pola-rs/polars · error

File to have ended

Error message

File to have ended

What it means

ParquetFileWriter::into_inner_and_metadata returns the thrift FileMetaData that is only populated by end(), which writes the footer and sets self.metadata. Calling it before a successful end() finds metadata == None and panics with 'File to have ended'. The panic is documented in the rustdoc of the method.

Source

Thrown at crates/polars-parquet/src/parquet/write/file.rs:262

            None,
        );

        let len = end_file(&mut self.writer, &metadata)?;
        self.state = State::Finished;
        self.metadata = Some(metadata);
        Ok(self.offset + len)
    }

    /// Returns the underlying writer.
    pub fn into_inner(self) -> W {
        self.writer
    }

    /// Returns the underlying writer and [`ThriftFileMetadata`]
    /// # Panics
    /// This function panics if [`Self::end`] has not yet been called
    pub fn into_inner_and_metadata(self) -> (W, ThriftFileMetadata) {
        (self.writer, self.metadata.expect("File to have ended"))
    }
}

View on GitHub (pinned to df599052da)

Solutions

1. Call end(key_value_metadata) and propagate its Result before into_inner_and_metadata()
2. If end() returned Err, treat the writer as failed — use into_inner() to recover the underlying writer, not the metadata API
3. Only use into_inner_and_metadata() in code paths where end() has provably succeeded

Example fix

// before
let (w, meta) = writer.into_inner_and_metadata(); // panics: end() not called

// after
writer.end(None)?;
let (w, meta) = writer.into_inner_and_metadata();

// end() is the validator: its Ok proves metadata exists
writer.end(None)?;
let (w, meta) = writer.into_inner_and_metadata();

When it happens

Trigger: Calling writer.into_inner_and_metadata() without calling writer.end(...) first, or ignoring an Err returned by end() (e.g. footer write failed) and then unwrapping metadata anyway.

Common situations: Custom Parquet sinks that finish writing row groups and forget the footer step; error paths where end() failed (disk full, IO error) and cleanup code still asks for metadata.

Related errors

• expr expected, did you call when().then().otherwise?
• invalid `POLARS_MAX_CACHED_METADATA_SCANS` value
• integer
• not implemented
• not implemented

AI-assisted analysis of pola-rs/polars@df599052da (2026-08-16). Data as JSON: /api/errors/20510d12e8359cf8. Report an issue: GitHub.