On Mon, Nov 28, 2016 at 5:28 AM, Tomas Orsava <torsava@redhat.com> wrote: [...]
Specification =============
When, for any reason, a standard library module is not to be included with the rest, a file with its name and the extension ``.missing.py`` shall be created and placed in the directory the module itself would have occupied. This file can contain any Python code, however, it *should* raise a ModuleNotFoundError_ with a helpful error message.
Currently, when Python tries to import a module ``XYZ``, the ``FileFinder`` path hook goes through the entries in ``sys.path``, and in each location looks for a file whose name is ``XYZ`` with one of the valid suffixes (e.g. ``.so``, ..., ``.py``, ..., ``.pyc``). The suffixes are tried in order. If none of them are found, Python goes on to try the next directory in ``sys.path``.
The ``.missing.py`` extension will be added to the end of the list, and configured to be handled by ``SourceFileLoader``. Thus, if a module is not found in its proper location, the ``XYZ.missing.py`` file is found and executed, and further locations are not searched.
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. This also suggests that the overall error-handling flow for 'import foo' should look like: 1) run foo.missing.py 2) if it raises an exception: propagate that 3) otherwise, if sys.modules["foo"] is missing: raise some variety of ImportError. 4) otherwise, use sys.modules["foo"] as the object that should be bound to 'foo' in the original invoker's namespace I think this might make everyone who was worried about exception handling downthread happy -- it allows a .missing.py file to successfully import if it really wants to, but only if it explicitly fulfills 'import' requirement that the module should somehow be made available. -n -- Nathaniel J. Smith -- https://vorpus.org