I'm trying to create a custom module type for a custom loader where the returned modules are immutable.  But I'm running into an issue where the immutable module type can't be used as a module for a package.  That's because the import machinery calls setattr to set the module as an attribute on it's parent in _boostrap.py

        # Set the module as an attribute on its parent.
        parent_module = sys.modules[parent]
        setattr(parent_module, name.rpartition('.')[2], module)

I'd be okay if for these immutable module types they simply didn't have their children packages published on them.

A simple simulation of this is a package which replaces its self with an object which doesn't support adding arbitrary attributes:

x/__init__.py:
import sys

class MyMod(object):
    __slots__ = ['__builtins__', '__cached__', '__doc__', '__file__', '__loader__', '__name__', '__package__', '__path__', '__spec__']
    def __init__(self):
        for attr in self.__slots__:
            setattr(self, attr, globals()[attr])


sys.modules['x'] = MyMod()

x/y.py:
# Empty file

>>> from x import y
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<frozen importlib._bootstrap>", line 983, in _find_and_load
  File "<frozen importlib._bootstrap>", line 971, in _find_and_load_unlocked
AttributeError: 'MyMod' object has no attribute 'y'

There's a few different options I could see on how this could be supported:
    1) Simply handle the attribute error and allow things to continue
    2) Add ability for the modules loader to perform the set, and fallback to setattr if one isn't available.  Such as:
         getattr(parent_module, 'add_child_module', setattr)(parent_module, name.rpartition('.')[2], module)
    3) Add the ability for the module type to handle the setattr:
         getattr(type(parent_module), 'add_child_module', fallback)(parent_module, 
, name.rpartition('.')[2], module)

So I think this is way too marginal a use-case to expand the API to let loaders inject themselves into the semantics of it.

I assume going with option 1 but raising an ImportWarning would be too noisy for your use-case? If not I'm totally fine with that solution.

I think the warning shouldn't be too bad.  It looks like ImportWarnings are filtered by default already, and the extra overhead of raising a warning in this case probably is nothing compared to the actual work in loading the module.

I apologize for the noise caused by the wrong PR connection.

New changeset 9b6fec46513006d7b06fcb645cca6e4f5bf7c7b8 by Dino Viehland in branch 'master':
bpo-39336: Allow packages to not let their child modules be set on them (#18006)
https://github.com/python/cpython/commit/9b6fec46513006d7b06fcb645cca6e4f5bf7c7b8

test_unwritable_module() fails on AMD64 Fedora Stable Clang Installed 3.x: bpo-39459.

commit 2528a6c3d0660c03ae43d796628462ccf8e58190
Author: Dino Viehland <dinoviehland@gmail.com>
Date:   Mon Jan 27 14:04:56 2020 -0800

    Add test.test_import.data.unwritable package to makefile (#18211)