micropython-hmac uses micropython-hashlib, which attempts to import various hash classes (sha1, sha256 etc) from uhashlib if they exist. For any missing class, it imports a pure Python implementation.

Now, the pure Python differ a bit in the API they expose, comparing to the implementations in uhashlib itself. That wouldn't be of a problem, but the hmac module later uses some of these APIs... So if your uhashlib has e.g sha1 then you can't use hmac(digestmod=sha1).

Specifically, digest_size, block_size and copy are missing (at least from what I've seen, possibly more missing attributes exist)