Fix auto CSV fallback

docs/llms-full.txt

@@ -29212,7 +29212,7 @@ The tables below are generated from the input type enum, parser routing code, sc
 | MCP tool schemas | `mcp-tools` | `JsonSchemaParser after MCP conversion` | Converted to JSON Schema; `draft-04`, `draft-06`, `draft-07`, `2019-09`, `2020-12`, `auto` | Explicit only |
 | JSON data | `json` | `JsonSchemaParser after genson conversion` | Converted to JSON Schema; `draft-04`, `draft-06`, `draft-07`, `2019-09`, `2020-12`, `auto` | Mapping that is not a schema/OpenAPI/AsyncAPI/Avro document |
 | YAML data | `yaml` | `JsonSchemaParser after genson conversion` | Converted to JSON Schema; `draft-04`, `draft-06`, `draft-07`, `2019-09`, `2020-12`, `auto` | Explicit for YAML sample data |
-| CSV data | `csv` | `JsonSchemaParser after genson conversion` | Converted to JSON Schema; `draft-04`, `draft-06`, `draft-07`, `2019-09`, `2020-12`, `auto` | Fallback when text cannot parse as YAML |
+| CSV data | `csv` | `JsonSchemaParser after genson conversion` | Converted to JSON Schema; `draft-04`, `draft-06`, `draft-07`, `2019-09`, `2020-12`, `auto` | Detected from CSV-like text with consistent comma counts across non-empty lines or explicit `csv` |
 | Python dictionary data | `dict` | `JsonSchemaParser after genson conversion` | Converted to JSON Schema; `draft-04`, `draft-06`, `draft-07`, `2019-09`, `2020-12`, `auto` | Explicit for mapping input |
 | Python input model | `--input-model` | `JsonSchemaParser` after Python schema conversion | JSON Schema after conversion; dict input can select another explicit schema type | Explicit only |
 
@@ -29542,7 +29542,7 @@ The tables below are generated from the input type enum, parser routing code, sc
 | MCP tool schemas | `mcp-tools` | `JsonSchemaParser after MCP conversion` | Converted to JSON Schema; `draft-04`, `draft-06`, `draft-07`, `2019-09`, `2020-12`, `auto` | Explicit only |
 | JSON data | `json` | `JsonSchemaParser after genson conversion` | Converted to JSON Schema; `draft-04`, `draft-06`, `draft-07`, `2019-09`, `2020-12`, `auto` | Mapping that is not a schema/OpenAPI/AsyncAPI/Avro document |
 | YAML data | `yaml` | `JsonSchemaParser after genson conversion` | Converted to JSON Schema; `draft-04`, `draft-06`, `draft-07`, `2019-09`, `2020-12`, `auto` | Explicit for YAML sample data |
-| CSV data | `csv` | `JsonSchemaParser after genson conversion` | Converted to JSON Schema; `draft-04`, `draft-06`, `draft-07`, `2019-09`, `2020-12`, `auto` | Fallback when text cannot parse as YAML |
+| CSV data | `csv` | `JsonSchemaParser after genson conversion` | Converted to JSON Schema; `draft-04`, `draft-06`, `draft-07`, `2019-09`, `2020-12`, `auto` | Detected from CSV-like text with consistent comma counts across non-empty lines or explicit `csv` |
 | Python dictionary data | `dict` | `JsonSchemaParser after genson conversion` | Converted to JSON Schema; `draft-04`, `draft-06`, `draft-07`, `2019-09`, `2020-12`, `auto` | Explicit for mapping input |
 | Python input model | `--input-model` | `JsonSchemaParser` after Python schema conversion | JSON Schema after conversion; dict input can select another explicit schema type | Explicit only |
 

docs/supported-data-types.md

@@ -40,7 +40,7 @@ The tables below are generated from the input type enum, parser routing code, sc
 | MCP tool schemas | `mcp-tools` | `JsonSchemaParser after MCP conversion` | Converted to JSON Schema; `draft-04`, `draft-06`, `draft-07`, `2019-09`, `2020-12`, `auto` | Explicit only |
 | JSON data | `json` | `JsonSchemaParser after genson conversion` | Converted to JSON Schema; `draft-04`, `draft-06`, `draft-07`, `2019-09`, `2020-12`, `auto` | Mapping that is not a schema/OpenAPI/AsyncAPI/Avro document |
 | YAML data | `yaml` | `JsonSchemaParser after genson conversion` | Converted to JSON Schema; `draft-04`, `draft-06`, `draft-07`, `2019-09`, `2020-12`, `auto` | Explicit for YAML sample data |
-| CSV data | `csv` | `JsonSchemaParser after genson conversion` | Converted to JSON Schema; `draft-04`, `draft-06`, `draft-07`, `2019-09`, `2020-12`, `auto` | Fallback when text cannot parse as YAML |
+| CSV data | `csv` | `JsonSchemaParser after genson conversion` | Converted to JSON Schema; `draft-04`, `draft-06`, `draft-07`, `2019-09`, `2020-12`, `auto` | Detected from CSV-like text with consistent comma counts across non-empty lines or explicit `csv` |
 | Python dictionary data | `dict` | `JsonSchemaParser after genson conversion` | Converted to JSON Schema; `draft-04`, `draft-06`, `draft-07`, `2019-09`, `2020-12`, `auto` | Explicit for mapping input |
 | Python input model | `--input-model` | `JsonSchemaParser` after Python schema conversion | JSON Schema after conversion; dict input can select another explicit schema type | Explicit only |
 

