'pydoc -k' fails when some module's loader is not found

[dlax]

BPO	46353
Nosy	@tiran, @dlax
PRs	bpo-46353: Fix "pydoc -k" when a module fails to load #30557

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2022-01-12.08:53:18.397>
labels = ['type-bug', 'library', '3.9']
title = "'pydoc -k' fails when some module's loader is not found"
updated_at = <Date 2022-01-12.13:04:25.878>
user = 'https://github.com/dlax'

bugs.python.org fields:

activity = <Date 2022-01-12.13:04:25.878>
actor = 'dlax'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2022-01-12.08:53:18.397>
creator = 'dlax'
dependencies = []
files = []
hgrepos = []
issue_num = 46353
keywords = ['patch']
message_count = 2.0
messages = ['410388', '410389']
nosy_count = 2.0
nosy_names = ['christian.heimes', 'dlax']
pr_nums = ['30557']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue46353'
versions = ['Python 3.9']

[dlax]

On my (Debian 10) system, 'pydoc -k' crashes as follows:

    $ python3 -m pydoc -k foo
    Traceback (most recent call last):
      File "/usr/lib/python3.9/runpy.py", line 197, in _run_module_as_main
        return _run_code(code, main_globals, None,
      File "/usr/lib/python3.9/runpy.py", line 87, in _run_code
        exec(code, run_globals)
      File "/usr/lib/python3.9/pydoc.py", line 2862, in <module>
        cli()
      File "/usr/lib/python3.9/pydoc.py", line 2795, in cli
        apropos(val)
      File "/usr/lib/python3.9/pydoc.py", line 2287, in apropos
        ModuleScanner().run(callback, key, onerror=onerror)
      File "/usr/lib/python3.9/pydoc.py", line 2248, in run
        loader = spec.loader
    AttributeError: 'NoneType' object has no attribute 'loader'

(The same happens with current 'main' of CPython.)

The module that is tried to be loaded is 'ansible.galaxy.data', installed in /usr/lib/python3/dist-packages/ and it uses a custom (Ansible) finder _AnsiblePathHookFinder.

Maybe this finder has problems, but since Finder.find_module() may return None, I believe this should be handled by pydoc's ModuleScanner.

If agreed, I'll prepare a fix accordingly.

[tiran]

An exception is not a crash. We reserve the word "crash" for segfaults.