usethis-python · nathanjmcdougall · Apr 1, 2026 · Apr 1, 2026 · Apr 1, 2026 · Apr 1, 2026
diff --git a/src/usethis/_integrations/pydantic/dump.py b/src/usethis/_integrations/pydantic/dump.py
@@ -126,29 +126,35 @@ def _fancy_model_dump_base_model(
 
     d: dict[str, ModelRepresentation] = {}
     for key, value in model:
-        default_value = model.__class__.model_fields[key].default
-
         # The value for the reference (for recursion)
         value_ref = _get_value_ref(reference, key=key)
 
-        # If the model has default value, we usually won't dump it.
-        # There is an exception though: if we have a reference which we are trying
-        # to minimize the diff against, then if the diff includes the default
-        # explicitly, we should include it too.
-        # This is technically a limitation in what kind of diffs we can express in
-        # the dump but it's a relatively minor one.
-
-        if value_ref is not None:
-            ref_has_default = value_ref == default_value
+        field_info = model.__class__.model_fields.get(key)
+        if field_info is not None:
+            default_value = field_info.default
+
+            # If the model has default value, we usually won't dump it.
+            # There is an exception though: if we have a reference which we are trying
+            # to minimize the diff against, then if the diff includes the default
+            # explicitly, we should include it too.
+            # This is technically a limitation in what kind of diffs we can express in
+            # the dump but it's a relatively minor one.
+
+            if value_ref is not None:
+                ref_has_default = value_ref == default_value
+            else:
+                ref_has_default = False
+
+            if (value == default_value) and not ref_has_default:
+                continue
+
+            # Find the key for display - there might be an alias
+            display_key = field_info.alias
+            if display_key is None:
+                display_key = key
         else:
-            ref_has_default = False
-
-        if (value == default_value) and not ref_has_default:
-            continue
-
-        # Find the key for display - there might be an alias
-        display_key = model.__class__.model_fields[key].alias
-        if display_key is None:
+            # Extra field not defined in the model schema (e.g. from prek syntax).
+            # Always include it using the raw key.
             display_key = key
 
         d[display_key] = fancy_model_dump(

diff --git a/tests/usethis/_integrations/pre_commit/test_hooks.py b/tests/usethis/_integrations/pre_commit/test_hooks.py
@@ -1,6 +1,7 @@
 from pathlib import Path
 
 import pytest
+from ruamel.yaml import YAML
 
 from usethis._config_file import files_manager
 from usethis._integrations.pre_commit import schema
@@ -237,6 +238,93 @@ def test_hook_order_constant_is_respected_multi(self, tmp_path: Path):
                 "codespell",
             ]
 
+    def test_prek_extra_fields_preserved(self, tmp_path: Path):
+        """Extra keys like `priority` (from prek syntax) are preserved."""
+        # Arrange
+        (tmp_path / ".pre-commit-config.yaml").write_text("""\
+minimum_prek_version: 0.2.23
+repos:
+  - repo: https://github.com/astral-sh/ruff-pre-commit
+    rev: v0.14.0
+    hooks:
+      - id: ruff-check
+        args: [--fix]
+        priority: 0
+      - id: ruff-format
+        priority: 0
+""")
+
+        # Act
+        with change_cwd(tmp_path), files_manager():
+            add_repo(
+                schema.LocalRepo(
+                    repo="local",
+                    hooks=[
+                        schema.HookDefinition(
+                            id="deptry",
+                            name="deptry",
+                            entry="uv run --frozen deptry src",
+                            language=schema.Language("system"),
+                            always_run=True,
+                        )
+                    ],
+                )
+            )
+
+        # Assert - parse YAML to verify structure
+        yaml = YAML()
+        parsed = yaml.load((tmp_path / ".pre-commit-config.yaml").read_text())
+        assert parsed["minimum_prek_version"] == "0.2.23"
+        ruff_repo = parsed["repos"][0]
+        assert ruff_repo["hooks"][0]["priority"] == 0
+        assert ruff_repo["hooks"][1]["priority"] == 0
+        assert any(
+            hook["id"] == "deptry" for repo in parsed["repos"] for hook in repo["hooks"]
+        )
+
+    def test_prek_arbitrary_extra_keys(self, tmp_path: Path):
+        """Arbitrary extra keys on hooks, repos, and top-level are preserved."""
+        # Arrange
+        (tmp_path / ".pre-commit-config.yaml").write_text("""\
+custom_top_level_key: some_value
+repos:
+  - repo: https://github.com/astral-sh/ruff-pre-commit
+    rev: v0.14.0
+    custom_repo_key: 42
+    hooks:
+      - id: ruff-format
+        custom_hook_key: true
+""")
+
+        # Act
+        with change_cwd(tmp_path), files_manager():
+            add_repo(
+                schema.LocalRepo(
+                    repo="local",
+                    hooks=[
+                        schema.HookDefinition(
+                            id="codespell",
+                            name="codespell",
+                            entry="codespell .",
+                            language=schema.Language("system"),
+                        )
+                    ],
+                )
+            )
+
+        # Assert - parse YAML to verify extra keys are at correct levels
+        yaml = YAML()
+        parsed = yaml.load((tmp_path / ".pre-commit-config.yaml").read_text())
+        assert parsed["custom_top_level_key"] == "some_value"
+        ruff_repo = parsed["repos"][0]
+        assert ruff_repo["custom_repo_key"] == 42
+        assert ruff_repo["hooks"][0]["custom_hook_key"] is True
+        assert any(
+            hook["id"] == "codespell"
+            for repo in parsed["repos"]
+            for hook in repo["hooks"]
+        )
+
 
