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

[Steve Dower]

On 28Nov2016 1433, Steve Dower wrote:
> On 28Nov2016 1419, Nathaniel Smith wrote:
>> I'd suggest that we additional specify that if we find a
>> foo.missing.py, then the code is executed but -- unlike a regular
>> module load -- it's not automatically inserted into
>> sys.modules["foo"]. That seems like it could only create confusion.
>> And it doesn't restrict functionality, because if someone really wants
>> to implement some clever shenanigans, they can always modify
>> sys.modules["foo"] by hand.
>
> In before Brett says "you can do this with an import hook", because,
> well, we can do this with an import hook :)

And since I suggested it, here's a rough proof-of-concept:

import importlib.abc
import os
import sys

class MissingPathFinder(importlib.abc.MetaPathFinder):
     def find_spec(self, fullname, path, target=None):
         for p in (path or sys.path):
             file = os.path.join(p, fullname + ".missing")
             if os.path.isfile(file):
                 with open(file, 'r', encoding='utf-8') as f:
                     raise ModuleNotFoundError(f.read())

sys.meta_path.append(MissingPathFinder())
import foo


Add a "foo.missing" file to your working directory and you'll get the 
message from that instead of the usual one.

Cheers,
Steve