Handle common cases of mutable default arguments explicitly

flytekit/core/promise.py

@@ -5,7 +5,7 @@
 import typing
 from copy import deepcopy
 from enum import Enum
-from typing import Any, Coroutine, Dict, Hashable, List, Optional, Set, Tuple, Union, cast, get_args
+from typing import Any, Coroutine, Dict, List, Optional, Set, Tuple, Union, cast, get_args
 
 from google.protobuf import struct_pb2 as _struct
 from typing_extensions import Protocol
@@ -1116,8 +1116,13 @@ def create_and_link_node(
                 or UnionTransformer.is_optional_type(interface.inputs_with_defaults[k][0])
             ):
                 default_val = interface.inputs_with_defaults[k][1]
-                if not isinstance(default_val, Hashable):
-                    raise _user_exceptions.FlyteAssertion("Cannot use non-hashable object as default argument")
+                # Common cases of mutable default arguments, as described in https://www.pullrequest.com/blog/python-pitfalls-the-perils-of-using-lists-and-dicts-as-default-arguments/
+                # or https://florimond.dev/en/posts/2018/08/python-mutable-defaults-are-the-source-of-all-evil, are not supported.
+                # As of 2024-08-05, Python native sets are not supported in Flytekit. However, they are included here for completeness.
+                if isinstance(default_val, list) or isinstance(default_val, dict) or isinstance(default_val, set):
+                    raise _user_exceptions.FlyteAssertion(
+                        f"Argument {k} for function {entity.name} is a mutable default argument, which is a python anti-pattern and not supported in flytekit tasks"
+                    )
                 kwargs[k] = default_val
             else:
                 error_msg = f"Input {k} of type {interface.inputs[k]} was not specified for function {entity.name}"

tests/flytekit/unit/core/test_serialization.py

@@ -1,3 +1,4 @@
+import re
 import os
 import typing
 from collections import OrderedDict
@@ -775,7 +776,10 @@ def wf_no_input() -> typing.List[int]:
     def wf_with_input() -> typing.List[int]:
         return t1(a=input_val)
 
-    with pytest.raises(FlyteAssertion, match="Cannot use non-hashable object as default argument"):
+    with pytest.raises(
+        FlyteAssertion,
+        match=r"Argument a for function .*test_serialization\.t1 is a mutable default argument, which is a python anti-pattern and not supported in flytekit tasks"
+    ):
         get_serializable(OrderedDict(), serialization_settings, wf_no_input)
 
     wf_with_input_spec = get_serializable(OrderedDict(), serialization_settings, wf_with_input)
@@ -810,7 +814,10 @@ def wf_no_input() -> typing.Dict[str, int]:
     def wf_with_input() -> typing.Dict[str, int]:
         return t1(a=input_val)
 
-    with pytest.raises(FlyteAssertion, match="Cannot use non-hashable object as default argument"):
+    with pytest.raises(
+        FlyteAssertion,
+        match=r"Argument a for function .*test_serialization\.t1 is a mutable default argument, which is a python anti-pattern and not supported in flytekit tasks"
+    ):
         get_serializable(OrderedDict(), serialization_settings, wf_no_input)
 
     wf_with_input_spec = get_serializable(OrderedDict(), serialization_settings, wf_with_input)
@@ -910,7 +917,10 @@ def wf_no_input() -> typing.Optional[typing.List[int]]:
     def wf_with_input() -> typing.Optional[typing.List[int]]:
         return t1(a=input_val)
 
-    with pytest.raises(FlyteAssertion, match="Cannot use non-hashable object as default argument"):
+    with pytest.raises(
+        FlyteAssertion,
+        match=r"Argument a for function .*test_serialization\.t1 is a mutable default argument, which is a python anti-pattern and not supported in flytekit tasks"
+    ):
         get_serializable(OrderedDict(), serialization_settings, wf_no_input)
 
     wf_with_input_spec = get_serializable(OrderedDict(), serialization_settings, wf_with_input)

tests/flytekit/unit/types/structured_dataset/test_structured_dataset.py

@@ -543,10 +543,11 @@ def test_reregister_encoder():
 
 
 def test_default_args_task():
+    default_val = pd.DataFrame({"name": ["Aegon"], "age": [27]})
     input_val = generate_pandas()
 
     @task
-    def t1(a: pd.DataFrame = pd.DataFrame()) -> pd.DataFrame:
+    def t1(a: pd.DataFrame = default_val) -> pd.DataFrame:
         return a
 
     @workflow
@@ -557,11 +558,16 @@ def wf_no_input() -> pd.DataFrame:
     def wf_with_input() -> pd.DataFrame:
         return t1(a=input_val)
 
-    with pytest.raises(FlyteAssertion, match="Cannot use non-hashable object as default argument"):
-        get_serializable(OrderedDict(), serialization_settings, wf_no_input)
-
+    wf_no_input_spec = get_serializable(OrderedDict(), serialization_settings, wf_no_input)
     wf_with_input_spec = get_serializable(OrderedDict(), serialization_settings, wf_with_input)
 
+    assert wf_no_input_spec.template.nodes[0].inputs[
+        0
+    ].binding.value.structured_dataset.metadata == StructuredDatasetMetadata(
+        structured_dataset_type=StructuredDatasetType(
+            format="parquet",
+        ),
+    )
     assert wf_with_input_spec.template.nodes[0].inputs[
         0
     ].binding.value.structured_dataset.metadata == StructuredDatasetMetadata(
@@ -570,8 +576,12 @@ def wf_with_input() -> pd.DataFrame:
         ),
     )
 
+    assert wf_no_input_spec.template.interface.outputs["o0"].type == LiteralType(
+        structured_dataset_type=StructuredDatasetType()
+    )
     assert wf_with_input_spec.template.interface.outputs["o0"].type == LiteralType(
         structured_dataset_type=StructuredDatasetType()
     )
 
+    pd.testing.assert_frame_equal(wf_no_input(), default_val)
     pd.testing.assert_frame_equal(wf_with_input(), input_val)