PEP 810: Add rejected idea for object transformation approach (#4676)

Author: pablogsal

peps/pep-0810.rst

@@ -1735,6 +1735,55 @@ Past decisions that violated this principle of keeping core abstractions clean
 have caused significant pain in the CPython ecosystem, making optimization
 difficult and introducing subtle bugs.
 
+Transforming lazy objects via ``__class__`` mutation
+----------------------------------------------------
+
+An alternative implementation approach was proposed where lazy import objects
+would be transformed into their final form by mutating their internal state,
+rather than replacing the object entirely. Under this approach, a lazy object
+would be transformed in-place after the actual import completes.
+
+This approach was rejected for several reasons:
+
+1. This technique could potentially work for module objects, but breaks down
+   completely for arbitrary objects imported via ``from`` statements. When a
+   user writes ``lazy from foo import bar``, the object ``bar`` could be any
+   Python object (a function, class, constant, etc.), not just a module. Any
+   transformation approach would require that the lazy proxy object have
+   compatible memory layout and other considerations with the target object,
+   which is impossible to know before loading the module. This creates a
+   fundamental asymmetry where ``lazy import x`` and ``lazy from x import y``
+   would require completely different implementation strategies, with the latter
+   still needing the proxy replacement mechanism.
+
+2. Even for module objects, the approach has fundamental limitations. Some
+   implementations substitute custom classes in ``sys.modules`` that inherit
+   from or replace the standard module type. These custom module classes can
+   have different memory layouts and sizes than ``PyModuleObject``. The
+   transformation approach cannot work with such generic custom module
+   implementations, creating fragility and maintenance burden across the
+   ecosystem.
+
+3. While the transformation approach might appear simpler in some respects, this
+   is somewhat subjective. It introduces a bifurcated implementation: one path
+   for modules and a completely different path for non-module objects. Whether
+   this is simpler than a unified proxy mechanism depends on perspective and
+   implementation details.
+
+4. Any code holding a reference to the object before transformation will see a
+   different type after transformation. This can break code that checks object
+   types or relies on type stability, particularly C extensions that cache type
+   pointers or use ``PyObject_TypeCheck``. The transformation also requires
+   careful coordination between the lazy import machinery and the type system to
+   ensure that the object remains valid throughout the transformation process.
+   The current proxy-based design avoids these issues by maintaining clear
+   boundaries between the lazy proxy and the actual imported object.
+
+The current design, which uses object replacement through the ``LazyImportType``
+proxy pattern, provides a consistent mechanism that works uniformly for both
+``import`` and ``from ... import`` statements while maintaining cleaner
+separation between the lazy import machinery and Python's core object model.
+
 Making ``lazy`` imports find the module without loading it
 ----------------------------------------------------------