The error message for odd-length input to `bytes.fromhex` is cryptic

[Kodiologist]

Bug report

Bug description:

Python 3.14.0a2+ (main, Dec  8 2024, 10:50:32) [GCC 14.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> bytes.fromhex('deadbee')
Traceback (most recent call last):
  File "<python-input-0>", line 1, in <module>
    bytes.fromhex('deadbee')
    ~~~~~~~~~~~~~^^^^^^^^^^^
ValueError: non-hexadecimal number found in fromhex() arg at position 7
>>> bytes.fromhex('deadbeef')
b'\xde\xad\xbe\xef'

I suggest: ValueError: fromhex() arg must be of even length.

CPython versions tested on:

3.12, CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-127740: For odd-length input to bytes.fromhex(...) change the error message to ValueError: fromhex() arg must be of even length #127756
• gh-127740: Add some more tests for earlier PR #127756 #127818

[srinivasreddy]

@Kodiologist are you working on it ?

[Kodiologist]

Nope, it's up for grabs right now.

[srinivasreddy]

Thanks 🙏. I am taking it then

[picnixz]

Marking this one as a feature rather than a bug since changing exception messages may break CI (python/mypy#17849) and users may rely on exception messages.

[rhettinger]

users may rely on exception messages.

Users may, but users shouldn't ;-).

We don't want to change error messages unnecessarily; however, if the error message is bad, it should get fixed. To misquote Spock, the needs of many people reading error messages outweigh the needs of the few who overspecify their tests.

Bad error messages are a major source of user frustration.

[picnixz]

I'm not against changing the error message but I honestly think we don't need to backport it to 3.12 and 3.13 (personally I wouldn't be against the backports but I learned the hard way that changing error messages in bug fix releases could cause un-necessary additional work =/).