Support empty .pre-commit-config.yaml files (#906)

nathanjmcdougall · web-flow · commit 73770d2f2a38 · 2025-07-25T15:35:08.000+12:00
* Support empty `.pre-commit-config.yaml files`

* Don't modify empty files if unchanged
diff --git a/src/usethis/_integrations/file/yaml/io_.py b/src/usethis/_integrations/file/yaml/io_.py
@@ -474,7 +474,12 @@ def edit_yaml(
         except YAMLError as err:
             msg = f"Error reading '{yaml_path}':\n{err}"
             raise YAMLDecodeError(msg) from None
+
+    start_empty = not yaml_document.content
     yield yaml_document
+    if start_empty and not yaml_document.content:
+        # No change
+        return
     yaml_document.roundtripper.dump(yaml_document.content, stream=yaml_path)
 
 
diff --git a/src/usethis/_integrations/pre_commit/io_.py b/src/usethis/_integrations/pre_commit/io_.py
@@ -5,6 +5,7 @@
 from typing import TYPE_CHECKING
 
 from pydantic import ValidationError
+from ruamel.yaml.comments import CommentedMap
 
 from usethis._config import usethis_config
 from usethis._console import tick_print
@@ -15,8 +16,6 @@
 if TYPE_CHECKING:
     from collections.abc import Generator
 
-    from ruamel.yaml.comments import CommentedMap
-
     from usethis._integrations.file.yaml.io_ import YAMLLiteral
 
 
@@ -62,6 +61,9 @@ def edit_pre_commit_config_yaml() -> Generator[PreCommitConfigYAMLDocument, None
 
 
 def _validate_config(ruamel_content: YAMLLiteral) -> JsonSchemaForPreCommitConfigYaml:
+    if isinstance(ruamel_content, CommentedMap) and not ruamel_content:
+        ruamel_content = CommentedMap({"repos": []})
+
     try:
         return JsonSchemaForPreCommitConfigYaml.model_validate(ruamel_content)
     except ValidationError as err:
diff --git a/tests/usethis/_integrations/pre_commit/test_pre_commit_io_.py b/tests/usethis/_integrations/pre_commit/test_pre_commit_io_.py
@@ -1,11 +1,6 @@
 from pathlib import Path
 
-import pytest
-
-from usethis._integrations.pre_commit.io_ import (
-    PreCommitConfigYAMLConfigError,
-    edit_pre_commit_config_yaml,
-)
+from usethis._integrations.pre_commit.io_ import edit_pre_commit_config_yaml
 from usethis._test import change_cwd
 
 
@@ -35,14 +30,27 @@ def test_unchanged(self, tmp_path: Path):
         # Assert
         assert (tmp_path / ".pre-commit-config.yaml").read_text() == content_str
 
-    def test_empty_is_invalid(self, tmp_path: Path):
+    def test_start_with_empty_file(self, tmp_path: Path):
         # Arrange
         (tmp_path / ".pre-commit-config.yaml").write_text("")
 
-        # Act, Assert
+        # Act
         with (
             change_cwd(tmp_path),
-            pytest.raises(PreCommitConfigYAMLConfigError),
-            edit_pre_commit_config_yaml(),
+            edit_pre_commit_config_yaml() as doc,
         ):
+            doc.content["repos"] = []
+
+        # Assert
+        assert (tmp_path / ".pre-commit-config.yaml").read_text() == "repos: []\n"
+
+    def test_empty_valid_but_unchanged(self, tmp_path: Path):
+        # Arrange
+        (tmp_path / ".pre-commit-config.yaml").write_text("")
+
+        # Act
+        with change_cwd(tmp_path), edit_pre_commit_config_yaml():
             pass
+
+        # Assert
+        assert (tmp_path / ".pre-commit-config.yaml").read_text() == ""
