bpo-39947: Add PyInterpreterState_Get() function (GH-18979)

* Rename _PyInterpreterState_Get() to PyInterpreterState_Get() and
  move it the limited C API.
* Add _PyInterpreterState_Get() alias to PyInterpreterState_Get() for
  backward compatibility with Python 3.8.

2 files changed, 14 insertions, 0 deletions

diff --git a/Doc/c-api/init.rst b/Doc/c-api/init.rst
index 747278c..661ed59 100644
--- a/Doc/c-api/init.rst
+++ b/Doc/c-api/init.rst
@@ -1072,6 +1072,18 @@ All of the following functions must be called after :c:func:`Py_Initialize`.
     to :c:func:`PyThreadState_Clear`.
 
 
+.. c:function:: PyInterpreterState* PyInterpreterState_Get(void)
+
+   Get the current interpreter.
+
+   Issue a fatal error if there no current Python thread state or no current
+   interpreter. It cannot return NULL.
+
+   The caller must hold the GIL.
+
+   .. versionadded:: 3.9
+
+
 .. c:function:: PY_INT64_T PyInterpreterState_GetID(PyInterpreterState *interp)
 
    Return the interpreter's unique ID.  If there was any error in doing
diff --git a/Doc/whatsnew/3.9.rst b/Doc/whatsnew/3.9.rst
index 0b61fb8..4373e37 100644
--- a/Doc/whatsnew/3.9.rst
+++ b/Doc/whatsnew/3.9.rst
@@ -406,6 +406,8 @@ Optimizations
 Build and C API Changes
 =======================
 
+* New :c:func:`PyInterpreterState_Get` function.
+
 * Add ``--with-platlibdir`` option to the ``configure`` script: name of the
   platform-specific library directory, stored in the new :attr:`sys.platlibdir`
   attribute. See :attr:`sys.platlibdir` attribute for more information.