Author William.Schwartz
Recipients William.Schwartz, barry, jaraco
Date 2020-12-03.16:07:46
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> If the issue has been fixed on Python 3.9 but not on 3.8, then it was likely a redesign that enabled the improved behavior

That appears to be the case: path() shares code with files().

> a redesign that won't be ported back to Python 3.8 and earlier.

Nor should it.

> In these situations, the best recommendation is often to just rely on importlib_resources (the backport) for those older Python versions.

I do not need the files() API or anything else added in 3.9 at this time. I just need the existing 3.7/3.8 functionality to work as documented.

> have you considered using importlib_resources for Python 3.8 and earlier? That would likely also address the issue and you could adopt it sooner.

My application is sensitive to the size of the installed site-packages both in bytes and in just the number of packages. Yes, importlib_resources would very likely solve the problem reported in the OP. However I don't need the files() API, so adding an extra requirement feels like a heavy solution.

> To some extent, the behavior you've described could be considered a bug or could be considered a feature request (add support for path on packages that have no __spec__.origin). I am not aware whether this limitation was by design or incidental.

I agree there should be a high bar for patching old versions, but I posit that the behavior is an unintentional bug. In particular, I believe the behavior contradicts the documentation. Below I link and quote relevant documentation.

The function in question:

> importlib.resources.path(package, resource)¶
>     ...package is either a name or a module object which conforms to the
>     Package requirements.
https://docs.python.org/3.8/library/importlib.html#importlib.resources.path

So we jump to Package:

> importlib.resources.Package
>     The Package type is defined as Union[str, ModuleType]. This means that
>     where the function describes accepting a Package, you can pass in either a
>     string or a module. Module objects must have a resolvable
>     __spec__.submodule_search_locations that is not None.
https://docs.python.org/3.8/library/importlib.html#importlib.resources.Package


The Package type restricts the types of modules based on __spec__.submodule_search_locations. This suggests to me that the original author thought about which __spec__s to accept and which to reject but chose not to say anything about __spec__.origin, which is documented as possibly being None:

> class importlib.machinery.ModuleSpec(...)
>     ...module.__spec__.origin == module.__file__.... Normally “origin” should
>     be set, but it may be None (the default) which indicates it is unspecified
>     (e.g. for namespace packages).
https://docs.python.org/3.8/library/importlib.html#importlib.machinery.ModuleSpec.origin

In particular, __spec__.origin *should* be None in certain situations:

> __file__
>     __file__ is optional.... The import system may opt to leave __file__ unset
>     if it has no semantic meaning (e.g. a module loaded from a database).
https://docs.python.org/3.8/reference/import.html#__file__

Taken together, the foregoing passages describe an `import` API in which path() works for all modules that are packages (i.e., __spec__.submodule_search_locations is not None), and in which some packages' __spec__.origin is None. That path() fails in practice to support some packages is therefore a bug not a the absence of a feature.

Regardless of whether PR 23611 is accepted, the test that it adds should be added to Python master to guard against regressions. I can submit that as a separate PR. Before I do that, do I need to create a new bpo ticket, or can I just reference this one?
History
Date User Action Args
2020-12-03 16:07:46William.Schwartzsetrecipients: + William.Schwartz, barry, jaraco
2020-12-03 16:07:46William.Schwartzsetmessageid: <1607011666.47.0.977434034308.issue42531@roundup.psfhosted.org>
2020-12-03 16:07:46William.Schwartzlinkissue42531 messages
2020-12-03 16:07:46William.Schwartzcreate