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Tue Jul 27 19:21:15 CEST 2004 · https://mail.python.org/pipermail/python-dev/2004-July/046436.html

Casey Duncan wrote:
> On Tue, 27 Jul 2004 12:39:22 -0400
> Jim Fulton <jim at zope.com> wrote:
> 
> 
>>Tim Peters wrote:
>>
>>>[Jim Fulton]
>>>...
>>>
>>>
>>>>No, it won't.  For example, suppose foo imports B. B tries to import
>>>>C, but fails.  B is now broken, but it is still importable. 
>>>
>>>Actually,>both foo and B can be imported without errors, even though
>>>they are>broken.
>>>
>>>
>>>Then you're proposing a way for a highly knowledgable user to
>>>anticipate, and partially worm around,  that Python leaves behind
>>>insane module objects in sys.modules. 
>>
>>No.  I'm proposing a way for a Python developer to detect the
>>presence or absence of a module.
>>
>>Hm, perhaps it would be better to provide an API (if there isn't one
>>already) to test whether a module is present.
> 
> 
> If such an api were to exist, what question exactly would it answer?
> 
> 1. That there is a module by a particular name that could possibly be
> imported, but the import may not succeed when actually tried.

Yup

> 2. There is a module by a particular name which can be imported and the
> import will not
> fail.
> 
> I would vote for the former, but it might be surprising to run into
> behavior like this:
> 
> 
>>>>sys.hasmodule('foo')
> 
> True
> 
>>>>import foo
> 
> ImportError: foo blew

Of course it would be surprising, because foo would be broken.
That's why they call them exceptions. :)

> What if there was a new exception ModuleNotFoundError which subclassed
> ImportError. This would be raised by the import machinery when the
> module could not be found. Errors during import would continue to raise
> a standard ImportError.

Yup.

> I think this, coupled with Jim's original suggestion and Tim's to
> prevent broken modules being retained in os.modules would help a lot.

Yup. It would certainly address the imediate problem.

Jim

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[Python-Dev] Re: Fix import errors to have data