tracel-ai/burn · error

Should be able to create the new file '{}': {}

Error message

Should be able to create the new file '{}': {}

What it means

`FileLogger::new` panics when it cannot open (create/truncate) the log file at the given path with `std::fs::File::options().write(true).truncate(true).create(true)`. The panic message includes the path and the underlying `io::Error`. Burn assumes a file logger path must be creatable at startup, so it fails fast instead of returning `Result`.

Source

Thrown at crates/burn-train/src/logger/file.rs:28

    /// Create a new file logger.
    ///
    /// # Arguments
    ///
    /// * `path` - The path.
    ///
    /// # Returns
    ///
    /// The file logger.
    pub fn new(path: impl AsRef<Path>) -> Self {
        let path = path.as_ref();
        let mut options = std::fs::File::options();
        let file = options
            .write(true)
            .truncate(true)
            .create(true)
            .open(path)
            .unwrap_or_else(|err| {
                panic!(
                    "Should be able to create the new file '{}': {}",
                    path.display(),
                    err
                )
            });

        Self { file }
    }
}

impl<T> Logger<T> for FileLogger
where
    T: std::fmt::Display,
{
    fn log(&mut self, item: T) {
        writeln!(&mut self.file, "{item}").expect("Can log an item.");
    }
}

View on GitHub (pinned to d16f7ba2ed)

Solutions

1. Create the parent directory before constructing the logger: `std::fs::create_dir_all(path.parent().unwrap())`
2. Check permissions on the target directory and that the process user can write there
3. Confirm the path is a file path, not an existing directory, and uses valid characters for the OS
4. Verify the filesystem/mount is writable (not read-only, disk not full)
5. If the path is user-supplied, wrap `FileLogger::new` in `std::panic::catch_unwind` or validate writability beforehand

Example fix

// before
let logger = FileLogger::new("/var/log/train/metrics.log"); // parent dir missing
// after
std::fs::create_dir_all("/var/log/train").expect("create log dir");
let logger = FileLogger::new("/var/log/train/metrics.log");

fn ensure_log_path_ok(path: &std::path::Path) -> std::io::Result<()> {
    if path.is_dir() {
        return Err(std::io::Error::new(std::io::ErrorKind::InvalidInput, "path is a directory"));
    }
    if let Some(parent) = path.parent() {
        if !parent.as_os_str().is_empty() {
            std::fs::create_dir_all(parent)?;
        }
    }
    // probe writability with a temp create/truncate
    std::fs::OpenOptions::new().write(true).create(true).truncate(true).open(path)?;
    Ok(())
}

When it happens

Trigger: Calling `FileLogger::new(path)` (or a learner `log`/metric-logger setup using it, e.g. via `FileMetricLogger`) when the path's parent directory does not exist, the directory is not writable, the path is an existing directory, or the filesystem denies creation (permissions, read-only mount, disk full, invalid characters).

Common situations: Pointing the learner's checkpoint/log directory at a path whose parent folders were never created; running training in a container as a non-root user writing to a root-owned dir; read-only NFS/CI workspace; passing a directory instead of a file path; Windows path with illegal characters.

Understand the failure class

Background: "open() failed", "failed to open file", "cannot create file" — what a file open error means and how to fix it — this error's family across 42 libraries.

Related errors

• Can delete model checkpoint.
• Failed to load module from file
• Can delete optimizer checkpoint.
• Can delete learning rate scheduler checkpoint.
• Can save model checkpoint.

AI-assisted analysis of tracel-ai/burn@d16f7ba2ed (2026-09-05). Data as JSON: /api/errors/7f11b6fa8b0bdfc8. Report an issue: GitHub.