Return enum when PV exactly matches it, otherwise fail

src/ophyd_async/epics/_backend/common.py

@@ -7,12 +7,19 @@ def get_supported_enum_class(
     datatype: Optional[Type[Enum]],
     pv_choices: Tuple[Any, ...],
 ) -> Type[Enum]:
-    if datatype:
-        if not issubclass(datatype, Enum):
-            raise TypeError(f"{pv} has type Enum not {datatype.__name__}")
-        if not issubclass(datatype, str):
-            raise TypeError(f"{pv} has type Enum but doesn't inherit from String")
-        choices = tuple(v.value for v in datatype)
-        if set(choices).difference(pv_choices):
-            raise TypeError(f"{pv} has choices {pv_choices}: not all in {choices}")
-    return Enum("GeneratedChoices", {x or "_": x for x in pv_choices}, type=str)  # type: ignore
+    if not datatype:
+        return Enum("GeneratedChoices", {x or "_": x for x in pv_choices}, type=str)  # type: ignore
+
+    if not issubclass(datatype, Enum):
+        raise TypeError(f"{pv} has type Enum not {datatype.__name__}")
+    if not issubclass(datatype, str):
+        raise TypeError(f"{pv} has type Enum but doesn't inherit from String")
+    choices = tuple(v.value for v in datatype)
+    if set(choices) != set(pv_choices):
+        raise TypeError(
+            (
+                f"{pv} has choices {pv_choices}, "
+                f"which do not match {datatype}, which has {choices}"
+            )
+        )
+    return datatype

tests/epics/_backend/test_common.py

@@ -0,0 +1,58 @@
+from enum import Enum
+
+import pytest
+
+from ophyd_async.epics._backend.common import get_supported_enum_class
+
+
+def test_given_a_non_enum_passed_to_get_supported_enum_then_raises():
+    with pytest.raises(TypeError):
+        get_supported_enum_class("", int, ("test",))
+
+
+def test_given_an_enum_but_not_str_passed_to_get_supported_enum_then_raises():
+    class MyEnum(Enum):
+        TEST = "test"
+
+    with pytest.raises(TypeError):
+        get_supported_enum_class("", MyEnum, ("test",))
+
+
+def test_given_pv_has_choices_not_in_supplied_enum_then_raises():
+    class MyEnum(str, Enum):
+        TEST = "test"
+
+    with pytest.raises(TypeError):
+        get_supported_enum_class("", MyEnum, ("test", "unexpected_choice"))
+
+
+def test_given_supplied_enum_has_choices_not_in_pv_then_raises():
+    class MyEnum(str, Enum):
+        TEST = "test"
+        OTHER = "unexpected_choice"
+
+    with pytest.raises(TypeError):
+        get_supported_enum_class("", MyEnum, ("test",))
+
+
+def test_given_no_supplied_enum_then_returns_generated_choices_enum_with_pv_choices():
+    enum_class = get_supported_enum_class("", None, ("test",))
+
+    assert isinstance(enum_class, type(Enum("GeneratedChoices", {})))
+    all_values = [e.value for e in enum_class]  # type: ignore
+    assert len(all_values) == 1
+    assert "test" in all_values
+
+
+def test_given_a_supplied_enum_that_matches_the_pv_choices_then_enum_type_is_returned():
+    class MyEnum(str, Enum):
+        TEST_1 = "test_1"
+        TEST_2 = "test_2"
+
+    enum_class = get_supported_enum_class("", MyEnum, ("test_1", "test_2"))
+
+    assert isinstance(enum_class, type(MyEnum))
+    all_values = [e.value for e in enum_class]  # type: ignore
+    assert len(all_values) == 2
+    assert "test_1" in all_values
+    assert "test_2" in all_values

tests/epics/test_signals.py

@@ -322,7 +322,10 @@ class EnumNoString(Enum):
         (
             BadEnum,
             "enum",
-            "has choices ('Aaa', 'Bbb', 'Ccc'): not all in ('Aaa', 'B', 'Ccc')",
+            (
+                "has choices ('Aaa', 'Bbb', 'Ccc'), which do not match "
+                "<enum 'BadEnum'>, which has ('Aaa', 'B', 'Ccc')"
+            ),
         ),
         (int, "str", "has type str not int"),
         (str, "float", "has type float not str"),