Use Literal type and lower case for encodings

rdata/_write.py

@@ -11,6 +11,7 @@
     import os
     from typing import Any
 
+    from .conversion.to_r import Encoding
     from .unparser import Compression, FileFormat
 
 
@@ -20,7 +21,7 @@ def write_rds(
     *,
     file_format: FileFormat = "xdr",
     compression: Compression = "gzip",
-    encoding: str = "UTF-8",
+    encoding: Encoding = "utf-8",
     format_version: int = DEFAULT_FORMAT_VERSION,
 ) -> None:
     """
@@ -75,7 +76,7 @@ def write_rda(
     *,
     file_format: FileFormat = "xdr",
     compression: Compression = "gzip",
-    encoding: str = "UTF-8",
+    encoding: Encoding = "utf-8",
     format_version: int = DEFAULT_FORMAT_VERSION,
 ) -> None:
     """

rdata/conversion/to_r.py

@@ -25,12 +25,15 @@
 
 if TYPE_CHECKING:
     from collections.abc import Mapping
-    from typing import Any, Final, Protocol
+    from typing import Any, Final, Literal, Protocol
+
+    Encoding = Literal["utf-8", "cp1252"]
+
 
     class Converter(Protocol):
         """Protocol for Py-to-R conversion."""
 
-        def __call__(self, data: Any, *, encoding: str) -> RObject: # noqa: ANN401
+        def __call__(self, data: Any, *, encoding: Encoding) -> RObject: # noqa: ANN401
             """Convert Python object to R object."""
 
 
@@ -91,7 +94,7 @@ def build_r_object(
 def build_r_list(
         data: Mapping[str, Any] | list[Any],
         *,
-        encoding: str,
+        encoding: Encoding,
         convert_value: Converter | None = None,
 ) -> RObject:
     """
@@ -138,7 +141,7 @@ def build_r_list(
 def build_r_sym(
         data: str,
         *,
-        encoding: str,
+        encoding: Encoding,
 ) -> RObject:
     """
     Build R object representing symbol.
@@ -158,7 +161,7 @@ def build_r_sym(
 def build_r_data(
         r_object: RObject,
         *,
-        encoding: str = "UTF-8",
+        encoding: Encoding = "utf-8",
         format_version: int = DEFAULT_FORMAT_VERSION,
         r_version_serialized: int = DEFAULT_R_VERSION_SERIALIZED,
 ) -> RData:
@@ -184,7 +187,8 @@ def build_r_data(
     )
 
     minimum_version_with_encoding = 3
-    extra = (RExtraInfo(encoding) if versions.format >= minimum_version_with_encoding
+    extra = (RExtraInfo(encoding.upper())
+             if versions.format >= minimum_version_with_encoding
              else RExtraInfo(None))
 
     return RData(versions, extra, r_object)
@@ -193,7 +197,7 @@ def build_r_data(
 def convert_to_r_object_for_rda(
         data: Mapping[str, Any],
         *,
-        encoding: str = "UTF-8",
+        encoding: Encoding = "utf-8",
 ) -> RObject:
     """
     Convert Python dictionary to R object for RDA file.
@@ -217,7 +221,7 @@ def convert_to_r_object_for_rda(
 def convert_to_r_object(  # noqa: C901, PLR0912, PLR0915
         data: Any,  # noqa: ANN401
         *,
-        encoding: str = "UTF-8",
+        encoding: Encoding = "utf-8",
 ) -> RObject:
     """
     Convert Python data to R object.
@@ -318,9 +322,9 @@ def convert_to_r_object(  # noqa: C901, PLR0912, PLR0915
         r_type = RObjectType.CHAR
         if all(chr(byte) in string.printable for byte in data):
             gp = CharFlags.ASCII
-        elif encoding == "UTF-8":
+        elif encoding == "utf-8":
             gp = CharFlags.UTF8
-        elif encoding == "CP1252":
+        elif encoding == "cp1252":
             # Note!
             # CP1252 and Latin1 are not the same.
             # Does CharFlags.LATIN1 mean actually CP1252

rdata/tests/test_write.py

@@ -13,6 +13,7 @@
 from rdata.unparser import unparse_data
 
 if TYPE_CHECKING:
+    from rdata.conversion.to_r import Encoding
     from rdata.unparser import Compression, FileFormat, FileType
 
 
@@ -118,9 +119,12 @@ def test_convert_to_r(fname: str) -> None:
         except NotImplementedError as e:
             pytest.skip(str(e))
 
-        encoding = r_data.extra.encoding
+        encoding: Encoding
+        encoding = r_data.extra.encoding  # type: ignore [assignment]
         if encoding is None:
-            encoding = "CP1252" if "win" in fname else "UTF-8"
+            encoding = "cp1252" if "win" in fname else "utf-8"
+        else:
+            encoding = encoding.lower()  # type: ignore [assignment]
 
         try:
             if file_type == "rds":
@@ -168,13 +172,13 @@ def test_unparse_bad_rda() -> None:
 def test_convert_to_r_bad_encoding() -> None:
     """Test checking encoding."""
     with pytest.raises(LookupError, match="(?i)unknown encoding"):
-        rdata.conversion.convert_to_r_object("ä", encoding="non-existent")
+        rdata.conversion.convert_to_r_object("ä", encoding="non-existent")  # type: ignore [arg-type]
 
 
 def test_convert_to_r_unsupported_encoding() -> None:
     """Test checking encoding."""
     with pytest.raises(ValueError, match="(?i)unsupported encoding"):
-        rdata.conversion.convert_to_r_object("ä", encoding="CP1250")
+        rdata.conversion.convert_to_r_object("ä", encoding="cp1250")  # type: ignore [arg-type]
 
 
 def test_unparse_big_int() -> None: