nr/fix forward refs in base types (#51)

* fix: Fixed serde of types that have a parameterized generic base class. (First reported in NiklasRosenstein/pydoc-markdown#292)

* tests: Add a unit tests to demonstrate that deserializing a nested type cannot work.

* Updated PR references in 1 changelogs.

skip-checks: true

---------

Co-authored-by: GitHub Action <[email protected]>

Author: NiklasRosenstein

databind.core/.changelog/_unreleased.toml

@@ -0,0 +1,6 @@
+[[entries]]
+id = "b44947f0-8e70-4a29-b584-a20eeff51bba"
+type = "fix"
+description = "Fixed serde of types that have a parameterized generic base class. (First reported in NiklasRosenstein/pydoc-markdown#292)"
+author = "[email protected]"
+pr = "https://github.com/NiklasRosenstein/python-databind/pull/51"

databind.core/src/databind/core/schema.py

@@ -246,7 +246,7 @@ class A(Generic[T]):
 
         # Continue with the base classes.
         for base in hint.bases or hint.type.__bases__:
-            base_hint = TypeHint(base).parameterize(parameter_map)
+            base_hint = TypeHint(base, source=hint.type).evaluate().parameterize(parameter_map)
             assert isinstance(base_hint, ClassTypeHint), f"nani? {base_hint}"
             if dataclasses.is_dataclass(base_hint.type):
                 queue.append(base_hint)

databind.json/.changelog/_unreleased.toml

@@ -0,0 +1,6 @@
+[[entries]]
+id = "6ac5f7d0-aeb8-436c-bc2c-a178e2a82a74"
+type = "tests"
+description = "Add a unit tests to demonstrate that deserializing a nested type cannot work."
+author = "@NiklasRosenstein"
+pr = "https://github.com/NiklasRosenstein/python-databind/pull/51"

databind.json/src/databind/json/tests/converters_test.py

@@ -543,3 +543,74 @@ class Nt(t.NamedTuple):
 
     assert mapper.serialize(Nt(1, "2"), Nt) == {"a": 1, "b": "2"}
     assert mapper.deserialize({"a": 1, "b": "2"}, Nt) == Nt(1, "2")
+
+
+T_Page = t.TypeVar("T_Page", bound="Page[t.Any]")
+
+
+@dataclasses.dataclass
+class Page(t.Generic[T_Page]):
+    name: str
+    children: t.List[T_Page]
+
+
+@dataclasses.dataclass
+class SpecificPage(Page["SpecificPage"]):
+    pass
+
+
+def test__parameterized_base_type_with_forward_ref() -> None:
+    mapper = make_mapper([SchemaConverter(), PlainDatatypeConverter(), CollectionConverter()])
+    payload = {"name": "root", "children": [{"name": "child", "children": []}]}
+    expected = SpecificPage("root", [SpecificPage("child", [])])
+    assert mapper.deserialize(payload, SpecificPage) == expected
+    mapper.serialize(expected, SpecificPage) == payload
+
+
+def test__parameterized_self_seferential_generic_cannot_be_processed() -> None:
+    """
+    A self-referential generic type (or nested generic type) cannot be properly parameterized in Mypy. [1]
+
+    For the page type above, if you would like to use it as-is, you would need to infinitely parameterized it,
+    as in `Page[Page[Page[Page[Page[... etc]]]]]`. As far as I am aware (@NiklasRosenstein, 2023.06.10), this can
+    only be solved by creating a dedicated specialized type that is self-referential via its base-class:
+
+    ```py
+    class MyPage(Page["MyPage"]):
+        pass
+    ```
+
+    When we encounter a partially parameterized type like `Page[Page]` (the inner `Page` is missing a type parameter),
+    databind will not have a way of knowing the value for the type parameter of the inner `Page` and will therefore
+    fail with an error like this:
+
+    ```
+    databind.core.converter.NoMatchingConverter: no deserializer for `TypeHint(~T_Page)` and payload of type `dict`
+    ```
+
+    [1]: https://github.com/python/mypy/issues/13693
+    """
+
+    mapper = make_mapper([SchemaConverter(), PlainDatatypeConverter(), CollectionConverter()])
+
+    payload = {
+        "name": "root",
+        "children": [
+            {
+                "name": "child",
+                "children": [
+                    # This is the level at which the deserialization will fail.
+                    {"name": "grandchild", "children": []}
+                ],
+            }
+        ],
+    }
+
+    with pytest.raises(NoMatchingConverter) as excinfo:
+        mapper.deserialize(payload, Page[Page])  # type: ignore[type-arg]
+    assert str(excinfo.value).splitlines()[0] == "no deserializer for `TypeHint(~T_Page)` and payload of type `dict`"
+
+    # It works with an additional level of page parameterization.
+    assert mapper.deserialize(payload, Page[Page[Page[Page]]]) == Page(  # type: ignore[type-arg]
+        "root", [Page("child", [Page("grandchild", [])])]
+    )