Skip unused formatter construction

src/datamodel_code_generator/parser/base.py

@@ -3938,7 +3938,7 @@ def _reset_local_source_cache(self) -> None:
         self._cache_local_sources = False
         self._local_source_cache = None
 
-    def parse(  # noqa: PLR0913, PLR0914, PLR0917
+    def parse(  # noqa: PLR0912, PLR0913, PLR0914, PLR0917
         self,
         with_import: bool | None = True,  # noqa: FBT001, FBT002
         format_: bool | None = True,  # noqa: FBT001, FBT002
@@ -3980,12 +3980,20 @@ def parse(  # noqa: PLR0913, PLR0914, PLR0917
         ) = self._build_module_structure(sorted_data_models, require_update_action_models, module_split_mode)
 
         if format_:
-            config = config._replace(
-                code_formatter=self._build_code_formatter(
-                    settings_path,
-                    is_multi_module_output=self.defer_formatting or len(module_models) > 1,
-                )
-            )
+            match self.formatters:
+                case [] if (
+                    not self.custom_formatter
+                    and "_build_code_formatter" not in self.__dict__
+                    and type(self)._build_code_formatter is Parser._build_code_formatter  # noqa: SLF001
+                ):
+                    pass
+                case _:
+                    config = config._replace(
+                        code_formatter=self._build_code_formatter(
+                            settings_path,
+                            is_multi_module_output=self.defer_formatting or len(module_models) > 1,
+                        ),
+                    )
 
         results: dict[ModulePath, Result] = {}
         unused_models: list[DataModel] = []

tests/data/expected/main/cli_fast_paths/empty_formatters.txt

@@ -0,0 +1,4 @@
+{
+  "generated": "# generated by datamodel-codegen:\n#   filename:  person.json\n\nfrom __future__ import annotations\nfrom pydantic import BaseModel, Field, conint\nfrom typing import Any\n\n\nclass Person(BaseModel):\n    firstName: str | None = Field(None, description=\"The person's first name.\")\n    lastName: str | None = Field(None, description=\"The person's last name.\")\n    age: conint(ge=0) | None = Field(None, description='Age in years which must be equal to or greater than zero.')\n    friends: list[Any] | None = None\n    comment: None = Field(None)",
+  "imported_format": false
+}

tests/data/expected/main/generate_with_custom_formatter_and_empty_formatters.py

@@ -0,0 +1,15 @@
+# generated by datamodel-codegen:
+#   filename:  person.json
+
+# a comment
+from __future__ import annotations
+from pydantic import BaseModel, Field, conint
+from typing import Any
+
+
+class Person(BaseModel):
+    firstName: str | None = Field(None, description="The person's first name.")
+    lastName: str | None = Field(None, description="The person's last name.")
+    age: conint(ge=0) | None = Field(None, description='Age in years which must be equal to or greater than zero.')
+    friends: list[Any] | None = None
+    comment: None = Field(None)

tests/data/expected/main/generate_with_empty_formatters.py

@@ -0,0 +1,14 @@
+# generated by datamodel-codegen:
+#   filename:  person.json
+
+from __future__ import annotations
+from pydantic import BaseModel, Field, conint
+from typing import Any
+
+
+class Person(BaseModel):
+    firstName: str | None = Field(None, description="The person's first name.")
+    lastName: str | None = Field(None, description="The person's last name.")
+    age: conint(ge=0) | None = Field(None, description='Age in years which must be equal to or greater than zero.')
+    friends: list[Any] | None = None
+    comment: None = Field(None)

tests/data/expected/main/parser_formatter_builder_override_with_empty_formatters.py

@@ -0,0 +1,12 @@
+# a comment
+from __future__ import annotations
+from typing import Any, List, Optional
+from pydantic import BaseModel, Field, conint
+
+
+class Person(BaseModel):
+    firstName: Optional[str] = Field(None, description="The person's first name.")
+    lastName: Optional[str] = Field(None, description="The person's last name.")
+    age: Optional[conint(ge=0)] = Field(None, description='Age in years which must be equal to or greater than zero.')
+    friends: Optional[List[Any]] = None
+    comment: None = Field(None)

tests/data/expected/main/parser_formatter_builder_override_with_empty_formatters_calls.txt

@@ -0,0 +1 @@
+1

tests/main/test_cli_fast_paths.py

@@ -16,6 +16,8 @@
 
 import pytest
 
+from tests.conftest import assert_output
+
 ROOT = Path(__file__).parents[2]
 SRC = ROOT / "src"
 MISSING = object()
@@ -170,6 +172,31 @@ def _run_main_import_probe() -> dict[str, Any]:
     )
 
 
+def _run_no_formatter_generation_probe() -> dict[str, Any]:
+    return _run_probe(
+        textwrap.dedent(
+            """
+            import json
+            import sys
+            from pathlib import Path
+
+            from datamodel_code_generator import InputFileType, generate
+
+            generated = generate(
+                Path("tests/data/jsonschema/person.json"),
+                input_file_type=InputFileType.JsonSchema,
+                disable_timestamp=True,
+                formatters=[],
+            )
+            print(json.dumps({
+                "generated": generated,
+                "imported_format": "datamodel_code_generator.format" in sys.modules,
+            }, indent=2, sort_keys=True))
+            """
+        )
+    )
+
+
 def _run_invalid_args_probe() -> dict[str, Any]:
     return _run_probe(
         textwrap.dedent(
@@ -371,6 +398,16 @@ def test_main_import_skips_formatter_runtime() -> None:
     assert imported["imported_validators"] is False
 
