Add support for CLI kebab case flag. (#489)

docs/index.md

@@ -957,17 +957,13 @@ assert cmd.model_dump() == {
 For `BaseModel` and `pydantic.dataclasses.dataclass` types, `CliApp.run` will internally use the following
 `BaseSettings` configuration defaults:
 
-* `alias_generator=AliasGenerator(lambda s: s.replace('_', '-'))`
 * `nested_model_default_partial_update=True`
 * `case_sensitive=True`
 * `cli_hide_none_type=True`
 * `cli_avoid_json=True`
 * `cli_enforce_required=True`
 * `cli_implicit_flags=True`
-
-!!! note
-    The alias generator for kebab case does not propagate to subcommands or submodels and will have to be manually set
-    in these cases.
+* `cli_kebab_case=True`
 
 ### Mutually Exclusive Groups
 
@@ -1131,6 +1127,37 @@ print(Settings().model_dump())
 #> {'good_arg': 'hello world'}
 ```
 
+#### CLI Kebab Case for Arguments
+
+Change whether CLI arguments should use kebab case by enabling `cli_kebab_case`.
+
+```py
+import sys
+
+from pydantic import Field
+
+from pydantic_settings import BaseSettings
+
+
+class Settings(BaseSettings, cli_parse_args=True, cli_kebab_case=True):
+    my_option: str = Field(description='will show as kebab case on CLI')
+
+
+try:
+    sys.argv = ['example.py', '--help']
+    Settings()
+except SystemExit as e:
+    print(e)
+    #> 0
+"""
+usage: example.py [-h] [--my-option str]
+
+options:
+  -h, --help       show this help message and exit
+  --my-option str  will show as kebab case on CLI (required)
+"""
+```
+
 #### Change Whether CLI Should Exit on Error
 
 Change whether the CLI internal parser will exit on error or raise a `SettingsError` exception by using

pydantic_settings/main.py

@@ -4,7 +4,7 @@
 from types import SimpleNamespace
 from typing import Any, ClassVar, TypeVar
 
-from pydantic import AliasGenerator, ConfigDict
+from pydantic import ConfigDict
 from pydantic._internal._config import config_keys
 from pydantic._internal._signature import _field_name_for_signature
 from pydantic._internal._utils import deep_update, is_model_class
@@ -52,6 +52,7 @@ class SettingsConfigDict(ConfigDict, total=False):
     cli_flag_prefix_char: str
     cli_implicit_flags: bool | None
     cli_ignore_unknown_args: bool | None
+    cli_kebab_case: bool | None
     secrets_dir: PathType | None
     json_file: PathType | None
     json_file_encoding: str | None
@@ -133,6 +134,7 @@ class BaseSettings(BaseModel):
         _cli_implicit_flags: Whether `bool` fields should be implicitly converted into CLI boolean flags.
             (e.g. --flag, --no-flag). Defaults to `False`.
         _cli_ignore_unknown_args: Whether to ignore unknown CLI args and parse only known ones. Defaults to `False`.
+        _cli_kebab_case: CLI args use kebab case. Defaults to `False`.
         _secrets_dir: The secret files directory or a sequence of directories. Defaults to `None`.
     """
 
@@ -160,6 +162,7 @@ def __init__(
         _cli_flag_prefix_char: str | None = None,
         _cli_implicit_flags: bool | None = None,
         _cli_ignore_unknown_args: bool | None = None,
+        _cli_kebab_case: bool | None = None,
         _secrets_dir: PathType | None = None,
         **values: Any,
     ) -> None:
@@ -189,6 +192,7 @@ def __init__(
                 _cli_flag_prefix_char=_cli_flag_prefix_char,
                 _cli_implicit_flags=_cli_implicit_flags,
                 _cli_ignore_unknown_args=_cli_ignore_unknown_args,
+                _cli_kebab_case=_cli_kebab_case,
                 _secrets_dir=_secrets_dir,
             )
         )
@@ -242,6 +246,7 @@ def _settings_build_values(
         _cli_flag_prefix_char: str | None = None,
         _cli_implicit_flags: bool | None = None,
         _cli_ignore_unknown_args: bool | None = None,
+        _cli_kebab_case: bool | None = None,
         _secrets_dir: PathType | None = None,
     ) -> dict[str, Any]:
         # Determine settings config values
@@ -309,6 +314,7 @@ def _settings_build_values(
             if _cli_ignore_unknown_args is not None
             else self.model_config.get('cli_ignore_unknown_args')
         )
+        cli_kebab_case = _cli_kebab_case if _cli_kebab_case is not None else self.model_config.get('cli_kebab_case')
 
         secrets_dir = _secrets_dir if _secrets_dir is not None else self.model_config.get('secrets_dir')
 
@@ -371,6 +377,7 @@ def _settings_build_values(
