Feature request: better support for argumentless CLI boolean flags

[sersorrel]

Currently, you can declare a model like this:

class Settings(BaseSettings):
    dry_run: bool = False
    model_config = SettingsConfigDict(
        cli_parse_args=True,
        alias_generator=AliasGenerator(
            validation_alias=lambda s: AliasChoices(s, to_snake(s).replace("_", "-"))
        ),
    )

and end up with an application you can run in dry-run mode by running myapp --dry-run=true. But this is not exactly what is usually expected of CLI applications; usually, boolean flags are argumentless (--dry-run, --force, etc.) and their presence implies the associated value is true. Sometimes they have an inverse like --no-dry-run which implies the opposite.

I don't think pydantic-settings currently supports arguments like this. It would be very useful if it did :)

[hramezani]

Thanks @sersorrel for this feature request.

@kschwab Do you think we can support this in pydantic-settings?

[kschwab]

@sersorrel thanks for the request.

@hramezani, yes, we can support this. My thoughts are to add a cli_implicit_flags config that will convert optional bool fields into flags. We can then add CliExplicitFlag and CliImplicitFlag annotations to provide control at a more granular level. e.g.:

class Cfg(BaseSettings, cli_parse_args=True, cli_implicit_flags=True):
    req_flag: bool
    """--req_flag {True,False}"""

    implicit_flag: bool = False
    """--implicit_flag, --no-implicit_flag"""

    # Flags are implicit by default, so must override explicit flags with annotation
    explicit_flag: CliExplicitFlag[bool] = False
    """--explicit_flag {True,False}"""

class Cfg(BaseSettings, cli_parse_args=True, cli_implicit_flags=False):
    req_flag: bool
    """--req_flag {True,False}"""

    # Flags are explicit by default, so must override implicit flags with annotation
    implicit_flag: CliImplicitFlag[bool] = False
    """--implicit_flag, --no-implicit_flag"""

    explicit_flag: bool = False
    """--explicit_flag {True,False}"""

If the above seems reasonable, I'll move forward. The only question I have is what the default should be for cli_implicit_flags? I lean towards having it disabled by default (the current behavior) to keep it aligned with env vars, but am interested in your thoughts.

[hramezani]

Thanks @kschwab for the proposal.

If the above seems reasonable, I'll move forward. The only question I have is what the default should be for cli_implicit_flags? I lean towards having it disabled by default (the current behavior) to keep it aligned with env vars, but am interested in your thoughts.

Yes, LGTM. the default should be cli_implicit_flags=False because this won't introduce a breaking change.

I assigned the issue to you @kschwab. but feel free to unassign yourself if you don't want to continue working on this issue.

[kschwab]

Final implementation is slightly modified from above proposal. CLI implicit flags will also convert required fields for python >= 3.9:

class Cfg(BaseSettings, cli_parse_args=True, cli_implicit_flags=True):
    req_implicit_flag: bool
    """--req_implicit_flag, --no-req_implicit_flag (required)"""

    # Flags are implicit by default, so must override explicit flags with annotation
    req_explicit_flag: CliExplicitFlag[bool]
    """--req_explicit_flag bool (required)"""

    opt_implicit_flag: bool = False
    """--opt_implicit_flag, --no-opt_implicit_flag (default: False)"""

    # Flags are implicit by default, so must override explicit flags with annotation
    opt_explicit_flag: CliExplicitFlag[bool] = False
    """--opt_explicit_flag bool (default: False)"""

class Cfg(BaseSettings, cli_parse_args=True, cli_implicit_flags=False):
    # Flags are explicit by default, so must override implicit flags with annotation
    req_implicit_flag: CliImplicitFlag[bool]
    """--req_implicit_flag, --no-req_implicit_flag (required)"""

    req_explicit_flag: bool
    """--req_explicit_flag bool (required)"""

    # Flags are explicit by default, so must override implicit flags with annotation
    opt_implicit_flag: CliImplicitFlag[bool] = False
    """--opt_implicit_flag, --no-opt_implicit_flag (default: False)"""

    opt_explicit_flag: CliExplicitFlag[bool] = False
    """--opt_explicit_flag bool (default: False)"""

For python < 3.9, the --no-flag option will not be supported due to underlying argparse support, and implicit flags can only be applied to optional bool fields. Therefore, python 3.8 will look like original proposal:

class Cfg(BaseSettings, cli_parse_args=True, cli_implicit_flags=True):
    req_flag: bool
    """--req_flag bool (required)"""

    implicit_flag: bool = False
    """--implicit_flag (default: False)"""

    # Flags are implicit by default, so must override explicit flags with annotation
    explicit_flag: CliExplicitFlag[bool] = False
    """--explicit_flag bool (default: False)"""

class Cfg(BaseSettings, cli_parse_args=True, cli_implicit_flags=False):
    req_flag: bool
    """--req_flag bool (required)"""

    # Flags are explicit by default, so must override implicit flags with annotation
    implicit_flag: CliImplicitFlag[bool] = False
    """--implicit_flag (default: False)"""

    explicit_flag: bool = False
    """--explicit_flag bool (default: False)"""