.with_materializers() added helpful error message (#986)

* added helpful error message

* added unit test; specified condition

---------

Co-authored-by: zilto <tjean@DESKTOP-V6JDCS2>

hamilton/driver.py

@@ -9,6 +9,7 @@
 # required if we want to run this code stand alone.
 import typing
 import uuid
+from collections.abc import Sequence  # typing.Sequence is deprecated in >=3.9
 from datetime import datetime
 from types import ModuleType
 from typing import Any, Callable, Collection, Dict, List, Optional, Set, Tuple, Union
@@ -1757,6 +1758,18 @@ def with_materializers(
         :param materializers: materializers to add to the dataflow
         :return: self
         """
+        if any(
+            m for m in materializers if not isinstance(m, (ExtractorFactory, MaterializerFactory))
+        ):
+            if len(materializers) == 1 and isinstance(materializers[0], Sequence):
+                raise ValueError(
+                    "`.with_materializers()` received a sequence. Unpack it by prepending `*` e.g., `*[to.json(...), from_.parquet(...)]`"
+                )
+            else:
+                raise ValueError(
+                    f"`.with_materializers()` only accepts materializers. Received instead: {materializers}"
+                )
+
         self.materializers.extend(materializers)
         return self
 

tests/test_hamilton_driver.py

@@ -585,6 +585,31 @@ def test_builder_materializer_and_execution_materializer(tmp_path):
     assert "static_saver" in additional.keys()
 
 
+def test_builder_materializer_error():
+    static_saver = to.json(
+        id="static_saver",
+        dependencies=["json_to_save_1"],
+        path="/file.json",
+    )
+
+    materializers = [static_saver]
+    with pytest.raises(ValueError):
+        (
+            Builder()
+            .with_modules(tests.resources.test_for_materialization)
+            .with_materializers(materializers)
+            .build()
+        )
+
+    # also check that using `*` leads to no issue
+    (
+        Builder()
+        .with_modules(tests.resources.test_for_materialization)
+        .with_materializers(*materializers)
+        .build()
+    )
+
+
 def test_materialize_checks_required_input(tmp_path):
     dr = Builder().with_modules(tests.resources.dummy_functions).build()