cmd.Cmd fails tab completion for lines beginning with `! ` (bang space)

[godlygeek]

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:

cpython/Lib/cmd.py

Lines 274 to 280 in 2bc8365

	if begidx>0:
	cmd, args, foo = self.parseline(line)
	if cmd == '':
	compfunc = self.completedefault
	else:
	try:
	compfunc = getattr(self, 'complete_' + cmd)

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