docs/supported_formats.md

@@ -27,7 +27,7 @@ The tables below are generated from the input type enum, parser routing code, sc
 | MCP tool schemas | `mcp-tools` | `JsonSchemaParser after MCP conversion` | Converted to JSON Schema; `draft-04`, `draft-06`, `draft-07`, `2019-09`, `2020-12`, `auto` | Explicit only |
 | JSON data | `json` | `JsonSchemaParser after genson conversion` | Converted to JSON Schema; `draft-04`, `draft-06`, `draft-07`, `2019-09`, `2020-12`, `auto` | Mapping that is not a schema/OpenAPI/AsyncAPI/Avro document |
 | YAML data | `yaml` | `JsonSchemaParser after genson conversion` | Converted to JSON Schema; `draft-04`, `draft-06`, `draft-07`, `2019-09`, `2020-12`, `auto` | Explicit for YAML sample data |
-| CSV data | `csv` | `JsonSchemaParser after genson conversion` | Converted to JSON Schema; `draft-04`, `draft-06`, `draft-07`, `2019-09`, `2020-12`, `auto` | Fallback when text cannot parse as YAML |
+| CSV data | `csv` | `JsonSchemaParser after genson conversion` | Converted to JSON Schema; `draft-04`, `draft-06`, `draft-07`, `2019-09`, `2020-12`, `auto` | Detected from CSV-like text with consistent comma counts across non-empty lines or explicit `csv` |
 | Python dictionary data | `dict` | `JsonSchemaParser after genson conversion` | Converted to JSON Schema; `draft-04`, `draft-06`, `draft-07`, `2019-09`, `2020-12`, `auto` | Explicit for mapping input |
 | Python input model | `--input-model` | `JsonSchemaParser` after Python schema conversion | JSON Schema after conversion; dict input can select another explicit schema type | Explicit only |
 

scripts/build_schema_docs.py

@@ -154,7 +154,9 @@
     InputFileType.Json: "Mapping that is not a schema/OpenAPI/AsyncAPI/Avro document",
     InputFileType.Yaml: "Explicit for YAML sample data",
     InputFileType.Dict: "Explicit for mapping input",
-    InputFileType.CSV: "Fallback when text cannot parse as YAML",
+    InputFileType.CSV: (
+        "Detected from CSV-like text with consistent comma counts across non-empty lines or explicit `csv`"
+    ),
     InputFileType.GraphQL: "Explicit only",
 }
 

src/datamodel_code_generator/__init__.py

@@ -1524,8 +1524,11 @@ def infer_input_type(text: str) -> InputFileType:  # noqa: PLR0911, PLR0912
 
     try:
         data = load_yaml(text)
-    except get_yaml_parse_errors():
-        return InputFileType.CSV
+    except get_yaml_parse_errors() as exc:
+        if not _is_json_text(text) and _looks_like_csv_text(text):
+            return InputFileType.CSV
+        msg = _infer_input_type_error_message(parse_error=exc)
+        raise Error(msg) from exc
     if isinstance(data, dict):
         if is_asyncapi(data):
             return InputFileType.AsyncAPI
@@ -1546,17 +1549,44 @@ def infer_input_type(text: str) -> InputFileType:  # noqa: PLR0911, PLR0912
         if is_avro_schema_data(data):
             return InputFileType.Avro
     if isinstance(data, str):
+        if _looks_like_csv_text(text):
+            return InputFileType.CSV
         from datamodel_code_generator.parser._avro_detection import is_avro_schema_data  # noqa: PLC0415
 
         if is_avro_schema_data(data):
             return InputFileType.Avro
-    msg = (
-        "Can't infer input file type from the input data. "
-        "Please specify the input file type explicitly with --input-file-type option."
-    )
+    msg = _infer_input_type_error_message()
     raise Error(msg)
 
