Improve String validation

src/fastcs/datatypes/string.py

@@ -1,4 +1,5 @@
 from dataclasses import dataclass
+from typing import Any
 
 from fastcs.datatypes.datatype import DataType
 
@@ -8,7 +9,11 @@ class String(DataType[str]):
     """`DataType` mapping to builtin ``str``."""
 
     length: int | None = None
-    """Maximum length of string to display in transports"""
+    """Maximum length of string to display in transports. Must be >=1 or None."""
+
+    def __post_init__(self):
+        if self.length is not None and self.length < 1:
+            raise ValueError("String length must be >= 1")
 
     @property
     def dtype(self) -> type[str]:
@@ -17,3 +22,12 @@ def dtype(self) -> type[str]:
     @property
     def initial_value(self) -> str:
         return ""
+
+    def validate(self, value: Any) -> str:
+        """Truncate string to maximum length
+
+        Returns:
+            The string, truncated to the maximum length if set
+
+        """
+        return super().validate(value)[: self.length]

src/fastcs/transports/epics/ca/util.py

@@ -146,7 +146,10 @@ def cast_to_epics_type(datatype: DataType[DType_T], value: DType_T) -> Any:
             else:  # enum backed by string record
                 return datatype.validate(value).name
         case String() as string:
-            return value[: string.length]
+            if string.length is not None:
+                return value[: string.length]
+            else:
+                return value[:DEFAULT_STRING_WAVEFORM_LENGTH]
         case datatype if issubclass(type(datatype), EPICS_ALLOWED_DATATYPES):
             return value
         case _:

tests/test_datatypes.py

@@ -104,3 +104,12 @@ def test_dataset_equal(fastcs_datatype: DataType, value1, value2, expected):
 )
 def test_dataset_all_equal(fastcs_datatype: DataType, values, expected):
     assert fastcs_datatype.all_equal(values) is expected
+
+
+def test_string_length():
+    assert String(length=10).validate("12345678901") == "1234567890"
+
+    assert String().validate("12345678901") == "12345678901"
+
+    with pytest.raises(ValueError):
+        String(length=0)

tests/transports/epics/ca/test_ca_util.py

@@ -77,7 +77,8 @@ class ShortMixedEnum(enum.Enum):
         (Int(), 4, 4),
         (Float(), 1.0, 1.0),
         (Bool(), True, True),
-        (String(), "hey", "hey"),
+        (String(), "a" * 257, "a" * 256),
+        (String(length=3), "1234", "123"),
         # shorter enums can be represented by integers from 0-15
         (Enum(ShortMixedEnum), ShortMixedEnum.STRING_MEMBER, 0),
         (Enum(ShortMixedEnum), ShortMixedEnum.INT_MEMBER, 1),