Note that sys._getframe is not guaranteed to exist in all implementations of…

Note that sys._getframe is not guaranteed to exist in all implementations of Python, and a corresponding note in inspect.currentframe. Issue 6712.

Doc/library/inspect.rst

@@ -567,6 +567,11 @@ line.
 
    Return the frame object for the caller's stack frame.
 
+   This function relies on Python stack frame support in the interpreter, which
+   isn't guaranteed to exist in all implementations of Python. If running in
+   an implmentation without Python stack frame support this function returns
+   ``None``.
+
 
 .. function:: stack([context])
 

Doc/library/sys.rst

@@ -417,7 +417,8 @@ always available.
    that is deeper than the call stack, :exc:`ValueError` is raised.  The default
    for *depth* is zero, returning the frame at the top of the call stack.
 
-   This function should be used for internal and specialized purposes only.
+   This function should be used for internal and specialized purposes only. It
+   is not guaranteed to exist in all implementations of Python.
 
 
 .. function:: getprofile()