Fixes dataloader to use the correct type hinting

This makes it work with __future__.annotations. See #1259.

hamilton/function_modifiers/adapters.py

@@ -733,7 +733,7 @@ def load_json_data(json_path: str = "data/my_data.json") -> tuple[pd.DataFrame,
 
     def validate(self, fn: Callable):
         """Validates that the output type is correctly annotated."""
-        return_annotation = inspect.signature(fn).return_annotation
+        return_annotation = typing.get_type_hints(fn).get("return")
         if return_annotation is inspect.Signature.empty:
             raise InvalidDecoratorException(
                 f"Function: {fn.__qualname__} must have a return annotation."

tests/function_modifiers/test_adapters.py

@@ -19,6 +19,7 @@
     resolve_kwargs,
 )
 from hamilton.function_modifiers.base import DefaultNodeCreator
+from hamilton.htypes import custom_subclass_check
 from hamilton.io.data_adapters import DataLoader, DataSaver
 from hamilton.io.default_data_loaders import JSONDataSaver
 from hamilton.registry import LOADER_REGISTRY
@@ -792,6 +793,18 @@ def test_dataloader():
     }
 
 
+def test_dataloader_future_annotations():
+    from tests.resources import nodes_with_future_annotation
+
+    fn_to_collect = nodes_with_future_annotation.sample_dataloader
+    fg = graph.create_function_graph(
+        ad_hoc_utils.create_temporary_module(fn_to_collect),
+        config={},
+    )
+    # the data loaded is a list
+    assert custom_subclass_check(fg["sample_dataloader"].type, list)
+
+
 def test_datasaver():
     annotation = datasaver()
     (node1,) = annotation.generate_nodes(correct_ds_function, {})

tests/resources/nodes_with_future_annotation.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+from hamilton.function_modifiers import dataloader
 from hamilton.htypes import Collect, Parallelizable
 
 """Tests future annotations with common node types"""
@@ -17,3 +18,9 @@ def standard(parallelized: int) -> int:
 
 def collected(standard: Collect[int]) -> int:
     return sum(standard)
+
+
+@dataloader()
+def sample_dataloader() -> tuple[list[str], dict]:
+    """Grouping here as the rest test annotations"""
+    return ["a", "b", "c"], {}