Why delaying "No such file or directory" Error

[xcdnlgd]

I just follow the quick start section in readme.

$ cargo run
   Compiling hello_plotters v0.1.0 (/home/xcdnlgd/study/hello_plotters)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 1.47s
     Running `target/debug/hello_plotters`
Error: BackendError(DrawingError(ImageError(IoError(Os { code: 2, kind: NotFound, message: "No such file or directory" })))

Turns out the error is "thrown" by

    root.present()?;

I expect the error to be returned at

    let root = BitMapBackend::new("plotters-doc-data/0.png", (640, 480)).into_drawing_area();

Why does BitMapBackend::new() need the path while not using it immediately, which delays the error?
Is there any other delayed Error?

[AaronErhardt]

Why does BitMapBackend::new() need the path while not using it immediately, which delays the error?

The reasoning is probably that you don't need the file until you draw to it. Since all DrawingArea types share a common interface and some might not need a path because they don't write to a file, other methods like present() don't take a path, which means you have to get it somewhere else. The simplest approach is to just get the path at the beginning. If you need it, it's like one or two lines to check whether you can write to that path before creating the BitMapBackend.

[xcdnlgd]

Thanks for your reply

If you need it, it's like one or two lines to check whether you can write to that path before creating the BitMapBackend.

Maybe this should be added to the document.

I tried the example from https://plotters-rs.github.io/book/intro/getting_started.html

use plotters::prelude::*;

fn main() {
    let root_drawing_area = BitMapBackend::new("images/0.1.png", (1024, 768))
        .into_drawing_area();

    root_drawing_area.fill(&WHITE).unwrap();

    let mut chart = ChartBuilder::on(&root_drawing_area)
        .build_cartesian_2d(-3.14..3.14, -1.2..1.2)
        .unwrap();

    chart.draw_series(LineSeries::new(
        (-314..314).map(|x| x as f64 / 100.0).map(|x| (x, x.sin())),
        &RED
    )).unwrap();
}

This time, the program doesn't even panic! As if it has saved the file, which is not true.

Consider modifing the example to be something like this

    let root_drawing_area = {
        if !std::path::Path::new("image/").is_dir() {
            panic!("can't save file under 'image/'");
        }
        BitMapBackend::new("image/1.png", (1024, 768))
        .into_drawing_area()
    };