Implement new Bytes field

src/marshmallow/fields.py

@@ -2,6 +2,7 @@
 from __future__ import annotations
 
 import abc
+import base64
 import collections
 import copy
 import datetime as dt
@@ -48,6 +49,7 @@
     "AwareDateTime",
     "Bool",
     "Boolean",
+    "Bytes",
     "Constant",
     "Date",
     "DateTime",
@@ -878,6 +880,95 @@ def _deserialize(self, value, attr, data, **kwargs) -> str:
             raise self.make_error("invalid_utf8") from error
 
 
+class Bytes(Field[bytes]):
+    """
+    A field for deserializing strings into byte arrays.
+
+    :param encoding: Specifies the string encoding used when encoding/decoding to/from strings.
+    :param errors: Error behaviour when converting to/from a :class:`str`, inherited from it's constructor.
+    :param serialize: Specifies the return type when serializing.
+        `base64` and `str` use the value of `encoding` for the string.
+    :param kwargs: The same keyword arguments that :class:`Field` receives.
+
+    .. versionadded:: 4.3.0
+    """
+
+    #: Default error messages.
+    default_error_messages = {
+        "not_bytes": "Not a bytes-like object.",
+        "unicode": "Invalid unicode string.",
+    }
+
+    def __init__(
+        self,
+        encoding: str = "utf-8",
+        errors: str = "strict",
+        serialize: typing.Literal["int", "str", "bytes", "base64"] = "base64",
+        **kwargs: Unpack[_BaseFieldKwargs],
+    ):
+        super().__init__(**kwargs)
+        self.encoding = encoding
+        self.errors = errors
+        self.serialize = serialize
+
+    def _deserialize(
+        self,
+        value: typing.Any,
+        attr: str | None,
+        data: typing.Mapping[str, typing.Any] | None,
+        **kwargs: typing.Any,
+    ) -> bytes:
+        try:
+            match value:
+                case str() as s:
+                    return bytes(
+                        s,
+                        encoding=self.encoding,
+                        errors=self.errors,
+                    )
+                case int() as i:
+                    return i.to_bytes(
+                        length=max(1, (7 + i.bit_length()) // 8),
+                        byteorder="big",
+                        signed=i < 0,
+                    )
+                case _:
+                    return bytes(value)
+        except TypeError as e:
+            raise self.make_error("not_bytes") from e
+        except UnicodeError as e:
+            raise self.make_error("unicode") from e
+
+    def _serialize(
+        self,
+        value: bytes,
+        attr: str | None,
+        obj: typing.Any,
+        **kwargs: typing.Any,
+    ) -> str | int | bytes:
+        try:
+            match self.serialize:
+                case "str":
+                    return str(
+                        value,
+                        encoding=self.encoding,
+                        errors=self.errors,
+                    )
+                case "base64":
+                    return base64.standard_b64encode(value)
+                case "int":
+                    return int.from_bytes(
+                        value,
+                        byteorder="big",
+                    )
+                case "bytes":
+                    return value
+                case _:
+                    typing.assert_never(self.serialize)
+        except UnicodeError as e:
+            raise self.make_error("unicode") from e
+
+
 class UUID(Field[uuid.UUID]):
     """A UUID field."""
 

tests/test_deserialization.py

@@ -322,6 +322,23 @@ def test_string_field_deserialization(self):
         with pytest.raises(ValidationError):
             field.deserialize({})
 
+    def test_bytes_field_deserialization(self):
+        field = fields.Bytes()
+        assert field.deserialize(b"foo") == b"foo"
+        assert field.deserialize(bytearray(b"foo")) == b"foo"
+        assert field.deserialize("foo") == b"foo"
+        assert field.deserialize(0xDEAD) == b"\xde\xad"
+        assert field.deserialize([0xBE, 0xEF]) == b"\xbe\xef"
+        assert field.deserialize((0xB, 0xA, 0xB, 0xE)) == b"\x0b\x0a\x0b\x0e"
+
+        with pytest.raises(ValidationError) as excinfo:
+            field.deserialize({"hi": 222})
+        assert excinfo.value.args[0] == "not a bytes-like object"
+
+        with pytest.raises(ValidationError) as excinfo:
+            field.deserialize(["12345"])
+        assert excinfo.value.args[0] == "not a bytes-like object"
+
     def test_boolean_field_deserialization(self):
         field = fields.Boolean()
         assert field.deserialize(True) is True