 
+def test_empty_formatters_skip_formatter_runtime() -> None:
+    """Explicit empty formatters keep the formatter runtime out of a fresh process."""
+    result = _run_no_formatter_generation_probe()
+
+    assert_output(
+        f"{json.dumps(result, indent=2, sort_keys=True)}\n",
+        ROOT / "tests/data/expected/main/cli_fast_paths/empty_formatters.txt",
+    )
+
+
 @pytest.mark.allow_direct_assert
 def test_invalid_args_skip_pydantic_import() -> None:
     """Argparse errors exit before loading CLI Config or Pydantic."""

tests/main/test_main_general.py

@@ -55,6 +55,7 @@
     assert_generated_modules_output,
     assert_httpx_get_kwargs,
     assert_no_uncommented_generated_code,
+    assert_output,
     assert_runtime_import_package,
     assert_warnings_contain,
     assert_warnings_do_not_contain,
@@ -2944,6 +2945,108 @@ def test_generate_accepts_path_input(output_file: Path) -> None:
     )
 
 
+@pytest.mark.parametrize("custom_formatters", [None, []], ids=["custom-unset", "custom-empty"])
+def test_generate_with_empty_formatters(output_file: Path, custom_formatters: list[str] | None) -> None:
+    """Skip formatter work when the explicit formatter list is empty."""
+    run_generate_file_and_assert(
+        input_path=JSON_SCHEMA_DATA_PATH / "person.json",
+        output_path=output_file,
+        input_file_type=InputFileType.JsonSchema,
+        disable_timestamp=True,
+        formatters=[],
+        custom_formatters=custom_formatters,
+        assert_func=assert_file_content,
+        expected_file="generate_with_empty_formatters.py",
+    )
+
+
+def test_generate_with_custom_formatter_and_empty_formatters(output_file: Path) -> None:
+    """Keep custom formatting when the built-in formatter list is empty."""
+    run_generate_file_and_assert(
+        input_path=JSON_SCHEMA_DATA_PATH / "person.json",
+        output_path=output_file,
+        input_file_type=InputFileType.JsonSchema,
+        disable_timestamp=True,
+        formatters=[],
+        custom_formatters=["tests.data.python.custom_formatters.add_comment"],
+        assert_func=assert_file_content,
+        expected_file="generate_with_custom_formatter_and_empty_formatters.py",
+    )
+
+
+def test_parser_formatter_builder_override_with_empty_formatters() -> None:
+    """Keep subclass formatter-builder hooks active with an empty formatter list."""
+    from datamodel_code_generator.parser.jsonschema import JsonSchemaParser
+    from tests.data.python.custom_formatters.add_comment import CodeFormatter as AddCommentFormatter
+
+    class ConfiguringJsonSchemaParser(JsonSchemaParser):
+        code_formatter_build_count = 0
+
+        def _build_code_formatter(
+            self,
+            settings_path: Path | None,
+            *,
+            is_multi_module_output: bool,
+        ) -> CodeFormatter:
+            code_formatter = super()._build_code_formatter(
+                settings_path,
+                is_multi_module_output=is_multi_module_output,
+            )
+            code_formatter.custom_formatters.append(AddCommentFormatter(formatter_kwargs={}))
+            self.code_formatter_build_count += 1
+            return code_formatter
+
+    parser = ConfiguringJsonSchemaParser(
+        source=(JSON_SCHEMA_DATA_PATH / "person.json").resolve(),
+        formatters=[],
+    )
+    assert_output(
+        f"{parser.parse()}\n",
+        EXPECTED_MAIN_PATH / "parser_formatter_builder_override_with_empty_formatters.py",
+    )
+    assert_output(
+        f"{parser.code_formatter_build_count}\n",
+        EXPECTED_MAIN_PATH / "parser_formatter_builder_override_with_empty_formatters_calls.txt",
+    )
+
+
+def test_parser_instance_formatter_builder_with_empty_formatters() -> None:
+    """Keep an instance-injected formatter builder active with an empty formatter list."""
+    from datamodel_code_generator.parser.jsonschema import JsonSchemaParser
+    from tests.data.python.custom_formatters.add_comment import CodeFormatter as AddCommentFormatter
+
+    parser = JsonSchemaParser(
+        source=(JSON_SCHEMA_DATA_PATH / "person.json").resolve(),
+        formatters=[],
+    )
+    default_builder = parser._build_code_formatter
+    code_formatter_build_count = 0
+
+    def build_code_formatter(
+        settings_path: Path | None,
+        *,
+        is_multi_module_output: bool,
+    ) -> CodeFormatter:
+        nonlocal code_formatter_build_count
+        code_formatter = default_builder(
+            settings_path,
+            is_multi_module_output=is_multi_module_output,
+        )
+        code_formatter.custom_formatters.append(AddCommentFormatter(formatter_kwargs={}))
+        code_formatter_build_count += 1
+        return code_formatter
+
+    parser._build_code_formatter = build_code_formatter  # type: ignore[method-assign]
+    assert_output(
+        f"{parser.parse()}\n",
+        EXPECTED_MAIN_PATH / "parser_formatter_builder_override_with_empty_formatters.py",
+    )
+    assert_output(
+        f"{code_formatter_build_count}\n",
+        EXPECTED_MAIN_PATH / "parser_formatter_builder_override_with_empty_formatters_calls.txt",
+    )
+
+
 def test_generate_keeps_existing_path_string_input() -> None:
     """Test generate() keeps existing path strings as inline source text."""
     run_generate_and_assert(