{"record":{"id":"fcf55eed0a92444b","repo":"pathwaycom/pathway","slug":"can-t-generate-schema-based-on-an-empty-csv-file","errorCode":null,"errorMessage":"can't generate Schema based on an empty CSV file","messagePattern":"can't generate Schema based on an empty CSV file","errorType":"exception","errorClass":"ValueError","httpStatus":null,"severity":"error","filePath":"python/pathway/internals/schema.py","lineNumber":977,"sourceCode":"        Schema\n    \"\"\"\n\n    def remove_comments_from_file(f: Iterable[str], comment_char: str | None):\n        for line in f:\n            if line.lstrip()[0] != comment_char:\n                yield line\n\n    with open(path) as f:\n        csv_reader = csv.DictReader(\n            remove_comments_from_file(f, comment_character),\n            delimiter=delimiter,\n            escapechar=escape,\n            quoting=csv.QUOTE_ALL,\n            quotechar=quote,\n            doublequote=double_quote_escapes,\n        )\n        if csv_reader.fieldnames is None:\n            raise ValueError(\"can't generate Schema based on an empty CSV file\")\n        column_names = csv_reader.fieldnames\n        if num_parsed_rows is None:\n            csv_data = list(csv_reader)\n        else:\n            csv_data = list(itertools.islice(csv_reader, num_parsed_rows))\n\n    def choose_type(entries: list[str]):\n        if len(entries) == 0:\n            return Any\n        if all(_is_parsable_to(s, int) for s in entries):\n            return int\n        if all(_is_parsable_to(s, float) for s in entries):\n            return float\n        return str\n\n    column_types = {\n        column_name: choose_type([row[column_name] for row in csv_data])\n        for column_name in column_names","sourceCodeStart":959,"sourceCodeEnd":995,"githubUrl":"https://github.com/pathwaycom/pathway/blob/fa2f74a4649b7c5908690cf60137263d8d80de5f/python/pathway/internals/schema.py#L959-L995","documentation":"Raised by Pathway's CSV schema autogeneration (python/pathway/internals/schema.py:977). It builds a csv.DictReader over the file (with comment lines stripped) and requires fieldnames — the header row. If csv_reader.fieldnames is None, the reader saw no header at all, so no schema can be inferred, and a ValueError is thrown.","triggerScenarios":"Calling pw.csv_schema / pw.schema_from_csv (or a CSV connector in schema-autodetect mode) on a file that is empty, contains only blank lines, or contains only lines stripped as comments (comment_character matching every line); pointing at a wrong/empty file path.","commonSituations":"Tail-ing a file that has been created but not yet written to; a download/ETL step that produced a 0-byte CSV; comment_character that accidentally matches the header line; passing num_parsed_rows with a file whose data rows are all consumed as comments.","solutions":["Check the file is non-empty and starts with a header row before generating the schema","Verify comment_character does not strip the header line","Supply an explicit schema (a pw.Schema class or pw.schema_from_types) instead of relying on autodetection","If the file appears later, wait for it to contain data before building the schema"],"exampleFix":"# before\nschema = pw.schema_from_csv(\"data.csv\")  # data.csv is empty -> ValueError\n\n# after\nimport os\nif os.path.getsize(\"data.csv\") == 0:\n    raise RuntimeError(\"data.csv is empty; cannot infer schema\")\nschema = pw.schema_from_csv(\"data.csv\")","handlingStrategy":"validation","validationCode":"import os\n\ndef csv_has_header(path: str) -> bool:\n    return os.path.getsize(path) > 0","typeGuard":null,"tryCatchPattern":"try:\n    schema = pw.schema_from_csv(path)\nexcept ValueError:\n    schema = MyExplicitSchema  # fall back to a declared pw.Schema","preventionTips":["Check file size before schema inference","Prefer explicit pw.Schema classes in production pipelines over CSV autodetection"],"tags":["csv","schema","io","pathway"],"backgroundTag":null,"analyzedSha":"fa2f74a4649b7c5908690cf60137263d8d80de5f","analyzedAt":"2026-08-15T01:48:17.006Z","schemaVersion":2},"datasetVersion":"2026-08-15T22:17:37.221Z"}