder.longIn(
+        case Bool():
+            record = builder.boolIn(
+                pv, **datatype_to_epics_fields(attribute.datatype), **attribute_fields
+            )
+        case Int():
+            record = builder.longIn(
                 pv,
-                EGU=units,
-                DRVL=min,
-                DRVH=max,
-                LOPR=min_alarm,
-                HOPR=max_alarm,
+                **datatype_to_epics_fields(attribute.datatype),
                 **attribute_fields,
             )
-        case Float(prec, units, min, max, min_alarm, max_alarm):
-            return builder.aIn(
+        case Float():
+            record = builder.aIn(
                 pv,
-                PREC=prec,
-                EGU=units,
-                DRVL=min,
-                DRVH=max,
-                LOPR=min_alarm,
-                HOPR=max_alarm,
+                **datatype_to_epics_fields(attribute.datatype),
                 **attribute_fields,
             )
         case String():
-            return builder.longStringIn(pv, **attribute_fields)
+            record = builder.longStringIn(
+                pv, **datatype_to_epics_fields(attribute.datatype), **attribute_fields
+            )
         case _:
             raise FastCSException(
                 f"Unsupported type {type(attribute.datatype)}: {attribute.datatype}"
             )
 
+    def datatype_updater(datatype: DataType):
+        for name, value in datatype_to_epics_fields(datatype).items():
+            record.set_field(name, value)
+
+    attribute.set_update_datatype_callback(datatype_updater)
+    return record
+
 
 def _create_and_link_write_pv(
     pv_prefix: str, pv_name: str, attr_name: str, attribute: AttrW[T]
@@ -262,48 +283,45 @@ def _get_output_record(pv: str, attribute: AttrW, on_update: Callable) -> Any:
         )
 
     match attribute.datatype:
-        case Bool(znam, onam):
-            return builder.boolOut(
+        case Bool():
+            record = builder.boolOut(
                 pv,
-                ZNAM=znam,
-                ONAM=onam,
+                **datatype_to_epics_fields(attribute.datatype),
                 always_update=True,
                 on_update=on_update,
             )
-        case Int(units, min, max, min_alarm, max_alarm):
-            return builder.longOut(
+        case Int():
+            record = builder.longOut(
                 pv,
                 always_update=True,
                 on_update=on_update,
-                EGU=units,
-                DRVL=min,
-                DRVH=max,
-                LOPR=min_alarm,
-                HOPR=max_alarm,
+                **datatype_to_epics_fields(attribute.datatype),
                 **attribute_fields,
             )
-        case Float(prec, units, min, max, min_alarm, max_alarm):
-            return builder.aOut(
+        case Float():
+            record = builder.aOut(
                 pv,
                 always_update=True,
                 on_update=on_update,
-                PREC=prec,
-                EGU=units,
-                DRVL=min,
-                DRVH=max,
-                LOPR=min_alarm,
-                HOPR=max_alarm,
+                **datatype_to_epics_fields(attribute.datatype),
                 **attribute_fields,
             )
         case String():
-            return builder.longStringOut(
+            record = builder.longStringOut(
                 pv, always_update=True, on_update=on_update, **attribute_fields
             )
         case _:
             raise FastCSException(
                 f"Unsupported type {type(attribute.datatype)}: {attribute.datatype}"
             )
 
+    def datatype_updater(datatype: DataType):
+        for name, value in datatype_to_epics_fields(datatype).items():
+            record.set_field(name, value)
+
+    attribute.set_update_datatype_callback(datatype_updater)
+    return record
+
 
 def _create_and_link_command_pvs(pv_prefix: str, mapping: Mapping) -> None:
     for single_mapping in mapping.get_controller_mappings():

src/fastcs/datatypes.py

@@ -12,6 +12,7 @@
 AttrCallback = Callable[[T], Awaitable[None]]
 
 
+@dataclass(frozen=True)  # So that we can type hint with dataclass methods
 class DataType(Generic[T]):
     """Generic datatype mapping to a python type, with additional metadata."""
 

tests/backends/epics/test_ioc.py

@@ -459,3 +459,23 @@ def test_long_pv_names_discarded(mocker: MockerFixture):
             always_update=True,
             on_update=mocker.ANY,
         )
+
+
+def test_update_datatype(mocker: MockerFixture):
+    builder = mocker.patch("fastcs.backends.epics.ioc.builder")
+
+    pv_name = f"{DEVICE}:Attr"
+    attr = AttrR(Int())
+    record = _get_input_record(pv_name, attr)
+
+    builder.longIn.assert_called_once_with(pv_name, **DEFAULT_SCALAR_FIELD_ARGS)
+    record.set_field.assert_not_called()
+    attr.update_datatype(Int(units="m", min=-3))
+    record.set_field.assert_any_call("EGU", "m")
+    record.set_field.assert_any_call("DRVL", -3)
+
+    with pytest.raises(
+        ValueError,
+        match="Attribute datatype must be of type <class 'fastcs.datatypes.Int'>",
+    ):
+        attr.update_datatype(String())  # type: ignore