diff --git a/Doc/c-api/exceptions.rst b/Doc/c-api/exceptions.rst
index b7175166a6f009..e7805ba143c584 100644
--- a/Doc/c-api/exceptions.rst
+++ b/Doc/c-api/exceptions.rst
@@ -358,7 +358,7 @@ an error value).
 .. c:function:: int PyErr_ResourceWarning(PyObject *source, Py_ssize_t stack_level, const char *format, ...)
 
    Function similar to :c:func:`PyErr_WarnFormat`, but *category* is
-   :exc:`ResourceWarning` and pass *source* to :func:`warnings.WarningMessage`.
+   :exc:`ResourceWarning` and it passes *source* to :func:`warnings.WarningMessage`.
 
    .. versionadded:: 3.6
 
diff --git a/Doc/c-api/init.rst b/Doc/c-api/init.rst
index c34b1174a5913d..badea5a01362a5 100644
--- a/Doc/c-api/init.rst
+++ b/Doc/c-api/init.rst
@@ -1091,6 +1091,32 @@ All of the following functions must be called after :c:func:`Py_Initialize`.
 
    .. versionadded:: 3.8
 
+.. c:type:: PyObject* (*_PyFrameEvalFunction)(PyThreadState *tstate, PyFrameObject *frame, int throwflag)
+
+   Type of a frame evaluation function.
+
+   The *throwflag* parameter is used by the ``throw()`` method of generators:
+   if non-zero, handle the current exception.
+
+   .. versionchanged:: 3.9
+      The function now takes a *tstate* parameter.
+
+.. c:function:: _PyFrameEvalFunction _PyInterpreterState_GetEvalFrameFunc(PyInterpreterState *interp)
+
+   Get the frame evaluation function.
+
+   See the :pep:`523` "Adding a frame evaluation API to CPython".
+
+   .. versionadded:: 3.9
+
+.. c:function:: void _PyInterpreterState_SetEvalFrameFunc(PyInterpreterState *interp, _PyFrameEvalFunction eval_frame);
+
+   Set the frame evaluation function.
+
+   See the :pep:`523` "Adding a frame evaluation API to CPython".
+
+   .. versionadded:: 3.9
+
 
 .. c:function:: PyObject* PyThreadState_GetDict()
 
diff --git a/Doc/whatsnew/3.9.rst b/Doc/whatsnew/3.9.rst
index 9948539b01c586..0b61fb8f0deb95 100644
--- a/Doc/whatsnew/3.9.rst
+++ b/Doc/whatsnew/3.9.rst
@@ -356,7 +356,7 @@ running.
 sys
 ---
 
-Add a new :attr:`sys.platlitdir` attribute: name of the platform-specific
+Add a new :attr:`sys.platlibdir` attribute: name of the platform-specific
 library directory. It is used to build the path of platform-specific dynamic
 libraries and the path of the standard library. It is equal to ``"lib"`` on
 most platforms.  On Fedora and SuSE, it is equal to ``"lib64"`` on 64-bit
@@ -407,7 +407,7 @@ Build and C API Changes
 =======================
 
 * Add ``--with-platlibdir`` option to the ``configure`` script: name of the
-  platform-specific library directory, stored in the new :attr:`sys.platlitdir`
+  platform-specific library directory, stored in the new :attr:`sys.platlibdir`
   attribute. See :attr:`sys.platlibdir` attribute for more information.
   (Contributed by Jan Matějek, Matěj Cepl, Charalampos Stratakis and Victor Stinner in :issue:`1294959`.)
 
@@ -487,6 +487,10 @@ Build and C API Changes
 
   (Contributed by Victor Stinner in :issue:`38644` and :issue:`39542`.)
 
+* ``PyInterpreterState.eval_frame`` (:pep:`523`) now requires a new mandatory
+  *tstate* parameter (``PyThreadState*``).
+  (Contributed by Victor Stinner in :issue:`38500`.)
+
 
 Deprecated
 ==========
diff --git a/Include/cpython/ceval.h b/Include/cpython/ceval.h
index e601304589f762..f03b53ade92983 100644
--- a/Include/cpython/ceval.h
+++ b/Include/cpython/ceval.h
@@ -21,7 +21,7 @@ PyAPI_FUNC(PyObject *) _PyEval_GetBuiltinId(_Py_Identifier *);
    flag was set, else return 0. */
 PyAPI_FUNC(int) PyEval_MergeCompilerFlags(PyCompilerFlags *cf);
 
-PyAPI_FUNC(PyObject *) _PyEval_EvalFrameDefault(struct _frame *f, int exc);
+PyAPI_FUNC(PyObject *) _PyEval_EvalFrameDefault(PyThreadState *tstate, struct _frame *f, int exc);
 
