Option arg cannot have None as default value

[justinlovinger]

Please complete the following tasks

• I have searched the discussions
• I have searched the open and rejected issues

Rust Version

rustc 1.70.0 (90c541806 2023-05-31) (built from a source tarball)

Clap Version

4.3.21

Minimal reproducible code

#[derive(Parser)]
struct Args {
    #[arg(long, num_args = 0..=1, default_value = "[]")]
    foo: Option<usize>,
}

fn main() {
    let args = Args::parse();
}

Steps to reproduce the bug with the above code

cargo run

Actual Behaviour

The program exits with error: invalid value '[]' for '--foo [<FOO>]': invalid digit found in string.

Expected Behaviour

The program should run with args.foo set to None.

Additional Context

num_args = 0..=1 disables the implied required = false behavior of Option<T>.

--foo [] is valid if entered from the command line. It is only not valid when used as a default value.

default_value_t = None fails because Option does not implement Display.

Debug Output

[clap_builder::builder::command]Command::_do_parse
[clap_builder::builder::command]Command::_build: name="owm"
[clap_builder::builder::command]Command::_propagate:owm
[clap_builder::builder::command]Command::_check_help_and_version:owm expand_help_tree=false
[clap_builder::builder::command]Command::long_help_exists
[clap_builder::builder::command]Command::_check_help_and_version: Building default --help
[clap_builder::builder::command]Command::_propagate_global_args:owm
[clap_builder::builder::debug_asserts]Command::_debug_asserts
[clap_builder::builder::debug_asserts]Arg::_debug_asserts:foo
[clap_builder::builder::debug_asserts]Arg::_debug_asserts:help
[clap_builder::builder::debug_asserts]Command::_verify_positionals
[clap_builder::parser::parser]Parser::get_matches_with
[clap_builder::parser::parser]Parser::add_defaults
[clap_builder::parser::parser]Parser::add_defaults:iter:foo:
[clap_builder::parser::parser]Parser::add_default_value: doesn't have conditional defaults
[clap_builder::parser::parser]Parser::add_default_value:iter:foo: has default vals
[clap_builder::parser::parser]Parser::add_default_value:iter:foo: wasn't used
[clap_builder::parser::parser]Parser::react action=Set, identifier=None, source=DefaultValue
[clap_builder::parser::arg_matcher]ArgMatcher::start_custom_arg: id="foo", source=DefaultValue
[clap_builder::parser::parser]Parser::push_arg_values: ["[]"]
[clap_builder::parser::parser]Parser::add_single_val_to_arg: cur_idx:=1
[clap_builder::builder::command]Command::color: Color setting...
[clap_builder::builder::command]Auto
[clap_builder::builder::command]Command::color: Color setting...
[clap_builder::builder::command]Auto
error: invalid value '[]' for '--foo []': invalid digit found in string

[epage]

#[derive(Parser)]
struct Args {
    #[arg(long, num_args = 0..=1, default_value = "[]")]
    foo: Option<usize>,
}

says to parse [] as a usize

#[derive(Parser)]
struct Args {
    #[arg(long, num_args = 0..=1, default_value_t = None)]
    foo: Option<usize>,
}

Recognizes usize as the core type and expects default_value_t to be for the core type. There are other advanced features you can use to override that to make it None which is why this makes sense.

#[derive(Parser)]
struct Args {
    #[arg(long, num_args = 0..=1, default_value_t)]
    foo: Option<usize>,
}

Says to use `Default::default

--foo [] is valid if entered from the command line. It is only not valid when used as a default value.

Not for me:

$ ./clap-5078.rs --foo []
warning: `package.edition` is unspecified, defaulting to `2021`
warning: unused variable: `args`
  --> clap-5078.rs:19:9
   |
19 |     let args = Args::parse();
   |         ^^^^ help: if this is intentional, prefix it with an underscore: `_args`
   |
   = note: `#[warn(unused_variables)]` on by default

warning: `clap-5078` (bin "clap-5078") generated 1 warning (run `cargo fix --bin "clap-5078"` to apply 1 suggestion)
    Finished dev [unoptimized + debuginfo] target(s) in 0.07s
     Running `/home/epage/.cargo/target/92/58a7b79897d359/debug/clap-5078 --foo '[]'`
error: invalid value '[]' for '--foo [<FOO>]': invalid digit found in string

For more information, try '--help'.

You might be running into shell specific logic where [] becomes "". If you do '[]', then you will likely bypass shell processing and see what is actually happening when it is passed into clap.

num_args = 0..=1 disables the implied required = false behavior of Option.

I'm not sure what this is referring to. It doesn't do that for me. I removed any defaulting and it runs as I would expect unless I'm missing something about what you are trying to accomplish.

[justinlovinger]

num_args = 0..=1 disables the implied required = false behavior of Option.

I'm not sure what this is referring to. It doesn't do that for me. I removed any defaulting and it runs as I would expect unless I'm missing something about what you are trying to accomplish.

This is referring to overriding the "assumed intent", https://docs.rs/clap/latest/clap/_derive/index.html#arg-types. From my testing, it makes the core type Option<usize> instead of it being an optional usize. It is like Option<Option<usize>>, but without the extra Option.

You might be running into shell specific logic where [] becomes "". If you do '[]', then you will likely bypass shell processing and see what is actually happening when it is passed into clap.

You may be right about this being shell-specific behavior. It appears equivalent to --foo. In which case, the core issue may be how to pass an explicit None value to an Option arg. --foo "" is not valid.

P.S. Vec<T> may have the same issue as I cannot find any way to pass an empty vector when using value_delimiter = ','.

[epage]

It might help if you stepped back and explained what you are trying to accomplish. What behavior do you want out of --foo?

[justinlovinger]

I want --foo to either take some value, like --foo 10, or a "none" value, like --foo None or --foo "".

For context, my application wraps a function that takes max_width: Option<usize> and max_height: Option<usize>. These can either be Some(value), in which case the function will run with a max width or height, or None, in which case it will run with no max width or height. These values can be set with the CLI, and I am trying to reflect this behavior in the CLI.

Additionally, by default, the CLI sets a max width, 1920, and no max height. The user should be able to set max width to None and set a value for max height. I want these clearly reflected in --help, so a user can look at the --max-height default, see it is set to "no max height", understand that as a possibility, and understand they can pass the same value to --max-width to get the same behavior for width.

[epage]

So you want to opt-out of special Option handling and have the value_parser return an Option<usize>. Currently special handling is purely textual. You can use std::option::Option or type OptionUsize = Option<usize> to work around this (see also #4626).

You can then do:

fn parser(raw: &str) -> Result<OptionUsize, String> {
    if raw == "None" || raw.is_empty() {
        Ok(None)
    } else {
        raw.parse::<usize>().map(Some)
    }
}


...

#[derive(Parser)]
struct Args {
    #[arg(long, value_parser = parser)]
    foo: OptionUsize,
}

[justinlovinger]

That work for my case. I suppose the question then is, why does Clap not support --foo "" for Option types out of the box?

[epage]

We don't know what the intention is for an empty string.