Require a value for flag even with default_value

[davidhewitt]

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• Searched the discussions
• Searched the closed issues

Describe your use case

Consider this program with clap 3.0 beta:

use clap::Clap;

#[derive(Clap, Debug)]
struct Opts {
    #[clap(long)]
    batch_size: usize
}

fn main() {
    dbg!(Opts::parse());
}

Invoking this program with cargo run -- --batch-size (without specifying the value) will fail with an intuitive error:

error: The argument '--batch-size <batch-size>' requires a value but none was supplied

That's nice 👍. Now, consider I want to add a default value:

#[derive(Clap, Debug)]
struct Opts {
    #[clap(long, default_value = "5")]
    batch_size: usize
}

Now I can invoke as cargo run, and there's no error. The default value 5 is used. This is what I want.

However, I can now also invoke the original form cargo run -- --batch-size and it no longer errors. Again, the default value 5 is used.

Describe the solution you'd like

To me, a user writing --batch-size without the value indicated that the user intended to specify the batch size, but forgot to pass a value. For example they might have written a shell substitution --batch-size $BATCH_SIZE but forgot to set the variable in their shell.

So I think that --batch-size without a following value should always be a failure, regardless of whether a default_value is set.

(To me, having a default_value should allow omitting the flag entirely, but not allow this potentially malformed invocation.)

Alternatives, if applicable

Discussion in #1545 implies that this behaviour is supposedly expected, and so clap could consider doing nothing here.

However, I would ask you to please reconsider this (at least for the clap 3.0 derive macro, which is why I've opened this as a separate issue). In my eyes, this behaviour allows users to silently make mistakes while using my CLI, e.g. by a malformed shell subsitution.

[x10an14]

I'd like to lend my support to this proposal.

I often - both in private and at work - experience this footgun situation when helping others with their scripts:

Them: "That can't be where the problem lies, the command ran fine!"
Me: "Did you double-check that it would complain if any of the variable-inputs had an unexpected/wrong value?"
Them: "No, then it shouldn't run!"

[pksunkara]

@CreepySkeleton Can you please take a look at this?

[pksunkara]

We have default_missing_value for this.

#[clap(long, default_value = "5", default_missing_value = "5")]

[davidhewitt]

@pksunkara thanks but default_missing_value does not solve the problem I describe. (Testing with 3.0 beta 2)

With the suggested #[clap(long, default_value = "5", default_missing_value = "5")], I get the following outputs:

cargo run                    // OK
batch_size = 5

cargo run -- --batch-size    // <--- I don't want this to work!
batch_size = 5

cargo run -- --batch-size 6  // OK
batch_size = 6

What I want my program to do:

cargo run
batch_size = 5

cargo run -- --batch-size
error: The argument '--batch-size <batch-size>' requires a value but none was supplied

cargo run -- --batch-size 6
batch_size = 6

[pksunkara]

Have you tried it with the latest master? It should fail now since you haven't added AllowEmptyValues setting.

[davidhewitt]

@pksunkara I found the same issue on master, created a PR #2172 to fix it! 🚀