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fix: partition_email raises KeyError on a multipart/* attachment
email.contentmanager has no get_content() handler registered for any multipart/* content-type (handlers exist for text/*, application/*, image/*, message/rfc822, etc., but multipart sub-parts are normally consumed via .iter_parts(), not .get_content()). A multipart sub-part can still surface as an "attachment" via iter_attachments() though -- e.g. a PGP/MIME-signed forwarded message (multipart/signed) nested inside a multipart/mixed envelope, or a multipart/mixed forwarded message itself. Resolving such an attachment's bytes in _AttachmentPartitioner._file_bytes raised KeyError('multipart/signed') (or 'multipart/mixed'), crashing the entire partition_email() call instead of processing or skipping just that one attachment. Fixed by falling back to the MIME part's raw serialized bytes (as_bytes()) when its content-type is multipart/*, rather than calling get_content(). This is a root-cause fix at the actual failure point, not a broadening of the shared, deliberately narrow EXPECTED_ATTACHMENT_ERRORS tuple (its own comment: "Intentionally narrow... we do not catch RuntimeError... would otherwise be silently skipped") -- a bare KeyError there would mask unrelated bugs elsewhere in partition(). Fixes Unstructured-IO#3922.
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‎CHANGELOG.md‎

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### Fixes
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- **Reject an empty `tokenizer` when chunking by `max_tokens`**: `""` is not `None`, so it slipped past the "tokenizer is required" check while still leaving the chunkers without a token counter — the window was then silently measured in characters, making `max_tokens=20` mean 20 characters. It now raises the same `ValueError` as omitting `tokenizer` altogether.
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- **`partition_email()` no longer crashes on a `multipart/*` attachment**: a multipart sub-part surfaced as an "attachment" (e.g. a PGP/MIME-signed forwarded message) previously raised an uncaught `KeyError` from `email.contentmanager`, which has no `get_content()` handler for any `multipart/*` content-type. `_AttachmentPartitioner._file_bytes` now falls back to the part's raw serialized bytes for this case instead.
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## 0.25.1
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‎example-docs/eml/mime-attach-multipart-signed.eml‎

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From: [email protected]
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To: [email protected]
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Subject: Test with PGP-signed forwarded message attachment
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MIME-Version: 1.0
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Content-Type: multipart/mixed; boundary="===============6075316962646898834=="
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--===============6075316962646898834==
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Content-Type: text/plain; charset="utf-8"
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Content-Transfer-Encoding: 7bit
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This is the main email body.
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--===============6075316962646898834==
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Content-Type: multipart/signed
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Content-Transfer-Encoding: base64
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Content-Disposition: attachment; filename="signed-message.eml"
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MIME-Version: 1.0
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TUlNRS1WZXJzaW9uOiAxLjAKQ29udGVudC1UeXBlOiBtdWx0aXBhcnQvc2lnbmVkOyBwcm90b2Nv
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bD0iYXBwbGljYXRpb24vcGdwLXNpZ25hdHVyZSI7CiBtaWNhbGc9InBncC1zaGEyNTYiOyBib3Vu
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ZGFyeT0iPT09PT09PT09PT09PT09MzU2NzcxNDQ4MjMwOTczNzk3MT09IgoKLS09PT09PT09PT09
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PT09PT0zNTY3NzE0NDgyMzA5NzM3OTcxPT0KQ29udGVudC1UeXBlOiB0ZXh0L3BsYWluOyBjaGFy
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c2V0PSJ1dGYtOCIKQ29udGVudC1UcmFuc2Zlci1FbmNvZGluZzogN2JpdApNSU1FLVZlcnNpb246
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IDEuMAoKVGhpcyBpcyB0aGUgUEdQLXNpZ25lZCBmb3J3YXJkZWQgbWVzc2FnZSBib2R5LgoKLS09
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PT09PT09PT09PT09PT0zNTY3NzE0NDgyMzA5NzM3OTcxPT0KTUlNRS1WZXJzaW9uOiAxLjAKQ29u
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dGVudC1UeXBlOiBhcHBsaWNhdGlvbi9wZ3Atc2lnbmF0dXJlCgotLS0tLUJFR0lOIFBHUCBTSUdO
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QVRVUkUtLS0tLQpmYWtlZmFrZWZha2UKLS0tLS1FTkQgUEdQIFNJR05BVFVSRS0tLS0tCgotLT09
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PT09PT09PT09PT09PTM1Njc3MTQ0ODIzMDk3Mzc5NzE9PS0tCg==
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--===============6075316962646898834==--

‎test_unstructured/partition/test_email.py‎

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]
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def test_partition_email_does_not_raise_on_multipart_attachment():
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"""A multipart/* "attachment" (e.g. a PGP/MIME-signed forwarded message) must not crash.
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`email.contentmanager` has no `get_content()` handler for any `multipart/*` content-type, so
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without special-casing this in `_AttachmentPartitioner._file_bytes`, resolving such an
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attachment's bytes raises `KeyError` -- crashing the whole `partition_email()` call instead
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of gracefully processing or skipping just that one attachment. Regression test for #3922.
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"""
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elements = partition_email(
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example_doc_path("eml/mime-attach-multipart-signed.eml"), process_attachments=True
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)
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# -- No exception; the email body is still partitioned. --
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assert elements == [NarrativeText("This is the main email body.")]
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# ================================================================================================
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# ISOLATED UNIT TESTS
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# ================================================================================================

‎unstructured/partition/email.py‎

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@cached_property
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def _file_bytes(self) -> bytes:
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"""The bytes of the attached file."""
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# -- `email.contentmanager` has no `get_content()` handler registered for any
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# -- `multipart/*` content-type (handlers exist for text/*, application/*, image/*,
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# -- message/rfc822, etc., but multipart sub-parts are normally consumed via
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# -- `.iter_parts()`, not `.get_content()`). A multipart sub-part can still appear as an
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# -- "attachment" though, e.g. a PGP/MIME-signed forwarded message (`multipart/signed`)
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# -- nested inside a `multipart/mixed` envelope. Fall back to the part's raw serialized
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# -- bytes in that case rather than letting `get_content()` raise `KeyError`.
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if self._attachment.get_content_type().startswith("multipart/"):
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return self._attachment.as_bytes()
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content = self._attachment.get_content()
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if isinstance(content, str):

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