 PyAPI_FUNC(void) _PyEval_SetSwitchInterval(unsigned long microseconds);
 PyAPI_FUNC(unsigned long) _PyEval_GetSwitchInterval(void);
diff --git a/Include/cpython/pystate.h b/Include/cpython/pystate.h
index d1792575c97378..fbb0899186f607 100644
--- a/Include/cpython/pystate.h
+++ b/Include/cpython/pystate.h
@@ -186,6 +186,16 @@ PyAPI_FUNC(void) PyThreadState_DeleteCurrent(void);
 
 typedef struct _frame *(*PyThreadFrameGetter)(PyThreadState *self_);
 
+/* Frame evaluation API */
+
+typedef PyObject* (*_PyFrameEvalFunction)(PyThreadState *tstate, struct _frame *, int);
+
+PyAPI_FUNC(_PyFrameEvalFunction) _PyInterpreterState_GetEvalFrameFunc(
+    PyInterpreterState *interp);
+PyAPI_FUNC(void) _PyInterpreterState_SetEvalFrameFunc(
+    PyInterpreterState *interp,
+    _PyFrameEvalFunction eval_frame);
+
 /* cross-interpreter data */
 
 struct _xid;
diff --git a/Include/internal/pycore_ceval.h b/Include/internal/pycore_ceval.h
index 70ce0ee5f70d34..23d80916fde389 100644
--- a/Include/internal/pycore_ceval.h
+++ b/Include/internal/pycore_ceval.h
@@ -40,7 +40,7 @@ void _PyEval_Fini(void);
 static inline PyObject*
 _PyEval_EvalFrame(PyThreadState *tstate, struct _frame *f, int throwflag)
 {
-    return tstate->interp->eval_frame(f, throwflag);
+    return tstate->interp->eval_frame(tstate, f, throwflag);
 }
 
 extern PyObject *_PyEval_EvalCode(
diff --git a/Include/internal/pycore_pystate.h b/Include/internal/pycore_pystate.h
index b5f509547207dd..0a835463625580 100644
--- a/Include/internal/pycore_pystate.h
+++ b/Include/internal/pycore_pystate.h
@@ -54,8 +54,6 @@ struct _ceval_runtime_state {
 
 /* interpreter state */
 
-typedef PyObject* (*_PyFrameEvalFunction)(struct _frame *, int);
-
 #define _PY_NSMALLPOSINTS           257
 #define _PY_NSMALLNEGINTS           5
 
diff --git a/Lib/_aix_support.py b/Lib/_aix_support.py
index 2c5cd3297de30d..45504934063df8 100644
--- a/Lib/_aix_support.py
+++ b/Lib/_aix_support.py
@@ -1,35 +1,20 @@
 """Shared AIX support functions."""
 
 import sys
-from sysconfig import get_config_var
+import sysconfig
 
-# subprocess is not necessarily available early in the build process
-# if not available, the config_vars are also definitely not available
-# supply substitutes to bootstrap the build
 try:
     import subprocess
-    _have_subprocess = True
-    _tmp_bd = get_config_var("AIX_BUILDDATE")
-    _bgt = get_config_var("BUILD_GNU_TYPE")
 except ImportError:  # pragma: no cover
-    _have_subprocess = False
-    _tmp_bd = None
-    _bgt = "powerpc-ibm-aix6.1.7.0"
-
-# if get_config_var("AIX_BUILDDATE") was unknown, provide a substitute,
-# impossible builddate to specify 'unknown'
-_MISSING_BD = 9898
-try:
-    _bd = int(_tmp_bd)
-except TypeError:
-    _bd = _MISSING_BD
-
-# Infer the ABI bitwidth from maxsize (assuming 64 bit as the default)
-_sz = 32 if sys.maxsize == (2**31-1) else 64
+    # _aix_support is used in distutils by setup.py to build C extensions,
+    # before subprocess dependencies like _posixsubprocess are available.
+    import _bootsubprocess as subprocess
 
 
 def _aix_tag(vrtl, bd):
     # type: (List[int], int) -> str
+    # Infer the ABI bitwidth from maxsize (assuming 64 bit as the default)
+    _sz = 32 if sys.maxsize == (2**31-1) else 64
     # vrtl[version, release, technology_level]
     return "aix-{:1x}{:1d}{:02d}-{:04d}-{}".format(vrtl[0], vrtl[1], vrtl[2], bd, _sz)
 
