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Name the key type when rejecting non-string CBOR/MessagePack map keys (#5594)

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* Name the key type when rejecting non-string CBOR/MessagePack map keys

CBOR and MessagePack allow map keys of any type, but JSON object keys
are always strings, so such maps are rejected. The error so far was the
one for a malformed string (e.g. "expected length specification
(0xA0-0xBF, 0xD9-0xDB); last byte: 0xC0" for a nil key), which does not
tell the user what went wrong. Report the type of the key instead:

  syntax error while parsing MessagePack object key: only string keys
  are supported, but found nil; last byte: 0xC0

The exception id (parse_error.113) and type are unchanged. Malformed
string keys and a missing key keep their previous messages. Document
the restriction on the CBOR and MessagePack pages.

Refs #2766, #3381

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

* Point the MessagePack key note to the spec's profile section

The note linked to "Serialization: type to format conversion", which says nothing about key types. Restricting map keys to strings is only mentioned in the "Profile" section (under "Future discussion") as an example of a JSON-compatible profile, so link there and describe it as such instead of as a permission.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

---------

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann
2026-09-28 17:51:07 +02:00
committed by GitHub
parent 373005f7ac
commit 9e1a09eec0
10 changed files with 484 additions and 17 deletions
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docs/mkdocs/docs/features/binary_formats/messagepack.md
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@@ -138,6 +138,21 @@ The library maps MessagePack types to JSON value types as follows:
Any MessagePack output created by `to_msgpack` can be successfully parsed by `from_msgpack`.
!!! warning "Object keys"
MessagePack allows map keys of any type, whereas JSON only allows strings as keys in object values. Like the
JSON-compatible [profile](https://github.com/msgpack/msgpack/blob/master/spec.md#profile) sketched in the
MessagePack specification, this library restricts map keys to `str` values. Maps with keys of any other type are
rejected with a [`parse_error.113`](../../home/exceptions.md#jsonexceptionparse_error113) exception (or, with
`allow_exceptions` set to `false`, a discarded value) naming the type of the key that was found, for instance:
```
[json.exception.parse_error.113] parse error at byte 2: syntax error while parsing MessagePack object key: only string keys are supported, but found nil; last byte: 0xC0
```
This applies to the [SAX interface](../parsing/sax_interface.md) as well, as the key is read before it is passed
on. Such input needs a general-purpose MessagePack library instead.
!!! warning "UTF-8 validation of string values"
The MessagePack specification requires `str` values (`fixstr`, `str 8`, `str 16`, `str 32`) to be valid UTF-8.