{"record":{"id":"58f142b2fe135670","repo":"pola-rs/polars","slug":"left-join-column-name-r-not-found","errorCode":null,"errorMessage":"left join column {name!r} not found","messagePattern":"left join column (.+?) not found","errorType":"validation","errorClass":"ValueError","httpStatus":null,"severity":"error","filePath":"py-polars/src/polars/lazyframe/frame.py","lineNumber":9068,"sourceCode":"                    msg = \"missing join columns for left frame\"\n                    raise ValueError(msg)\n                if right_on is None:\n                    msg = \"missing join columns for right frame\"\n                    raise ValueError(msg)\n        else:\n            # move on into left/right_on to simplify logic\n            left_on = right_on = on\n\n        if isinstance(left_on, str):\n            left_on = [left_on]\n        if isinstance(right_on, str):\n            right_on = [right_on]\n\n        left_schema = self.collect_schema()\n        for name in left_on:\n            if name not in left_schema:\n                msg = f\"left join column {name!r} not found\"\n                raise ValueError(msg)\n        right_schema = other.collect_schema()\n        for name in right_on:\n            if name not in right_schema:\n                msg = f\"right join column {name!r} not found\"\n                raise ValueError(msg)\n\n        # no need to join if *only* join columns are in other (inner/left update only)\n        if how != \"full\" and len(right_schema) == len(right_on):\n            if row_index_name is not None:\n                return self.drop(row_index_name)\n            return self\n\n        # only use non-idx right columns present in left frame\n        right_other = set(right_schema).intersection(left_schema) - set(right_on)\n\n        # When include_nulls is True, we need to distinguish records after the join that\n        # were originally null in the right frame, as opposed to records that were null\n        # because the key was missing from the right frame.","sourceCodeStart":9050,"sourceCodeEnd":9086,"githubUrl":"https://github.com/pola-rs/polars/blob/fe841f959ef4d2ceefc05a310d33ed7b1ab24e5e/py-polars/src/polars/lazyframe/frame.py#L9050-L9086","documentation":"Raised as a ValueError when a column named in `left_on` does not exist in the left frame's schema. The check runs against collect_schema() before the join executes, failing eagerly and naming the offending column.","triggerScenarios":"Calling `lf.join(other, left_on=\"cust_id\", ...)` where the left frame has no column 'cust_id' (renamed, dropped, or misspelled).","commonSituations":"Upstream column renames; lazy frame schema differing from what the join assumes; typos in column names.","solutions":["Verify the column exists via `lf.collect_schema().names()` and fix the name.","Add the column upstream via `with_columns`/`rename` before joining.","Check that preceding select/drop/rename steps did not remove the key column."],"exampleFix":"// before\nlf.join(other, left_on=\"cust_id\", right_on=\"id\")  # 'cust_id' missing\n// after\nlf = lf.rename({\"customer_id\": \"cust_id\"})\nlf.join(other, left_on=\"cust_id\", right_on=\"id\")","handlingStrategy":"validation","validationCode":"left_schema = lf.collect_schema()\nmissing = [c for c in left_on if c not in left_schema]\nif missing:\n    raise ValueError(f\"left frame missing join columns: {missing}\")","typeGuard":null,"tryCatchPattern":"try:\n    out = lf.join(other, left_on=left_on, right_on=right_on, how=how)\nexcept ValueError as e:\n    if \"left join column\" in str(e):\n        raise KeyError(f\"left frame columns: {lf.collect_schema().names()}\") from e\n    raise","preventionTips":["Call collect_schema() on both frames and assert join keys exist before joining.","Avoid renaming/dropping key columns in upstream select steps without updating joins.","Keep join key names in shared constants."],"tags":["python","polars","join","schema","column-not-found"],"backgroundTag":"schema-validation-failed","analyzedSha":"fe841f959ef4d2ceefc05a310d33ed7b1ab24e5e","analyzedAt":"2026-09-18T22:14:11.667Z","contentChangedAt":"2026-09-18T22:14:11.667Z","schemaVersion":2},"datasetVersion":"2026-09-23T08:17:48.524Z"}