                     cli_flag_prefix_char=cli_flag_prefix_char,
                     cli_implicit_flags=cli_implicit_flags,
                     cli_ignore_unknown_args=cli_ignore_unknown_args,
+                    cli_kebab_case=cli_kebab_case,
                     case_sensitive=case_sensitive,
                 )
                 sources = (cli_settings,) + sources
@@ -418,6 +425,7 @@ def _settings_build_values(
         cli_flag_prefix_char='-',
         cli_implicit_flags=False,
         cli_ignore_unknown_args=False,
+        cli_kebab_case=False,
         json_file=None,
         json_file_encoding=None,
         yaml_file=None,
@@ -497,13 +505,13 @@ def run(
 
             class CliAppBaseSettings(BaseSettings, model_cls):  # type: ignore
                 model_config = SettingsConfigDict(
-                    alias_generator=AliasGenerator(lambda s: s.replace('_', '-')),
                     nested_model_default_partial_update=True,
                     case_sensitive=True,
                     cli_hide_none_type=True,
                     cli_avoid_json=True,
                     cli_enforce_required=True,
                     cli_implicit_flags=True,
+                    cli_kebab_case=True,
                 )
 
             model = CliAppBaseSettings(**model_init_data)

pydantic_settings/sources.py

@@ -1063,6 +1063,7 @@ class CliSettingsSource(EnvSettingsSource, Generic[T]):
         cli_implicit_flags: Whether `bool` fields should be implicitly converted into CLI boolean flags.
             (e.g. --flag, --no-flag). Defaults to `False`.
         cli_ignore_unknown_args: Whether to ignore unknown CLI args and parse only known ones. Defaults to `False`.
+        cli_kebab_case: CLI args use kebab case. Defaults to `False`.
         case_sensitive: Whether CLI "--arg" names should be read with case-sensitivity. Defaults to `True`.
             Note: Case-insensitive matching is only supported on the internal root parser and does not apply to CLI
             subcommands.
@@ -1093,6 +1094,7 @@ def __init__(
         cli_flag_prefix_char: str | None = None,
         cli_implicit_flags: bool | None = None,
         cli_ignore_unknown_args: bool | None = None,
+        cli_kebab_case: bool | None = None,
         case_sensitive: bool | None = True,
         root_parser: Any = None,
         parse_args_method: Callable[..., Any] | None = None,
@@ -1152,6 +1154,9 @@ def __init__(
             if cli_ignore_unknown_args is not None
             else settings_cls.model_config.get('cli_ignore_unknown_args', False)
         )
+        self.cli_kebab_case = (
+            cli_kebab_case if cli_kebab_case is not None else settings_cls.model_config.get('cli_kebab_case', False)
+        )
 
         case_sensitive = case_sensitive if case_sensitive is not None else True
         if not case_sensitive and root_parser is not None:
@@ -1613,7 +1618,9 @@ def _add_parser_args(
             preferred_alias = alias_names[0]
             if _CliSubCommand in field_info.metadata:
                 for model in sub_models:
-                    subcommand_alias = model.__name__ if len(sub_models) > 1 else preferred_alias
+                    subcommand_alias = self._check_kebab_name(
+                        model.__name__ if len(sub_models) > 1 else preferred_alias
+                    )
                     subcommand_name = f'{arg_prefix}{subcommand_alias}'
                     subcommand_dest = f'{arg_prefix}{preferred_alias}'
                     self._cli_subcommands[f'{arg_prefix}:subcommand'][subcommand_name] = subcommand_dest
@@ -1677,17 +1684,17 @@ def _add_parser_args(
                     else f'{arg_prefix}{preferred_alias}'
                 )
 
-                if kwargs['dest'] in added_args:
+                arg_names = self._get_arg_names(arg_prefix, subcommand_prefix, alias_prefixes, alias_names, added_args)
+                if not arg_names or (kwargs['dest'] in added_args):
                     continue
 
                 if is_append_action:
                     kwargs['action'] = 'append'
                     if _annotation_contains_types(field_info.annotation, (dict, Mapping), is_strip_annotated=True):
                         self._cli_dict_args[kwargs['dest']] = field_info.annotation
 
-                arg_names = self._get_arg_names(arg_prefix, subcommand_prefix, alias_prefixes, alias_names)
