gh-148588: Clarify __lazy_modules__ must be a container (__contains__), not any iterable

Author: johnslavik

Doc/reference/datamodel.rst

@@ -1124,7 +1124,8 @@ the following writable attributes:
 
 .. attribute:: module.__lazy_modules__
 
-   A container of fully qualified module name strings.  When defined
+   A container (an object implementing :meth:`~object.__contains__`) of fully
+   qualified module name strings.  When defined
    at module scope, any regular :keyword:`import` statement in that module whose
    target module name appears in this container is treated as a
    :ref:`lazy import <lazy-imports>`, as if the :keyword:`lazy` keyword had

Doc/reference/simple_stmts.rst

@@ -931,7 +931,8 @@ Compatibility mode via ``__lazy_modules__``
 As an alternative to using the :keyword:`lazy` keyword, a module can opt
 into lazy loading for specific imports by defining a module-level
 :attr:`~module.__lazy_modules__` variable.  When present, it must be a
-container of fully qualified module name strings.  Any regular (non-``lazy``)
+container (an object implementing :meth:`~object.__contains__`) of fully
+qualified module name strings.  Any regular (non-``lazy``)
 :keyword:`import` statement at module scope whose target appears in
 :attr:`!__lazy_modules__` is treated as a lazy import, exactly as if the
 :keyword:`lazy` keyword had been used.

Doc/whatsnew/3.15.rst

@@ -185,8 +185,8 @@ raise :exc:`SyntaxError`).
 For code that cannot use the ``lazy`` keyword directly (for example, when
 supporting Python versions older than 3.15 while still leveraging lazy
 imports on 3.15+), a module can define
-:attr:`~module.__lazy_modules__` as an :term:`iterable` of fully qualified module
-name strings.  Regular ``import`` statements for those modules are then treated
+:attr:`~module.__lazy_modules__` as a container (an object implementing
+:meth:`~object.__contains__`) of fully qualified module name strings.  Regular ``import`` statements for those modules are then treated
 as lazy, with the same semantics as the ``lazy`` keyword::
 
    __lazy_modules__ = ["json", "pathlib"]