hmac won't fall back if OpenSSL is available, the requested algorithm isn't in OpenSSL, but the algorithm is inhashlib.
If you [monkey]patchhashlib to include a new algorithm, you can't use that algorithm fromhmac by name.
It appears that the OpenSSL implementation (known as_hashlib from insidehashlib, or_hashopenssl from insidehmac) doesn't actually return anUnsupportedDigestmodError, but rather it's base classValueError.

MRE

# The following is MRE-specific to easily introduce a new name# My use case involves a monkeypatch, but imagine any algorithm NOT implemented by OpenSSL, ONLY by hashlib>>> hashlib.__builtin_constructor_cache['myhashalg'] = hashlib.md5>>> hashlib.new('myhashalg', b'').digest().hex() # confirm hashlib can use that name'd41d8cd98f00b204e9800998ecf8427e'>>> hmac.digest(b'key', b'message', 'myhashalg')Traceback (most recent call last):  File"<pyshell#nnn>", line1, in<module>    hmac.digest(b'key',b'message','myhashalg')  File"C:\Python311\Lib\hmac.py", line198, indigestreturn _hashopenssl.hmac_digest(key, msg, digest)ValueError:unsupported hash type myhashalg

The exception goes unhandled at

This also shows up in the stateful (non-oneshot) code at

Environment

Windows 10 64-bit
Python 3.11.2

Possible fixes

• Change_hashopenssl.hmac_digest to correctly raise anUnsupportedDigestmodError (this looks like what was intended, givenbpo-40645: use C implementation of HMAC #24920)
• Catch aValueError instead (asUnsupportedDigestmodError is derived fromValueError this would work, but may not be what is truly intended)
• Something closer to what existed before likehttps://github.com/tiran/cpython/blob/837f9e42e3a1ad03b340661afe85e67d2719334f/Lib/hmac.py#L181 ??

Linked PRs

• gh-103256: Fix hmac algorithm to support fallback implementation #103286
• [3.11] gh-103256: Fix hmac algorithm to support fallback implementation (gh-103286) #103328