The classmethod constructors on builtin types are absent

Found 2026-08-16 by a tools/pydiff.py sweep over the builtin surface.

bytes.fromhex("6162")     # CPython b'ab'    pxx: undefined variable (bytes)
float.fromhex("0x1p3")    # CPython 8.0      pxx: undefined variable (float)
int.from_bytes(b, "big")  # works — it is in the table
dict.fromkeys(xs)         # works — it is in the table
str.maketrans("a", "b")   # works — it is in the table

So the mechanism exists (PyStdlibCallProc's dotted table) and these two names are simply not in it. The diagnostic is misleading in a specific way: it says "undefined variable (bytes)", blaming the TYPE NAME the program used correctly, and bytes IS a working callable one line earlier — see [[project_nilpy_five_builtin_type_names_are_also_pylib_procs]] for why that name in particular reads oddly.

bytes.fromhex is the one worth having: hex-string-to-bytes is ordinary protocol and test code. float.fromhex is rare.

Work

Add bytes.fromhex (and float.fromhex) to the dotted table with pylib implementations, exactly as int.from_bytes is done. Watch the ODD-length and non-hex cases: CPython raises ValueError, and a silent partial parse would be the wrong trade.

Gate

A .npy diffed against CPython: even and odd lengths, embedded spaces (CPython allows them between byte pairs), an invalid digit raising ValueError, and a user def fromhex still shadowing nothing it should not.