Error vocabulary

🟢 Beginner · Lesson 06

Failing well on day one - every exception melder raises at you is catchable from the root namespace. No internal paths, ever.

Before you run

Use the Beginner guide for prerequisite concepts. Run from a checkout with Melder installed and Python 3.14 free-threading selected. The collection download includes the level's local helper modules.

Run the saved script

python UX_and_AIX_experiences/01_beginner/06_error_vocabulary.py

py -3.14t UX_and_AIX_experiences/01_beginner/06_error_vocabulary.py

Download this collection · Source on GitHub

Public surface

md.SpellbookValidationError, md.MeldExecutionError

Code

"""
TIER: beginner (06)
GOAL: Failing well on day one - every exception melder raises at you is
      catchable from the root namespace. No internal paths, ever.
SURFACE EXERCISED: md.SpellbookValidationError, md.MeldExecutionError
"""
import melder as md


class Registered:
    pass


class NeverRegistered:
    pass


def main() -> None:
    book = md.Spellbook()
    book.bind(spell=Registered, existence=md.Existence.unique)
    conduit = book.conjure()

    try:
        conduit.meld(spell=NeverRegistered)
    except KeyError as err:
        # the documented "not found anywhere" contract is one stable KeyError
        print("unregistered spell -> KeyError:", err)

    # the full catchable family, importable without knowing ANY internal path
    family = [
        md.SpellbookValidationError, md.MeldExecutionError,
        md.SpellSpaceScopeError, md.HookExecutionError,
        md.InternalRegistrationError, md.PhaseSchedulerError,
        md.PhaseExecutionError, md.PhaseTimeoutError, md.DeadReferenceError,
    ]
    assert all(issubclass(e, BaseException) for e in family)
    print("error vocabulary size:", len(family))


if __name__ == "__main__":
    main()

Check the outcome

The script contains its own assertions or demonstrated refusal paths. Run it to evaluate those checks against your installed version. The code above is taken directly from the saved file; no run output is invented here.

More beginner examples · Level guide