Bug report

Bug description:

As noted in #133351 (comment) PDB allows commands that start with !, but Cmd fails to tab complete them if there's a space after the !, because it falls down this code path:

which expects cmd to always be a string, but parsecmd is returning None instead when the line starts with ! and do_shell is not defined.

CPython versions tested on:

CPython main branch

Operating systems tested on:

No response

Linked PRs

• gh-133363: Fix Cmd completion for lines beginning with !  #133364