Issue #23014: Make importlib.abc.Loader.create_module() required when

importlib.abc.Loader.exec_module() is also defined.

Before this change, create_module() was optional **and** could return
None to trigger default semantics. This change now reduces the
options for choosing default semantics to one and in the most
backporting-friendly way (define create_module() to return None).

3 files changed, 29 insertions, 11 deletions

diff --git a/Doc/library/importlib.rst b/Doc/library/importlib.rst
index 3af8d62..9c6b280 100644
--- a/Doc/library/importlib.rst
+++ b/Doc/library/importlib.rst
@@ -347,13 +347,16 @@ ABC hierarchy::
 
     .. method:: create_module(spec)
 
-       An optional method that returns the module object to use when
-       importing a module.  create_module() may also return ``None``,
-       indicating that the default module creation should take place
-       instead.
+       A method that returns the module object to use when
+       importing a module.  This method may return ``None``,
+       indicating that default module creation semantics should take place.
 
        .. versionadded:: 3.4
 
+       .. versionchanged:: 3.5
+          Starting in Python 3.6, this method will not be optional when
+          :meth:`exec_module` is defined.
+
     .. method:: exec_module(module)
 
        An abstract method that executes the module in its own namespace
@@ -417,7 +420,7 @@ ABC hierarchy::
 
         .. deprecated:: 3.4
            The recommended API for loading a module is :meth:`exec_module`
-           (and optionally :meth:`create_module`).  Loaders should implement
+           (and :meth:`create_module`).  Loaders should implement
            it instead of load_module().  The import machinery takes care of
            all the other responsibilities of load_module() when exec_module()
            is implemented.
@@ -1136,9 +1139,9 @@ an :term:`importer`.
 
 .. function:: module_from_spec(spec)
 
-   Create a new module based on **spec**.
+   Create a new module based on **spec** and ``spec.loader.create_module()``.
 
-   If the module object is from ``spec.loader.create_module()``, then any
+   If ``spec.loader.create_module()`` does not return ``None``, then any
    pre-existing attributes will not be reset. Also, no :exc:`AttributeError`
    will be raised if triggered while accessing **spec** or setting an attribute
    on the module.
@@ -1234,9 +1237,10 @@ an :term:`importer`.
    module has an attribute accessed.
 
    This class **only** works with loaders that define
-   :meth:`importlib.abc.Loader.exec_module` as control over what module type
-   is used for the module is required. For the same reasons, the loader
-   **cannot** define :meth:`importlib.abc.Loader.create_module`. Finally,
+   :meth:`~importlib.abc.Loader.exec_module` as control over what module type
+   is used for the module is required. For those same reasons, the loader's
+   :meth:`~importlib.abc.Loader.create_module` method will be ignored (i.e., the
+   loader's method should only return ``None``). Finally,
    modules which substitute the object placed into :attr:`sys.modules` will
    not work as there is no way to properly replace the module references
    throughout the interpreter safely; :exc:`ValueError` is raised if such a
diff --git a/Doc/reference/import.rst b/Doc/reference/import.rst
index 99b77be..2d60e62 100644
--- a/Doc/reference/import.rst
+++ b/Doc/reference/import.rst
@@ -339,6 +339,7 @@ of what happens during the loading portion of import::
 
     module = None
     if spec.loader is not None and hasattr(spec.loader, 'create_module'):
+        # It is assumed 'exec_module' will also be defined on the loader.
         module = spec.loader.create_module(spec)
     if module is None:
         module = ModuleType(spec.name)
@@ -427,7 +428,7 @@ Module loaders may opt in to creating the module object during loading
 by implementing a :meth:`~importlib.abc.Loader.create_module` method.
 It takes one argument, the module spec, and returns the new module object
 to use during loading.  ``create_module()`` does not need to set any attributes
-on the module object.  If the loader does not define ``create_module()``, the
+on the module object.  If the method returns ``None``, the
 import machinery will create the new module itself.
 
 .. versionadded:: 3.4
@@ -462,6 +463,11 @@ import machinery will create the new module itself.
       module(s), and only if the loader itself has loaded the module(s)
       explicitly.
 
+.. versionchanged:: 3.5
+   A :exc:`DeprecationWarning` is raised when ``exec_module()`` is defined but
+   ``create_module()`` is not. Starting in Python 3.6 it will be an error to not
+   define ``create_module()`` on a loader attached to a ModuleSpec.
+
 Module spec
 -----------
 
diff --git a/Doc/whatsnew/3.5.rst b/Doc/whatsnew/3.5.rst
index def4add..f3cc514 100644
--- a/Doc/whatsnew/3.5.rst
+++ b/Doc/whatsnew/3.5.rst
@@ -456,6 +456,14 @@ Changes in the Python API
   `http.client` and `http.server` remain available for backwards compatibility.
   (Contributed by Demian Brecht in :issue:`21793`.)
 
+* When an import loader defines :meth:`~importlib.machinery.Loader.exec_module`
+  it is now expected to also define
+  :meth:`~importlib.machinery.Loader.create_module` (raises a
+  :exc:`DeprecationWarning` now, will be an error in Python 3.6). If the loader
+  inherits from :class:`importlib.abc.Loader` then there is nothing to do, else
+  simply define :meth:`~importlib.machinery.Loader.create_module` to return
+  ``None`` (:issue:`23014`).
+
 Changes in the C API
 --------------------