a-b-street/abstreet · error

bad map path

Error message

bad map path: {}

What it means

map_gui's map_name derives the MapName either from the stored map_path (via MapName::from_path) or from the default maps.json in the player data directory. If a map_path is set but cannot be parsed into a MapName, it panics with "bad map path: {path}". from_path returns Option (no error detail), so the panic fires for any unparseable path.

Solutions

  1. Check the map path matches MapName::from_path's expected format (importer-generated map under the standard data dir with the conventional filename).
  2. Use the app's map picker / change_map_btn to select a valid imported map instead of typing the path.
  3. Re-import the map with the importer so it lands in the canonical location with a canonical name.
  4. Patch from_path to return Result with a reason, or fall back to the default map instead of panicking.

Example fix

// before
MapName::from_path(path).unwrap_or_else(|| panic!("bad map path: {}", path))
// after
match MapName::from_path(path) {
    Some(name) => name,
    None => {
        eprintln!("unrecognized map path {}, using default", path);
        default_map_name()
    }
}
Defensive patterns

Strategy: validation

Validate before calling

let path = std::env::args().find(|a| a.starts_with("--map"));
if let Some(p) = path {
    assert!(std::path::Path::new(&p).is_file(), "map file missing: {}", p);
}

Try / catch

// Panics uncatchably; pre-narrow the path yourself:
fn valid_map_path(p: &str) -> Option<MapName> {
    MapName::from_path(p).or_else(|| {
        eprintln!("bad map path {}, falling back to default", p);
        None
    })
}

Prevention

When it happens

Trigger: Launching the app with a --map path (or loading a saved map_path) whose filename doesn't match the expected `<name>.bin`-style layout that MapName::from_path understands — e.g. wrong directory structure, missing city/country components, or a renamed map file.

Common situations: Passing an arbitrary OSM/other .bin file not produced by the importer; moving map files out of the expected data/maps directory; typos in the CLI flag; maps.json absent while map_path is also unset is a separate path — here map_path IS set but malformed.

Understand the failure class

Background: "Invalid ... format", "must be in format X", "does not look like a ..." — invalid argument format errors across CLI tools and libraries — this error's family across 17 libraries.

Related errors

AI-assisted analysis of a-b-street/abstreet@0964f29315 (2026-09-13). Data as JSON: /api/errors/ba3d09e00ebe525a. Report an issue: GitHub.

Appendix: source

Thrown at map_gui/src/simple_app.rs:84

            opts.toggle_day_night_colors = false;
        }
    }

    pub fn update_widgetry_settings(&self, mut settings: Settings) -> Settings {
        settings = settings
            .read_svg(Box::new(abstio::slurp_bytes))
            .window_icon(abstio::path("system/assets/pregame/icon.png"));
        if let Some(s) = self.scale_factor {
            settings = settings.scale_factor(s);
        }
        settings
    }

    pub fn map_name(&self) -> MapName {
        self.map_path
            .as_ref()
            .map(|path| {
                MapName::from_path(path).unwrap_or_else(|| panic!("bad map path: {}", path))
            })
            .or_else(|| {
                abstio::maybe_read_json::<crate::tools::DefaultMap>(
                    abstio::path_player("maps.json"),
                    &mut Timer::throwaway(),
                )
                .ok()
                .map(|x| x.last_map)
            })
            .unwrap_or_else(|| MapName::seattle("montlake"))
    }
}

impl<T: 'static> SimpleApp<T> {
    pub fn new<
        F: 'static + Fn(&mut EventCtx, &mut SimpleApp<T>) -> Vec<Box<dyn State<SimpleApp<T>>>>,
    >(
        ctx: &mut EventCtx,

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