 class TestInsertRepo:
     def test_predecessor_is_none(

diff --git a/tests/usethis/_integrations/pydantic/test_dump.py b/tests/usethis/_integrations/pydantic/test_dump.py
@@ -1,6 +1,6 @@
 from typing import Literal
 
-from pydantic import BaseModel, Field, RootModel
+from pydantic import BaseModel, ConfigDict, Field, RootModel
 
 from usethis._integrations.pydantic.dump import fancy_model_dump
 from usethis._integrations.pydantic.typing_ import ModelRepresentation
@@ -294,3 +294,71 @@ class MyModel(BaseModel):
 
             # Assert
             assert output == {}
+
+    class TestExtraFields:
+        def test_extra_field_included(self):
+            # Arrange
+            class MyModel(BaseModel):
+                model_config = ConfigDict(extra="allow")
+                x: int
+
+            mm = MyModel(x=1, **{"priority": 0})
+
+            # Act
+            output = fancy_model_dump(mm)
+
+            # Assert
+            assert output == {"x": 1, "priority": 0}
+
+        def test_extra_field_with_default_fields(self):
+            # Arrange
+            class MyModel(BaseModel):
+                model_config = ConfigDict(extra="allow")
+                x: int
+                y: float = 2.0
+
+            mm = MyModel(x=1, **{"priority": 0})
+
+            # Act
+            output = fancy_model_dump(mm)
+
+            # Assert
+            assert output == {"x": 1, "priority": 0}
+
+        def test_extra_field_with_reference(self):
+            # Arrange
+            class MyModel(BaseModel):
+                model_config = ConfigDict(extra="allow")
+                x: int
+                y: float = 2.0
+
+            mm = MyModel(x=1, **{"priority": 0})
+            ref = {"x": 0, "y": 2.0, "priority": 0}
+
+            # Act
+            output = fancy_model_dump(mm, reference=ref)
+
+            # Assert
+            assert output == {"x": 1, "y": 2.0, "priority": 0}
+
+        def test_nested_extra_field(self):
+            # Arrange
+            class MyInner(BaseModel):
+                model_config = ConfigDict(extra="allow")
+                id: str | None = None
+
+            class MyOuter(BaseModel):
+                model_config = ConfigDict(extra="allow")
+                items: list[MyInner]
+
+            inner = MyInner(id="test", **{"priority": 0})
+            outer = MyOuter(items=[inner], **{"minimum_prek_version": "0.2.23"})
+
+            # Act
+            output = fancy_model_dump(outer)
+
+            # Assert
+            assert output == {
+                "items": [{"id": "test", "priority": 0}],
+                "minimum_prek_version": "0.2.23",
+            }