NilPy: Python's builtin exception classes, and int(s, base) that raises

Hangs off [[feature-nilpy-corpus-uforth]]. This is uforth's wall as of 2026-07-20, at uforth.py:359, after os.path/one-line-suites landed.

The wall, and why it is bigger than it looks

try:
    val = int(s, base)      # two-argument int() with a radix — not supported
    ...
except ValueError:          # ValueError is not a declared class — not supported
    pass

Two separate gaps, and they must land together to be useful:

1. int(s, base) — the two-argument radix form. parser.inc's NilPy int( handler (search isNilPy and (name = 'int')) parses exactly one argument and then expects ). The one-argument forms already lower to the -203 (Trunc) and -200 (parse/unbox) intrinsics.
2. Builtin exception classes. except ValueError: resolves the class name through FindUClass, so only Exception (via PyEnsureExceptionClass) and user-declared classes work. Python's builtins are not declared anywhere.

Do not implement int(s, base) with the usual pylib error path. Every other pylib failure does WriteLn(...); Halt(1). That is fatal here: a Forth interpreter tries EVERY input word as a number first, so a non-numeric token is the common case, not an error case. pyint_parse must raise something except ValueError can catch, or uforth dies on its first ordinary word.

Census — the exception classes uforth catches

AttributeError, EOFError, Exception, ForthThrow (its own, already works), KeyboardInterrupt, OSError, ValueError.

Shape

• Predeclare the builtin hierarchy the way PyEnsureExceptionClass already creates Exception — lazily, on first reference, so a program that never names them pays nothing. All descend from Exception so a bare except Exception: still catches them.
• The open question is how pylib raises one: the classes are created by the FRONTEND at parse time, so a Pascal unit cannot name them directly. Options: (a) have the frontend lower int(s, base) to a pylib call returning a success flag and synthesise the raise in the AST at the call site; (b) give pylib a raise-by-class-name runtime helper. (a) keeps pylib free of frontend knowledge and is the recommended route.

Gate

make test-nilpy green with a .npy case diffed against CPython (must cover a FAILING parse caught by except ValueError, not just a succeeding one, since the catch is the whole point) + --tier quick + self-host byte-identical + make fpc-check clean relative to HEAD.

Log

• 2026-07-20 — resolved, commit HEAD.