update-shape.py: the kept-copy check reads an except ImportError: handler that binds the name and then re-raises as the copy's own fallback, so a copy that cannot start is kept #897

Goal: an except ImportError: handler whose reached body ends in raise answers no name to the kept-copy check.

Filed by lane openRepoShape-3, from its triage of #458 at the v1.1.1 release commit 5017830. The item comes from lane 2's review of #802 (issuecomment-6233233871), on #458 (issuecomment-6233551346). The v1.1.1 notes' known limits name it.

State at 5017830.

• _fallback_in (update-shape.py:4328-4337) returns _bound_at(handler.body) and never asks whether the handler raises.
• _reached (:4376) drops only the statements AFTER a raise.
• The docstring at :4272 says "A handler that exits, re-raises or passes binds nothing", which is false of a handler that binds and then raises.

What happens. Take a kept copy holding

try:
    from repo_shape import NEW
except ImportError:
    NEW = None
    raise

beside a repo_shape.py without NEW. It is not refused, and Python stops on the re-raised ImportError.

Evidence. A probe at 5017830:

_names_imported -> {}; python exit 1: ImportError: cannot import name 'NEW' from 'repo_shape'
control, a handler that only binds: {} and python exit 0
control, a handler that passes: {'repo_shape': {'NEW'}}

Direction.

• In _fallback_in, a handler whose _reached(handler.body) ends in an ast.Raise answers nothing.
• Add a row to tests/test_update_shape_kept_helper.py.
• update-shape.py is not a pinned copy, so nothing re-pins.

Size: S.

Lane: openreposhape-3 (openRepoShape-3)