Surprising behaviour: argfile reads @argfiles after --

[passcod]

Found in watchexec/watchexec#717.

When there's a trailing_var_arg (or in general, really), I would expect that argfile would not parse beyond the -- separator. That is, these should load an argfile:

command @argfile --option
command --option @argfile
command @argfile -- sub args

But this should not:

command --option -- @bazel-target-or-something

Here's a minimal repro:

#!/usr/bin/env cargo

//! ```cargo
//! [dependencies]
//! argfile = "0.1.6"
//! clap = { version = "4.4.11", features = ["derive"] }
//! ```

use clap::{Parser, ValueHint};

#[derive(Debug, Clone, Parser)]
pub struct Args {
	#[arg(
		trailing_var_arg = true,
		num_args = 1..,
		value_hint = ValueHint::CommandString,
		value_name = "COMMAND",
	)]
	pub command: Vec<String>,
}

fn main() {
    let args = argfile::expand_args(argfile::parse_fromfile, argfile::PREFIX)
		.expect("while reading @argfile");
    let args = Args::parse_from(args);
    dbg!(args);
}

Calling with ./repro.rs -- @not-an-argfile panics, while I would expect it to silently ignore and leave the @not-an-argfile in the args.

[epage]

parse_fromfile is specifically imitating what Python does. Can you confirm if Python ignore it after an escape (--)?

[passcod]

Alright if you saw the previous version of this post, didn't realise you were talking about specifically the argparse module from Python; the answer is no, argparse doesn't ignore fromfiles after --.

#!/usr/bin/env python

import argparse

parser = argparse.ArgumentParser(fromfile_prefix_chars='@')
parser.add_argument('--option')
parser.add_argument('--flag', action='store_true')
parser.add_argument('command', metavar='COMMAND', nargs='+')
print(parser.parse_args())

$ ./run.py -- foo
Namespace(option=None, flag=False, command=['foo'])

$ cat foo
--flag

$ ./run.py -- @foo
Namespace(option=None, flag=False, command=['--flag'])