What causes "Bad --alerts={}. Must be print|block|silence" in a-b-street/abstreet?

Change the --alerts value to exactly one of print, block, or silence

Bad --alerts= . Must be print|block|silence

Error message

Bad --alerts={}. Must be print|block|silence

What it means

This error comes from the CLI argument parser for the --alerts flag in the simulator. The flag controls how in-game alerts are handled, and it accepts exactly three values: print, block, or silence. Any other string fails with this message, echoing the bad value and the valid choices.

Solutions

Change the --alerts value to exactly one of print, block, or silence
Check for case sensitivity and stray whitespace in the flag value
If you need a new handler, add a match arm in parse_alert_handler in sim/src/sim/mod.rs

Example fix

// before
--alerts=Blocks
// after
--alerts=block

Defensive patterns

Strategy: validation

Validate before calling

const VALID_ALERTS: [&str; 3] = ["print", "block", "silence"];
if !VALID_ALERTS.contains(&alerts.as_str()) {
    eprintln!("--alerts must be print|block|silence, got: {}", alerts);
    std::process::exit(2);
}

Type guard

fn is_valid_alert_handler(x: &str) -> bool {
    matches!(x, "print" | "block" | "silence")
}

Prevention

Wrap simulator invocations in a script that validates flags before launch
Use an enum-typed CLI parser (e.g. clap with ValueEnum) instead of raw string matching
Add a shell-completion or help text listing the three valid values

When it happens

Trigger: Calling the sim binary (or SimOptions parsing path) with --alerts set to anything other than print, block, or silence, including typos like 'blockq', capitalized 'Print', or an empty string.

Common situations: Typos in shell scripts or run configs, copy-pasting flag values from docs of other tools, environment-specific wrappers that interpolate an unexpected value into --alerts.

Understand the failure class

Background: "Unknown argument", "Invalid value", and "must be one of": invalid CLI argument errors explained — this error's family across 35 libraries.

Related errors

AI-assisted analysis of a-b-street/abstreet@0964f29315 (2026-09-13). Data as JSON: /api/errors/c9424435f59812bb. Report an issue: GitHub.

Appendix: source

Thrown at sim/src/sim/mod.rs:182

    Print,
    /// Print the alert to STDOUT and don't proceed until the UI calls clear_alerts()
    Block,
    /// Don't do anything
    Silence,
}

impl Default for AlertHandler {
    fn default() -> AlertHandler {
        AlertHandler::Print
    }
}

fn parse_alert_handler(x: &str) -> Result<AlertHandler> {
    match x {
        "print" => Ok(AlertHandler::Print),
        "block" => Ok(AlertHandler::Block),
        "silence" => Ok(AlertHandler::Silence),
        _ => bail!("Bad --alerts={}. Must be print|block|silence", x),
    }
}

// Setup
impl Sim {
    pub fn new(map: &Map, mut opts: SimOptions) -> Sim {
        let mut timer = Timer::new("create blank sim");
        let mut scheduler = Scheduler::new();

        // Always disable parking for two maps. See
        // https://github.com/a-b-street/abstreet/issues/688 for discussion of how to set this
        // properly.
        if map.get_name() == &MapName::seattle("arboretum")
            || map.get_name().city == CityName::new("ir", "tehran")
            || map.get_name() == &MapName::new("gb", "poundbury", "center")
            || map.get_name() == &MapName::new("us", "phoenix", "tempe")
            || map.get_name() == &MapName::new("br", "sao_paulo", "sao_miguel_paulista")
        {

View on GitHub (pinned to 0964f29315)