cpython/Lib/collections/__init__.py module-level __getattr__

Raymond Hettinger talked about the module-level dunder method __getattr__ and then linked to an old version of the module-level attribute of the module collections.

This module-level __getattr__ is used to issue deprecation warnings when certain aliases from the module collections.abc are accessed. It wouldn't make sense to issue the deprecation warnings as soon as the module is imported, or when a different object is imported from the module, so the module-level __getattr__ is put in place for that effect:

def __getattr__(name):
    # For backwards compatibility, continue to make the collections ABCs
    # through Python 3.6 available through the collections module.
    # Note, no new collections ABCs were added in Python 3.7
    if name in _collections_abc.__all__:
        obj = getattr(_collections_abc, name)
        import warnings
        warnings.warn("Using or importing the ABCs from 'collections' instead "
                      "of from 'collections.abc' is deprecated since Python 3.3, "
                      "and in 3.10 it will stop working",
                      DeprecationWarning, stacklevel=2)
        globals()[name] = obj
        return obj
    raise AttributeError(f'module {__name__!r} has no attribute {name!r}')