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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"mimetypes CLI writes error messages to stdout instead of stderr","articleBody":"## Description\n\nThe `mimetypes` command-line interface documentation states:\n\n> For each `type` entry, the script writes a line into the standard output stream. If an unknown type occurs, it writes an error message into the **standard error stream** and exits with the return code `1`.\n\nHowever, the actual implementation writes *all* output — including error messages — to stdout via a single `print()` call:\n\n```python\n# Lib/mimetypes.py\nif __name__ == '__main__':\n    import sys\n    results = _main()\n    print(\"\\n\".join(results))  # errors and normal output mixed into stdout\n    sys.exit(any(result.startswith(\"error: \") for result in results))\n```\n\n**Verified behavior:**\n```\n$ python -m mimetypes foo.unknownext > out.txt 2> err.txt\n$ cat out.txt\nerror: media type unknown for foo.unknownext\n$ cat err.txt\n(empty)\n```\n\nError messages go to stdout, stderr is empty — contradicting the docs.\n\n## Additional issue: stale docs example\n\nThe documentation example shows:\n\n```\n$ python -m mimetypes filename.pict\nerror: unknown extension of filename.pict\n```\n\nBut `filename.pict` is now recognized:\n\n```\n$ python -m mimetypes filename.pict\ntype: image/pict encoding: None\n```\n\nThe example error message format also doesn't match the actual output (`\"error: unknown extension of...\"` vs `\"error: media type unknown for...\"`).\n\n## Proposed fix\n\nRoute error messages to stderr in the `__main__` block:\n\n```python\nif __name__ == '__main__':\n    import sys\n    results = _main()\n    has_error = False\n    for result in results:\n        if result.startswith(\"error: \"):\n            print(result, file=sys.stderr)\n            has_error = True\n        else:\n            print(result)\n    sys.exit(has_error)\n```\n\nAnd update the docs example to use a genuinely unrecognized extension.\n\n\u003c!-- gh-linked-prs -->\n### Linked PRs\n* gh-149683\n\u003c!-- /gh-linked-prs -->\n","author":{"url":"https://github.com/htjworld","@type":"Person","name":"htjworld"},"datePublished":"2026-05-11T14:28:44Z","url":"https://github.com/python/cpython/issues/149682"}