[Python-ideas] Better error messages for missing optional stdlib packages

[Terry Reedy]

On 10/3/2018 4:29 PM, Marcus Harnisch wrote:

> When trying to import lzma on one of my machines, I was suprised to get 
> a normal import error like for any other module. 
What was the traceback and message?  Did you get an import error for one 
of the three imports in lzma.py.  I don't know why you would expect 
anything else.  Any import in any stdlib module can potential fail if 
the file is buggy, corrupted, or missing.

 > According to the docs
> lzma has been part of stdlib since 3.3. Further digging revealed that 
> the error is due to the fact that xz wasn't compiled in when building 
> Python.

Perhaps this is a buggy build.  Where did you get it from?  Have you 
complained to the distributor?

lzma is documented as wrapping liblzma and supporting .xz files.  If 
'xz' is a separate library, then perhaps lzma should tolerate it missing.

> Since I suspect that there are other optional stdlib modules, 

I agree that modules that are necessarily optional should be documented 
as such, and as I mentioned on https://bugs.python.org/issue34895, many 
are so documented.

In the absence of such documentation, I would considered it to be not 
optional except as some distributor decides to omit it.  But then it is 
the responsibility of the distributor to document the omission.

> this made me think whether the message in those cases should look a 
> little more polished. Perhaps installing a stub module that prints some 
> informative text before raising the relevant exception or similar.

> Also, maybe add a little note in the docs, stating that despite being 
> part of stdlib this module might not be available on all systems.

ImportError messages and the documented status versus actual status of a 
module are two different issues.  Let's keep them separate.

-- 
Terry Jan Reedy