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

[Gallian Colombeau]

Hello,

the PEP 420 (Implicit Namespace Packages) is quite descriptive about the 
problem and the solution implemented back in Python 3.3 but I feel there 
may be a part missing (but maybe this is categorized as an 
implementation detail).

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:

Lib/test/namespace_pkgs
     project1
         parent
             child
                 one.py
     project2
         parent
             child
                 two.py

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

Again, I may have missed something or misinterpreted PEP 420 but this 
contributes to the "Implicit package directories introduce ambiguity 
into file system layouts." point by Nick Coghlan that was supposed to be 
addressed in PEP 395.

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

Gallian
-- 

*Gallian Colombeau *
*Software engineer *

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