gh-106137: Add SoftDeprecationWarning category

* Add SoftDeprecationWarning warning category.
* Add default warnings filters for SoftDeprecationWarning: ignore
  SoftDeprecationWarning by default, except in the __main__ module
  (similar to PEP 565).
* Add warnings._soft_deprecated(): only emit SoftDeprecationWarning
  in Python Development Mode and if Python is built in debug mode.
* Add PyExc_SoftDeprecationWarning to the limited C API.

Author: vstinner

Doc/data/stable_abi.dat

@@ -802,6 +802,19 @@ The following exceptions are used as warning categories; see the
    The deprecation policy is described in :pep:`387`.
 
 
+.. exception:: SoftDeprecationWarning
+
+   Base class for warnings about soft deprecated features when those warnings
+   are intended for other Python developers.
+
+   Ignored by the default warning filters, except in the ``__main__`` module.
+   Enabling the :ref:`Python Development Mode <devmode>` shows this warning.
+
+   The soft deprecation policy is described in :pep:`387`.
+
+   .. versionadded:: 3.13
+
+
 .. exception:: PendingDeprecationWarning
 
    Base class for warnings about features which are obsolete and

Doc/library/exceptions.rst

@@ -76,6 +76,9 @@ New Features
 Other Language Changes
 ======================
 
+* Add :exc:`SoftDeprecationWarning` warning category to implement soft
+  deprecation. The soft deprecation policy is described in :pep:`387`.
+  (Contributed by Victor Stinner in :gh:`106137`.)
 
 
 New Modules
@@ -441,6 +444,10 @@ New Features
   ``NULL`` if the referent is no longer live.
   (Contributed by Victor Stinner in :gh:`105927`.)
 
+* Add :exc:`PyExc_SoftDeprecationWarning` warning category to implement soft
+  deprecation. The soft deprecation policy is described in :pep:`387`.
+  (Contributed by Victor Stinner in :gh:`106137`.)
+
 Porting to Python 3.13
 ----------------------
 

Doc/whatsnew/3.13.rst

@@ -153,6 +153,7 @@ PyAPI_DATA(PyObject *) PyExc_Warning;
 PyAPI_DATA(PyObject *) PyExc_UserWarning;
 PyAPI_DATA(PyObject *) PyExc_DeprecationWarning;
 PyAPI_DATA(PyObject *) PyExc_PendingDeprecationWarning;
+PyAPI_DATA(PyObject *) PyExc_SoftDeprecationWarning;
 PyAPI_DATA(PyObject *) PyExc_SyntaxWarning;
 PyAPI_DATA(PyObject *) PyExc_RuntimeWarning;
 PyAPI_DATA(PyObject *) PyExc_FutureWarning;

Include/pyerrors.h

@@ -62,6 +62,7 @@ BaseException
            ├── PendingDeprecationWarning
            ├── ResourceWarning
            ├── RuntimeWarning
+           ├── SoftDeprecationWarning
            ├── SyntaxWarning
            ├── UnicodeWarning
            └── UserWarning

Lib/test/exception_hierarchy.txt

@@ -13,6 +13,13 @@ def import_deprecated(name):
         return importlib.import_module(name)
 
 
+def import_soft_deprecated(name):
+    """Import *name* while suppressing SoftDeprecationWarning."""
+    with warnings.catch_warnings():
+        warnings.simplefilter('ignore', category=SoftDeprecationWarning)
+        return importlib.import_module(name)
+
+
 def check_syntax_warning(testcase, statement, errtext='',
                          *, lineno=1, offset=None):
     # Test also that a warning is emitted only once.

Lib/test/support/warnings_helper.py

@@ -1242,8 +1242,10 @@ def test_default_filter_configuration(self):
                 main_module_filter = "__main__"
             expected_default_filters = [
                 ('default', None, DeprecationWarning, main_module_filter, 0),
+                ('default', None, SoftDeprecationWarning, main_module_filter, 0),
                 ('ignore', None, DeprecationWarning, None, 0),
                 ('ignore', None, PendingDeprecationWarning, None, 0),
+                ('ignore', None, SoftDeprecationWarning, None, 0),
                 ('ignore', None, ImportWarning, None, 0),
                 ('ignore', None, ResourceWarning, None, 0),
             ]

Lib/test/test_stable_abi_ctypes.py

@@ -529,6 +529,13 @@ def _deprecated(name, message=_DEPRECATED_MSG, *, remove, _version=sys.version_i
         warn(msg, DeprecationWarning, stacklevel=3)
 
 
+def _soft_deprecated(msg):
+    # only emit SoftDeprecationWarning in the Python Development Mode
+    # and if Python is built in debug mode
+    if sys.flags.dev_mode or hasattr(sys, 'gettotalrefcount'):
+        warn(msg, SoftDeprecationWarning, stacklevel=3)
+
+
 # Private utility function called by _PyErr_WarnUnawaitedCoroutine
 def _warn_unawaited_coroutine(coro):
     msg_lines = [
@@ -587,8 +594,11 @@ def _filters_mutated():
     if not hasattr(sys, 'gettotalrefcount'):
         filterwarnings("default", category=DeprecationWarning,
                        module="__main__", append=1)
+        filterwarnings("default", category=SoftDeprecationWarning,
+                       module="__main__", append=1)
         simplefilter("ignore", category=DeprecationWarning, append=1)
         simplefilter("ignore", category=PendingDeprecationWarning, append=1)
+        simplefilter("ignore", category=SoftDeprecationWarning, append=1)
         simplefilter("ignore", category=ImportWarning, append=1)
         simplefilter("ignore", category=ResourceWarning, append=1)
 

Lib/test/test_warnings/__init__.py

@@ -0,0 +1,3 @@
+Add :exc:`PyExc_SoftDeprecationWarning` warning category to implement soft
+deprecation. The soft deprecation policy is described in :pep:`387`. Patch
+by Victor Stinner.