fix: preserve ByteStream subclasses during deserialization

haystack/dataclasses/byte_stream.py

@@ -4,7 +4,7 @@
 
 from dataclasses import dataclass, field
 from pathlib import Path
-from typing import Any
+from typing import Any, Self
 
 from haystack.utils.dataclasses import _warn_on_inplace_mutation
 from haystack.utils.misc import _guess_mime_type
@@ -41,7 +41,7 @@ def from_file_path(
         mime_type: str | None = None,
         meta: dict[str, Any] | None = None,
         guess_mime_type: bool = False,
-    ) -> "ByteStream":
+    ) -> Self:
         """
         Create a ByteStream from the contents read from a file.
 
@@ -58,7 +58,7 @@ def from_file_path(
     @classmethod
     def from_string(
         cls, text: str, encoding: str = "utf-8", mime_type: str | None = None, meta: dict[str, Any] | None = None
-    ) -> "ByteStream":
+    ) -> Self:
         """
         Create a ByteStream encoding a string.
 
@@ -113,12 +113,12 @@ def _to_trace_dict(self) -> dict[str, Any]:
         return {"data": f"Binary data ({len(self.data)} bytes)", "meta": self.meta, "mime_type": self.mime_type}
 
     @classmethod
-    def from_dict(cls, data: dict[str, Any]) -> "ByteStream":
+    def from_dict(cls, data: dict[str, Any]) -> Self:
         """
         Create a ByteStream from a dictionary representation.
 
         :param data: A dictionary with keys 'data', 'meta', and 'mime_type'.
 
-        :returns: A ByteStream instance.
+        :returns: An instance of this class.
         """
-        return ByteStream(data=bytes(data["data"]), meta=data.get("meta", {}), mime_type=data.get("mime_type"))
+        return cls(data=bytes(data["data"]), meta=data.get("meta", {}), mime_type=data.get("mime_type"))

releasenotes/notes/preserve-bytestream-subclass-8f2c408199657a06.yaml

@@ -0,0 +1,4 @@
+fixes:
+  - |
+    ``ByteStream.from_dict()`` now preserves subclasses when deserializing a
+    dictionary, instead of always returning a ``ByteStream`` instance.

test/dataclasses/test_byte_stream.py

@@ -159,6 +159,17 @@ def test_from_dict():
     assert str(b2) == str(b)
 
 
+def test_from_dict_preserves_subclass():
+    class CustomByteStream(ByteStream):
+        pass
+
+    original = CustomByteStream.from_string("Hello, world!", mime_type="text/plain", meta={"foo": "bar"})
+    restored = CustomByteStream.from_dict(original.to_dict())
+
+    assert isinstance(restored, CustomByteStream)
+    assert restored.to_dict() == original.to_dict()
+
+
 def test_no_warning_on_init():
     with warnings.catch_warnings():
         warnings.simplefilter("error", Warning)