{"record":{"id":"66c5f85b234a8f3f","repo":"pola-rs/polars","slug":"n-chunks-must-be-a-multiple-of-the-number-of-chu","errorCode":null,"errorMessage":"`n_chunks` must be a multiple of the number of chunks of this column ({total_n_chunks})","messagePattern":"`n_chunks` must be a multiple of the number of chunks of this column \\((.+?)\\)","errorType":"exception","errorClass":"ValueError","httpStatus":null,"severity":"error","filePath":"py-polars/src/polars/interchange/column.py","lineNumber":137,"sourceCode":"        -----\n        When `n_chunks` is higher than the number of chunks in the column, a slice\n        must be performed that is not on the chunk boundary. This will trigger some\n        compute if the column contains null values or if the column is of data type\n        boolean.\n        \"\"\"\n        total_n_chunks = self.num_chunks()\n        chunks = self._col.get_chunks()\n\n        if (n_chunks is None) or (n_chunks == total_n_chunks):\n            for chunk in chunks:\n                yield PolarsColumn(chunk, allow_copy=self._allow_copy)\n\n        elif (n_chunks <= 0) or (n_chunks % total_n_chunks != 0):\n            msg = (\n                \"`n_chunks` must be a multiple of the number of chunks of this column\"\n                f\" ({total_n_chunks})\"\n            )\n            raise ValueError(msg)\n\n        else:\n            subchunks_per_chunk = n_chunks // total_n_chunks\n            for chunk in chunks:\n                size = len(chunk)\n                step = size // subchunks_per_chunk\n                if size % subchunks_per_chunk != 0:\n                    step += 1\n                for start in range(0, step * subchunks_per_chunk, step):\n                    yield PolarsColumn(\n                        chunk[start : start + step], allow_copy=self._allow_copy\n                    )\n\n    def get_buffers(self) -> ColumnBuffers:\n        \"\"\"Return a dictionary containing the underlying buffers.\"\"\"\n        dtype = self._col.dtype\n\n        if dtype == String and not self._allow_copy:","sourceCodeStart":119,"sourceCodeEnd":155,"githubUrl":"https://github.com/pola-rs/polars/blob/df599052daf96e7a9cc30a3b0c6bd25d6947e3c0/py-polars/src/polars/interchange/column.py#L119-L155","documentation":"Raised by PolarsColumn.get_chunks when the requested n_chunks is <= 0 or is not an integer multiple of the column's actual chunk count. The interchange chunking logic can only evenly subdivide existing chunks, so arbitrary counts are rejected.","triggerScenarios":"column.get_chunks(3) on a column with 2 chunks (3 % 2 != 0); get_chunks(0) or a negative value; consumers computing n_chunks from a target batch size without aligning to the source chunk count.","commonSituations":"Streaming/batching code that asks for a fixed number of chunks (e.g. batch_size-driven); interleaving chunk requests across columns assuming all columns share one chunk count; calling num_chunks() once and then reusing a stale value after the frame changed.","solutions":["Pass n_chunks=None to iterate the natural chunks, or n_chunks equal to a multiple of column.num_chunks()","Compute the request dynamically: n_chunks = column.num_chunks() * k for the subdivision factor k you need","Validate n_chunks > 0 before calling"],"exampleFix":"// before\nchunks = list(col.get_chunks(3))  # col has 2 chunks\n// after\nk = math.ceil(3 / col.num_chunks())\nchunks = list(col.get_chunks(col.num_chunks() * k))\n# or simply: chunks = list(col.get_chunks())","handlingStrategy":"validation","validationCode":"total = column.num_chunks()\nok = n_chunks is None or (n_chunks > 0 and n_chunks % total == 0)","typeGuard":"def is_valid_chunk_request(n_chunks: int, total: int) -> bool:\n    return n_chunks > 0 and n_chunks % total == 0","tryCatchPattern":"try:\n    chunks = list(column.get_chunks(n))\nexcept ValueError:\n    chunks = list(column.get_chunks())  # fall back to natural chunking","preventionTips":["Default to n_chunks=None and batch downstream","Compute chunk requests from num_chunks() at call time, never cached"],"tags":["polars","interchange-protocol","chunking","argument-validation"],"backgroundTag":null,"analyzedSha":"df599052daf96e7a9cc30a3b0c6bd25d6947e3c0","analyzedAt":"2026-08-16T12:10:03.978Z","schemaVersion":2},"datasetVersion":"2026-08-16T13:17:31.715Z"}