 
+def _infer_input_type_error_message(*, parse_error: Exception | None = None) -> str:
+    message = "Can't infer input file type from the input data."
+    hint = "Please specify the input file type explicitly with --input-file-type option."
+    if parse_error is None:
+        return f"{message} {hint}"
+    return f"{message} YAML parser error: {type(parse_error).__name__}: {parse_error}. {hint}"
+
+
+_MIN_CSV_NON_EMPTY_LINES = 2
+
+
+def _looks_like_csv_text(text: str) -> bool:
+    comma_count: int | None = None
+    matched_lines = 0
+    for raw_line in text.splitlines():
+        if not (line := raw_line.strip()):
+            continue
+        if (current_comma_count := line.count(",")) == 0:
+            return False
+        match comma_count:
+            case None:
+                comma_count = current_comma_count
+            case _ if current_comma_count != comma_count:
+                return False
+        matched_lines += 1
+    return matched_lines >= _MIN_CSV_NON_EMPTY_LINES
+
+
 inferred_message = (
     "The input file type was determined to be: {}\nThis can be specified explicitly with the "
     "`--input-file-type` option."

tests/data/expected/schema_docs/input_format_guide.txt

@@ -17,7 +17,7 @@ The tables below are generated from the input type enum, parser routing code, sc
 | MCP tool schemas | `mcp-tools` | `JsonSchemaParser after MCP conversion` | Converted to JSON Schema; `draft-04`, `draft-06`, `draft-07`, `2019-09`, `2020-12`, `auto` | Explicit only |
 | JSON data | `json` | `JsonSchemaParser after genson conversion` | Converted to JSON Schema; `draft-04`, `draft-06`, `draft-07`, `2019-09`, `2020-12`, `auto` | Mapping that is not a schema/OpenAPI/AsyncAPI/Avro document |
 | YAML data | `yaml` | `JsonSchemaParser after genson conversion` | Converted to JSON Schema; `draft-04`, `draft-06`, `draft-07`, `2019-09`, `2020-12`, `auto` | Explicit for YAML sample data |
-| CSV data | `csv` | `JsonSchemaParser after genson conversion` | Converted to JSON Schema; `draft-04`, `draft-06`, `draft-07`, `2019-09`, `2020-12`, `auto` | Fallback when text cannot parse as YAML |
+| CSV data | `csv` | `JsonSchemaParser after genson conversion` | Converted to JSON Schema; `draft-04`, `draft-06`, `draft-07`, `2019-09`, `2020-12`, `auto` | Detected from CSV-like text with consistent comma counts across non-empty lines or explicit `csv` |
 | Python dictionary data | `dict` | `JsonSchemaParser after genson conversion` | Converted to JSON Schema; `draft-04`, `draft-06`, `draft-07`, `2019-09`, `2020-12`, `auto` | Explicit for mapping input |
 | Python input model | `--input-model` | `JsonSchemaParser` after Python schema conversion | JSON Schema after conversion; dict input can select another explicit schema type | Explicit only |
 

tests/data/yaml/broken.yaml

@@ -0,0 +1,2 @@
+root:
+  child: [

tests/main/test_main_csv.py

@@ -12,6 +12,8 @@
 from tests.main.conftest import (
     CSV_DATA_PATH,
     EXPECTED_CSV_PATH,
+    JSON_DATA_PATH,
+    YAML_DATA_PATH,
     run_main_and_assert,
 )
 
@@ -33,6 +35,41 @@ def test_csv_file(output_file: Path) -> None:
     )
 
 
+def test_csv_file_auto_detection(output_file: Path, capsys: pytest.CaptureFixture[str]) -> None:
+    """Test CSV file input is detected by auto input type."""
+    run_main_and_assert(
+        input_path=CSV_DATA_PATH / "simple.csv",
+        output_path=output_file,
+        assert_func=assert_file_content,
+        expected_file="csv_file_simple.py",
+        capsys=capsys,
+        expected_stderr_contains="The input file type was determined to be: csv",
+    )
+
+
+@pytest.mark.parametrize(
+    "input_path",
+    [
+        JSON_DATA_PATH / "broken.json",
+        YAML_DATA_PATH / "broken.yaml",
+    ],
+)
+def test_auto_detection_rejects_malformed_non_csv_input(
+    input_path: Path,
+    output_file: Path,
+    capsys: pytest.CaptureFixture[str],
+) -> None:
+    """Test auto input type does not treat malformed JSON/YAML as CSV data."""
+    run_main_and_assert(
+        input_path=input_path,
+        output_path=output_file,
+        expected_exit=Exit.ERROR,
+        capsys=capsys,
+        expected_stderr_contains="Please specify the input file type explicitly with --input-file-type option.",
+        output_should_not_exist=True,
+    )
+
+
 def test_csv_stdin(monkeypatch: pytest.MonkeyPatch, output_file: Path) -> None:
     """Test CSV stdin input code generation."""
     run_main_and_assert(

tests/test_infer_input_type.py

@@ -10,7 +10,14 @@
 
 import pytest
 
-from datamodel_code_generator import Error, InputFileType, _is_json_text, _is_xml_text, infer_input_type
+from datamodel_code_generator import (
+    Error,
+    InputFileType,
+    _is_json_text,
+    _is_xml_text,
+    _looks_like_csv_text,
+    infer_input_type,
+)
 
