[AlexXuz]

Summary

format() documents accepting an exception instance, but its direct-instance dispatch only recognizes Exception. Consequently, an explicitly passed KeyboardInterrupt, SystemExit, or GeneratorExit raises ValueError, even though formatting the equivalent sys.exc_info()-style tuple already works:

exc = KeyboardInterrupt('message')
stackprinter.format(exc)  # ValueError before this change
stackprinter.format((type(exc), exc, exc.__traceback__))
# 'KeyboardInterrupt: message'

Use BaseException in the existing dispatch check so direct instances follow the same path as exception tuples. show() benefits through its existing call to format(). This does not change exception catching or traceback rendering.

The regressions cover instances with and without tracebacks, plaintext and colored output, suppression options, show() output, ordinary exceptions, and rejection of unrelated inputs/classes.

Validation

• New tests on the unchanged base: 16 failed, 15 passed; after the fix: 31 passed.
• Full suite: 39 passed on Python 3.12.14 and 3.13.5; 38 passed, 1 existing interpreter-specific skip on Python 3.10.22.
• CI critical flake8 check: passed. Warning-only lint is unchanged from the baseline.
• Built the wheel and sdist; an isolated wheel installation passed direct-instance/tuple and show() smoke checks.

AI-assisted implementation and review; no claim of production use. Existing exception-group rendering limitations are outside this change.

[cknd]

Thanks for the contribution!