fix: unravel Optional to inner generic arg from instance (#2172)

* fix: unravel Optional to inner generic arg from instance

* test: remove dependency on Incremental in common

* refactor: use extract_inner_type

* refactor: remove redundant conditional

dlt/common/typing.py

@@ -438,8 +438,7 @@ def get_generic_type_argument_from_instance(
     """
     orig_param_type = Any
     if cls_ := getattr(instance, "__orig_class__", None):
-        # instance of generic class
-        pass
+        cls_ = extract_inner_type(cls_)
     elif bases_ := get_original_bases(instance.__class__):
         # instance of class deriving from generic
         cls_ = bases_[0]

tests/common/test_typing.py

@@ -1,10 +1,13 @@
+from types import SimpleNamespace
+
 import pytest
 from dataclasses import dataclass
 from typing import (
     Any,
     Callable,
     ClassVar,
     Final,
+    Generic,
     List,
     Literal,
     Mapping,
@@ -44,6 +47,7 @@
     is_annotated,
     is_callable_type,
     add_value_to_literal,
+    get_generic_type_argument_from_instance,
 )
 
 
@@ -310,3 +314,22 @@ def test_add_value_to_literal() -> None:
     add_value_to_literal(TestSingleLiteral, "green")
     add_value_to_literal(TestSingleLiteral, "blue")
     assert get_args(TestSingleLiteral) == ("red", "green", "blue")
+
+
+def test_get_generic_type_argument_from_instance() -> None:
+    T = TypeVar("T")
+
+    class Foo(Generic[T]):
+        pass
+
+    # generic contains hint
+    instance = SimpleNamespace(__orig_class__=Foo[str])
+    assert get_generic_type_argument_from_instance(instance) is str
+    instance = SimpleNamespace(__orig_class__=Optional[Foo[str]])
+    assert get_generic_type_argument_from_instance(instance) is str
+
+    # with sample values
+    instance = SimpleNamespace(__orig_class__=Foo[Any])
+    assert get_generic_type_argument_from_instance(instance, 1) is int
+    instance = SimpleNamespace(__orig_class__=Optional[Foo[Any]])
+    assert get_generic_type_argument_from_instance(instance, 1) is int