From 44a88a5eae043314aa7124638d4da0dcf77c2630 Mon Sep 17 00:00:00 2001
From: Gary Yendell <gary.yendell@diamond.ac.uk>
Date: Thu, 7 Aug 2025 16:21:39 +0000
Subject: [PATCH] Update DataType.validate to attempt cast

---
 src/fastcs/datatypes.py               | 23 +++++++++++----
 tests/test_attribute.py               | 22 +-------------
 tests/test_datatypes.py               | 42 +++++++++++++++++++++++++++
 tests/transport/epics/ca/test_util.py |  6 ----
 4 files changed, 60 insertions(+), 33 deletions(-)
 create mode 100644 tests/test_datatypes.py

diff --git a/src/fastcs/datatypes.py b/src/fastcs/datatypes.py
index 9580c3574..498de64fe 100644
--- a/src/fastcs/datatypes.py
+++ b/src/fastcs/datatypes.py
@@ -5,7 +5,7 @@
 from collections.abc import Awaitable, Callable
 from dataclasses import dataclass
 from functools import cached_property
-from typing import Generic, TypeVar
+from typing import Any, Generic, TypeVar
 
 import numpy as np
 from numpy.typing import DTypeLike
@@ -36,12 +36,23 @@ class DataType(Generic[T]):
     def dtype(self) -> type[T]:  # Using property due to lack of Generic ClassVars
         pass
 
-    def validate(self, value: T) -> T:
-        """Validate a value against fields in the datatype."""
-        if not isinstance(value, self.dtype):
-            raise ValueError(f"Value '{value}' is not of type {self.dtype}")
+    def validate(self, value: Any) -> T:
+        """Validate a value against the datatype.
 
-        return value
+        The base implementation is to try the cast and raise a useful error if it fails.
+
+        Child classes can implement logic before calling ``super.validate(value)`` to
+        modify the value passed in and help the cast succeed or after to perform further
+        validation of the coerced type.
+
+        """
+        if isinstance(value, self.dtype):
+            return value
+
+        try:
+            return self.dtype(value)
+        except (ValueError, TypeError) as e:
+            raise ValueError(f"Failed to cast {value} to type {self.dtype}") from e
 
     @property
     @abstractmethod
diff --git a/tests/test_attribute.py b/tests/test_attribute.py
index 2a8e788c2..473501a75 100644
--- a/tests/test_attribute.py
+++ b/tests/test_attribute.py
@@ -1,11 +1,10 @@
 from functools import partial
 
-import numpy as np
 import pytest
 from pytest_mock import MockerFixture
 
 from fastcs.attributes import AttrHandlerR, AttrHandlerRW, AttrR, AttrRW, AttrW
-from fastcs.datatypes import Enum, Float, Int, String, Waveform
+from fastcs.datatypes import Int, String
 
 
 @pytest.mark.asyncio
@@ -87,22 +86,3 @@ async def test_handler_initialise(mocker: MockerFixture):
 
     # Assert no error in calling initialise on the TestUpdater handler
     await attr.initialise(mocker.ANY)
-
-
-@pytest.mark.parametrize(
-    ["datatype", "init_args", "value"],
-    [
-        (Int, {"min": 1}, 0),
-        (Int, {"max": -1}, 0),
-        (Float, {"min": 1}, 0.0),
-        (Float, {"max": -1}, 0.0),
-        (Float, {}, 0),
-        (String, {}, 0),
-        (Enum, {"enum_cls": int}, 0),
-        (Waveform, {"array_dtype": "U64", "shape": (1,)}, np.ndarray([1])),
-        (Waveform, {"array_dtype": "float64", "shape": (1, 1)}, np.ndarray([1])),
-    ],
-)
-def test_validate(datatype, init_args, value):
-    with pytest.raises(ValueError):
-        datatype(**init_args).validate(value)
diff --git a/tests/test_datatypes.py b/tests/test_datatypes.py
new file mode 100644
index 000000000..4ff7ed16f
--- /dev/null
+++ b/tests/test_datatypes.py
@@ -0,0 +1,42 @@
+from enum import IntEnum
+
+import numpy as np
+import pytest
+
+from fastcs.datatypes import DataType, Enum, Float, Int, Waveform
+
+
+def test_base_validate():
+    class TestInt(DataType[int]):
+        @property
+        def dtype(self) -> type[int]:
+            return int
+
+    class MyIntEnum(IntEnum):
+        A = 0
+        B = 1
+
+    test_int = TestInt()
+
+    assert test_int.validate("0") == 0
+    assert test_int.validate(MyIntEnum.B) == 1
+
+    with pytest.raises(ValueError, match="Failed to cast"):
+        test_int.validate("foo")
+
+
+@pytest.mark.parametrize(
+    ["datatype", "init_args", "value"],
+    [
+        (Int, {"min": 1}, 0),
+        (Int, {"max": -1}, 0),
+        (Float, {"min": 1}, 0.0),
+        (Float, {"max": -1}, 0.0),
+        (Enum, {"enum_cls": int}, 0),
+        (Waveform, {"array_dtype": "U64", "shape": (1,)}, np.ndarray([1])),
+        (Waveform, {"array_dtype": "float64", "shape": (1, 1)}, np.ndarray([1])),
+    ],
+)
+def test_validate(datatype, init_args, value):
+    with pytest.raises(ValueError):
+        datatype(**init_args).validate(value)
diff --git a/tests/transport/epics/ca/test_util.py b/tests/transport/epics/ca/test_util.py
index 25e3e7c1d..e26e4cf45 100644
--- a/tests/transport/epics/ca/test_util.py
+++ b/tests/transport/epics/ca/test_util.py
@@ -90,14 +90,8 @@ def test_casting_to_epics(datatype, input, output):
 @pytest.mark.parametrize(
     "datatype, input",
     [
-        (object(), 0),
         # TODO cover Waveform and Table cases
-        (Enum(ShortEnum), 0),  # can't use index
         (Enum(ShortEnum), LongEnum.TOO),  # wrong enum.Enum class
-        (Int(), 4.0),
-        (Float(), 1),
-        (Bool(), None),
-        (String(), 10),
     ],
 )
 def test_cast_to_epics_validations(datatype, input):