`ArgAction::Count` starts from `0` rather than `default_value`

[Eir Nym]

Please complete the following tasks

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

Rust Version

rustc 1.78.0 (9b00956e5 2024-04-29)

Clap Version

4.5.0

Minimal reproducible code

use clap::{ArgAction, Parser};

#[derive(Debug, Clone, Parser)]
pub struct CliArgs {
    #[clap(short = 'v', action = ArgAction::Count, default_value_t=5)]
    pub verbose: u8,
}
fn main() {
    let args = CliArgs::parse();
    println!("verbose count: {}", args.verbose);
}

Steps to reproduce the bug with the above code

$ cargo run -- -vvv

Actual Behaviour

default_value_t is not taken into consideration for the starting point of Count

output:

$ cargo run 
verbose count: 5
$ cargo run -- -vvv
verbose count: 3

Expected Behaviour

default_value_t is taken into consideration for the starting point of Count

output:

$ cargo run 
verbose count: 5
$ cargo run -- -vvv
verbose count: 8

Additional Context

Debug Output

No response

[Ed Page]

I am seeing the same behavior for both default_value and default_value_t and the behavior is correct.

Could you give more details, like copies of the output that is expected and unexpected?

#!/usr/bin/env nargo
---
[dependencies]
clap = { path = "../clap", features = ["derive"] }
---

use clap::{ArgAction, Parser};

#[derive(Debug, Clone, Parser)]
pub struct CliArgs {
    #[arg(short = 'v', action = ArgAction::Count, default_value_t=5)]
    pub verbose: u8,
}
fn main() {
    let args = CliArgs::parse();
    println!("verbose count: {}", args.verbose);
}

$ ./clap-5483.rs
verbose count: 5
$ ./clap-5483.rs -vvv
verbose count: 3

Running

#!/usr/bin/env nargo
---
[dependencies]
clap = { path = "../clap", features = ["derive"] }
---

use clap::{ArgAction, Parser};

#[derive(Debug, Clone, Parser)]
pub struct CliArgs {
    #[arg(short = 'v', action = ArgAction::Count, default_value="5")]
    pub verbose: u8,
}
fn main() {
    let args = CliArgs::parse();
    println!("verbose count: {}", args.verbose);
}

$ ./clap-5483.rs
verbose count: 5
$ ./clap-5483.rs -vvv
verbose count: 3

[Eir Nym]

@epage Thank you for your reply. I updated my comment above. TL;DR I expect, that ArgAction::Count will start count starting at given value. From documentation this action sets default_value to 0 if not provided, so I assumed, that it will start counting at default_value

[Ed Page]

In all ways within clap, default_value is assigned if nothing else is used. iirc Python's argparse has the same behavior and I've run into the same frustration :).

This means that your application needs to take this into account. https://github.com/clap-rs/clap-verbosity-flag is a crate that encodes all of this for that one specific use case.

I'm hesitant to change it and inclined to close this but I have just enough doubt that I think I'll leave this open to see what additional insights it might generate.

[Ed Page]

I've updated the Issue to reflect the latest clarification. To be clear, there is no difference in behavior between default_value and default_value_t. If you are seeing some, we'd need a reproduction case to better understand it.

[Eir Nym]

It could be a different action if there's no way to change this one. At least clarification for this action is required.

Example could be found in my recently published tool I'd like to start my verbose level with 2 (Info). -q is an opposite of -v and then I calculate actual verbosity level. (beginning of main.rs and args.rs)

[Ed Page]

Added a clarification in 11ff6cc

There is a cost for everything we add to the API. I'd be hesitant to add a variant that changes the default behavior especially when this can be worked around. I'd be more inclined towards the custom Actions that I've been considering.