feat: append mode for MetricWriter.open#54
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Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Add mode="a" to open(): on a missing/empty file the header is written (append-or-create); on a non-empty file the existing header is validated against the metric class's fields and skipped. mode defaults to "w", so existing callers are unaffected. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Lift the read/validate/decide logic into _append_writes_header and collapse open()'s nested conditional into a single branch on mode. No behavior change. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Replace the hand-rolled `readline()` + one-shot `csv.reader` + empty-string check in `_read_existing_header` with `csv.DictReader.fieldnames` — the symmetric counterpart to the `DictWriter` the writer emits with. It parses the first row identically to how it was written and natively returns `None` for an empty file, so the header read needs no manual empty handling. Also note the `utf-8` (writer) vs `utf-8-sig` (reader) default asymmetry in the `open()` docstring. No behavior change. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
`MetricWriter.open(mode="a")` promises append-or-create: an empty file gets a fresh header. For a zero-byte `.bz2`/`.xz` file this raised EOFError instead — the header-validation read handed the empty file to xopen, and the bz2/xz decompressors reject a truncated stream (plaintext and gzip tolerate it). Check the file size before decompressing: a zero-byte file of any format has no header, so we skip the read and write one. Genuine corruption in a non-empty file still surfaces rather than being silently swallowed. Cover append-to-empty across all four formats (the previously-broken case), and pin the documented trailing-newline concatenation behavior. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Replace the try/except stat in _read_existing_header with an explicit exists()/st_size check, narrow its return type to list[str] | None so the header comparison is list-to-list, and rename _append_writes_header to _append_needs_header — it reports whether a header is needed rather than writing one. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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`MetricWriter.open(path, mode="a")` previously raised unless the first row of an existing file exactly matched the metric's header. It now also accepts a headerless data file: if the first row validates as a record of the metric class (via `model_validate`, the same path `MetricReader` uses to parse a row), rows are appended without injecting a header into the middle of the file. A `ValueError` is raised only when the first row is neither the expected header nor a valid record. A row with the wrong number of columns cannot be a record, so it is rejected. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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Summary
Fixes #47
This PR adds a
modeargument toMetricWriter.open()so callers can append metric rows to an existing delimited file without duplicating the header (e.g. resumable steps, sharded writers, incremental logging, any other append-or-create context).open(...)callmode="w"(default)mode="a"mode="a"mode="a"mode="a"ValueErroron entry