[Python-ideas] PEP: Distributing a Subset of the Standard Library

[M.-A. Lemburg]

On 29.11.2016 00:50, Brett Cannon wrote:
> Seventh, these *.missing.py files if they are directly executed are totally
> going to be abused like *.pth files, I can just feel it in my bones. We
> need to be okay with this if we accept this PEP as-is.

Since the purpose of the PEP was to allow distributors to guide
users through the installation process of extra packages in order
to get access to parts of the stdlib which are not installed,
I think the PEP is overly broad in concept to address this one
use case.

Just as with .pth files, the possibility to hook arbitrary code
execution into the module search path will get abused for
all kinds of weird things, esp. if the whole sys.path is
scanned for the .missing.py module and not only the part
where the stdlib lives (as was suggested in the thread).

So why not limit the PEP to just the intended use case ?

I.e. define a static list of modules which do make up the Python
stdlib and then have the importer turn a ModuleNotFoundError error
into a nice distribution specific error message, if and only
if the imported module is from the set of stdlib modules.

The change of the error message could be done by having the
distributor patch the importer or we could have the importer
call a function defined via sitecustomize.py by the distributor
to return a message.

Thinking about this some more...

We don't even need a list of stdlib modules. Simply define
a general purpose import error formatting function, e.g.
sys.formatimporterror(), pass in the name of the module and
let it determine the error message based on the available
information.

A distributor could then provide a custom function that
knows about the installed Python packages and then guides
the user to install any missing ones.

-- 
Marc-Andre Lemburg
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