 DATA_PATH: Path = Path(__file__).parent / "data"
 HEAVY_INFERENCE_MODULES = (
@@ -166,6 +173,56 @@ def test_is_json_text() -> None:
     assert not _is_json_text("name: value")
 
 
+@pytest.mark.parametrize(
+    ("text", "expected"),
+    [
+        ("id,name\n1,taro\n", True),
+        ("\n id, name \n 1, taro \n", True),
+        ("id,name\n", False),
+        ("name\nvalue\n", False),
+        ("id,name,tel\n1,taro\n", False),
+        ("id,name\n1,taro\n2,jiro,extra\n", False),
+        ("", False),
+    ],
+)
+def test_looks_like_csv_text(text: str, expected: bool) -> None:
+    """Test bounded CSV text detection for auto inference fallback."""
+    assert _looks_like_csv_text(text) is expected
+
+
+@pytest.mark.parametrize(
+    ("text", "expected"),
+    [
+        ("id,name\n1,taro\n", InputFileType.CSV),
+        ("\n id, name \n 1, taro \n", InputFileType.CSV),
+    ],
+)
+def test_infer_input_type_detects_csv_text(text: str, expected: InputFileType) -> None:
+    """Test auto inference detects CSV data without treating all YAML errors as CSV."""
+    assert infer_input_type(text) == expected
+
+
+@pytest.mark.parametrize(
+    "text",
+    [
+        '{"Pet": {',
+        "root:\n  child: [",
+        "id,name,tel\n1,taro\n",
+        "id,name\n1,taro\n2,jiro,extra\n",
+    ],
+)
+def test_infer_input_type_rejects_non_csv_parse_errors(text: str) -> None:
+    """Test malformed JSON/YAML and uneven comma data do not infer as CSV."""
+    with pytest.raises(
+        Error,
+        match=(
+            r"(?s)Can't infer input file type from the input data\..*"
+            r"Please specify the input file type explicitly with --input-file-type option\."
+        ),
+    ):
+        infer_input_type(text)
+
+
 def test_infer_input_type_non_schema_xml() -> None:
     """Test non-schema XML does not infer as XML Schema."""
     with pytest.raises(

tests/test_yaml_backend.py

@@ -11,7 +11,7 @@
 import pytest
 import yaml
 
-from datamodel_code_generator import InputFileType, infer_input_type, load_yaml, load_yaml_dict_from_path
+from datamodel_code_generator import Error, InputFileType, infer_input_type, load_yaml, load_yaml_dict_from_path
 from datamodel_code_generator.util import (
     _is_yaml_deprecated_bool_warning_enabled,
     get_yaml_backend,
@@ -210,19 +210,31 @@ class TestInferInputType:
     def test_csv_with_pyyaml_error(self) -> None:
         """YAML parse error from PyYAML returns CSV type."""
         with patch.dict("sys.modules", {"ryaml": None}):
-            result = infer_input_type("a,b,c\n1,2,3\n::")
+            result = infer_input_type("a,b:\n1,2\n")
             assert result == InputFileType.CSV
 
     def test_csv_with_ryaml_error(self) -> None:
-        """YAML parse error from ryaml returns CSV type."""
+        """CSV-looking YAML parse error from ryaml returns CSV type."""
         mock_invalid_yaml_error = type("InvalidYamlError", (Exception,), {})
         mock_ryaml = MagicMock()
         mock_ryaml.InvalidYamlError = mock_invalid_yaml_error
         mock_ryaml.loads.side_effect = mock_invalid_yaml_error("parse error")
         with patch.dict("sys.modules", {"ryaml": mock_ryaml}):
-            result = infer_input_type(":::invalid yaml:::")
+            result = infer_input_type("a,b,c\n1,2,3\n")
             assert result == InputFileType.CSV
 
+    def test_non_csv_ryaml_error_raises_error(self) -> None:
+        """Non-CSV YAML parse error from ryaml raises inference error."""
+        mock_invalid_yaml_error = type("InvalidYamlError", (Exception,), {})
+        mock_ryaml = MagicMock()
+        mock_ryaml.InvalidYamlError = mock_invalid_yaml_error
+        mock_ryaml.loads.side_effect = mock_invalid_yaml_error("parse error")
+        with (
+            patch.dict("sys.modules", {"ryaml": mock_ryaml}),
+            pytest.raises(Error, match=r"YAML parser error: InvalidYamlError: parse error"),
+        ):
+            infer_input_type(":::invalid yaml:::")
+
     def test_openapi_detection(self) -> None:
         """OpenAPI input is detected correctly regardless of backend."""
         with patch.dict("sys.modules", {"ryaml": None}):