[Python-ideas] PEP 420: implicit namespace sub-package

[Barry Warsaw]

Gallian Colombeau wrote on 8/27/18 06:49:

> As I understand, for a package to allow being extended in this way, it 
> must be a namespace package and not contain a marker file. As a matter 
> of fact, no sub-package until the top level package can have a marker file:

No, that's not true.

> However, what is not discussed is "implicit namespace sub-package". In 

There really is no such thing.  Packages are either PEP 420 style 
namespace packages, or regular packages.  The latter contain __init__.py 
files.

The language reference goes into quite a bit of detail on the matter.

https://docs.python.org/3/reference/import.html#packages

> Python 3.6 (I guess since the first implementation), if you have this 
> layout:
> Lib/test/namespace_pkgs
>      project1
>          parent # Regular package
>              __init__.py
>              child # Namespace package
>                  one.py
> 
> you get "parent" as a regular package and "parent.child" as a namespace 
> package and it works (although now, every package data directory became 
> namespace packages and are importable, which may or may not be 
> desirable). The point is, does that add any value? 

Personally, I don't think so.  You can do it, but it's not the intended 
purpose, so you're on your own.

> I wasn't able to find 
> any discussion about this and, as far as I can see, there is actually no 
> use case for this as there is no possible way to contribute to the 
> "parent.child" namespace. Is that an intended behavior of PEP 420?

There can be use cases for subpackage namespace packages, although they 
are definitely more rare than top-level namespace packages.  One 
possibility would be a plugin system, say for application 'foo', where 
they reserve a subpackage for separate-distribution plugins,  E.g. 
foo.plugins.ext where foo/plugins/ext has no __init__.py file.

> Wouldn't it be more appropriate to enforce a sub-package to be a regular 
> package if the parent package is a regular package?

As Brett says, it's probably way too late to change this.

-Barry