                 if _CliPositionalArg in field_info.metadata:
-                    kwargs['metavar'] = preferred_alias.upper()
+                    kwargs['metavar'] = self._check_kebab_name(preferred_alias.upper())
                     arg_names = [kwargs['dest']]
                     del kwargs['dest']
                     del kwargs['required']
@@ -1726,6 +1733,11 @@ def _add_parser_args(
         self._add_parser_alias_paths(parser, alias_path_args, added_args, arg_prefix, subcommand_prefix, group)
         return parser
 
+    def _check_kebab_name(self, name: str) -> str:
+        if self.cli_kebab_case:
+            return name.replace('_', '-')
+        return name
+
     def _convert_bool_flag(self, kwargs: dict[str, Any], field_info: FieldInfo, model_default: Any) -> None:
         if kwargs['metavar'] == 'bool':
             default = None
@@ -1743,16 +1755,23 @@ def _convert_bool_flag(self, kwargs: dict[str, Any], field_info: FieldInfo, mode
                     )
 
     def _get_arg_names(
-        self, arg_prefix: str, subcommand_prefix: str, alias_prefixes: list[str], alias_names: tuple[str, ...]
+        self,
+        arg_prefix: str,
+        subcommand_prefix: str,
+        alias_prefixes: list[str],
+        alias_names: tuple[str, ...],
+        added_args: list[str],
     ) -> list[str]:
         arg_names: list[str] = []
         for prefix in [arg_prefix] + alias_prefixes:
             for name in alias_names:
-                arg_names.append(
+                arg_name = self._check_kebab_name(
                     f'{prefix}{name}'
                     if subcommand_prefix == self.env_prefix
                     else f'{prefix.replace(subcommand_prefix, "", 1)}{name}'
                 )
+                if arg_name not in added_args:
+                    arg_names.append(arg_name)
         return arg_names
 
     def _add_parser_submodels(

tests/test_source_cli.py

@@ -2292,3 +2292,86 @@ class WithUnion(BaseSettings):
         poly: Poly
 
     assert CliApp.run(WithUnion, ['--poly.type=a']).model_dump() == {'poly': {'a': 1, 'type': 'a'}}
+
+
+def test_cli_kebab_case(capsys, monkeypatch):
+    class DeepSubModel(BaseModel):
+        deep_pos_arg: CliPositionalArg[str]
+        deep_arg: str
+
+    class SubModel(BaseModel):
+        sub_subcmd: CliSubCommand[DeepSubModel]
+        sub_arg: str
+
+    class Root(BaseModel):
+        root_subcmd: CliSubCommand[SubModel]
+        root_arg: str
+
+    assert CliApp.run(
+        Root,
+        cli_args=[
+            '--root-arg=hi',
+            'root-subcmd',
+            '--sub-arg=hello',
+            'sub-subcmd',
+            'hey',
+            '--deep-arg=bye',
+        ],
+    ).model_dump() == {
+        'root_arg': 'hi',
+        'root_subcmd': {
+            'sub_arg': 'hello',
+            'sub_subcmd': {'deep_pos_arg': 'hey', 'deep_arg': 'bye'},
+        },
+    }
+
+    with monkeypatch.context() as m:
+        m.setattr(sys, 'argv', ['example.py', '--help'])
+        with pytest.raises(SystemExit):
+            CliApp.run(Root)
+        assert (
+            capsys.readouterr().out
+            == f"""usage: example.py [-h] --root-arg str {{root-subcmd}} ...
+
+{ARGPARSE_OPTIONS_TEXT}:
+  -h, --help      show this help message and exit
+  --root-arg str  (required)
+
+subcommands:
+  {{root-subcmd}}
+    root-subcmd
+"""
+        )
+
+        m.setattr(sys, 'argv', ['example.py', 'root-subcmd', '--help'])
+        with pytest.raises(SystemExit):
+            CliApp.run(Root)
+        assert (
+            capsys.readouterr().out
+            == f"""usage: example.py root-subcmd [-h] --sub-arg str {{sub-subcmd}} ...
+
+{ARGPARSE_OPTIONS_TEXT}:
+  -h, --help     show this help message and exit
+  --sub-arg str  (required)
+
+subcommands:
+  {{sub-subcmd}}
+    sub-subcmd
+"""
+        )
+
+        m.setattr(sys, 'argv', ['example.py', 'root-subcmd', 'sub-subcmd', '--help'])
+        with pytest.raises(SystemExit):
+            CliApp.run(Root)
+        assert (
+            capsys.readouterr().out
+            == f"""usage: example.py root-subcmd sub-subcmd [-h] --deep-arg str DEEP-POS-ARG
+
+positional arguments:
+  DEEP-POS-ARG
+
+{ARGPARSE_OPTIONS_TEXT}:
+  -h, --help      show this help message and exit
+  --deep-arg str  (required)
+"""
+        )