@@ -48,17 +33,12 @@ def _aix_bosmp64():
     The fileset bos.mp64 is the AIX kernel. It's VRMF and builddate
     reflect the current ABI levels of the runtime environment.
     """
-    if _have_subprocess:
-        # We expect all AIX systems to have lslpp installed in this location
-        out = subprocess.check_output(["/usr/bin/lslpp", "-Lqc", "bos.mp64"])
-        out = out.decode("utf-8").strip().split(":")  # type: ignore
-        # Use str() and int() to help mypy see types
-        return str(out[2]), int(out[-1])
-    else:
-        from os import uname
-
-        osname, host, release, version, machine = uname()
-        return "{}.{}.0.0".format(version, release), _MISSING_BD
+    # We expect all AIX systems to have lslpp installed in this location
+    out = subprocess.check_output(["/usr/bin/lslpp", "-Lqc", "bos.mp64"])
+    out = out.decode("utf-8")
+    out = out.strip().split(":")  # type: ignore
+    # Use str() and int() to help mypy see types
+    return (str(out[2]), int(out[-1]))
 
 
 def aix_platform():
@@ -87,8 +67,10 @@ def aix_platform():
 # extract vrtl from the BUILD_GNU_TYPE as an int
 def _aix_bgt():
     # type: () -> List[int]
-    assert _bgt
-    return _aix_vrtl(vrmf=_bgt)
+    gnu_type = sysconfig.get_config_var("BUILD_GNU_TYPE")
+    if not gnu_type:
+        raise ValueError("BUILD_GNU_TYPE is not defined")
+    return _aix_vrtl(vrmf=gnu_type)
 
 
 def aix_buildtag():
@@ -96,4 +78,12 @@ def aix_buildtag():
     """
     Return the platform_tag of the system Python was built on.
     """
-    return _aix_tag(_aix_bgt(), _bd)
+    # AIX_BUILDDATE is defined by configure with:
+    # lslpp -Lcq bos.mp64 | awk -F:  '{ print $NF }'
+    build_date = sysconfig.get_config_var("AIX_BUILDDATE")
+    try:
+        build_date = int(build_date)
+    except (ValueError, TypeError):
+        raise ValueError(f"AIX_BUILDDATE is not defined or invalid: "
+                         f"{build_date!r}")
+    return _aix_tag(_aix_bgt(), build_date)
diff --git a/Lib/test/test_buffer.py b/Lib/test/test_buffer.py
index 2ddca06b8b568a..d440bcf7e0faa1 100644
--- a/Lib/test/test_buffer.py
+++ b/Lib/test/test_buffer.py
@@ -2754,6 +2754,10 @@ def test_memoryview_cast_1D_ND(self):
         # be 1D, at least one format must be 'c', 'b' or 'B'.
         for _tshape in gencastshapes():
             for char in fmtdict['@']:
+                # Casts to _Bool are undefined if the source contains values
+                # other than 0 or 1.
+                if char == "?":
+                    continue
                 tfmt = ('', '@')[randrange(2)] + char
                 tsize = struct.calcsize(tfmt)
                 n = prod(_tshape) * tsize
diff --git a/Misc/NEWS.d/next/Build/2020-02-06-18-08-25.bpo-1294959.AZPg4R.rst b/Misc/NEWS.d/next/Build/2020-02-06-18-08-25.bpo-1294959.AZPg4R.rst
index 1642e6929bcef4..a90d6300f33bad 100644
--- a/Misc/NEWS.d/next/Build/2020-02-06-18-08-25.bpo-1294959.AZPg4R.rst
+++ b/Misc/NEWS.d/next/Build/2020-02-06-18-08-25.bpo-1294959.AZPg4R.rst
@@ -1,5 +1,5 @@
 Add ``--with-platlibdir`` option to the configure script: name of the
-platform-specific library directory, stored in the new :attr:`sys.platlitdir`
+platform-specific library directory, stored in the new :attr:`sys.platlibdir`
 attribute. It is used to build the path of platform-specific dynamic libraries
 and the path of the standard library. It is equal to ``"lib"`` on most
 platforms. On Fedora and SuSE, it is equal to ``"lib64"`` on 64-bit platforms.
diff --git a/Misc/NEWS.d/next/C API/2019-11-22-14-06-28.bpo-38500.nPEdjH.rst b/Misc/NEWS.d/next/C API/2019-11-22-14-06-28.bpo-38500.nPEdjH.rst
new file mode 100644
index 00000000000000..f1ccfacd2c8f96
--- /dev/null
+++ b/Misc/NEWS.d/next/C API/2019-11-22-14-06-28.bpo-38500.nPEdjH.rst	
@@ -0,0 +1,5 @@
+Add a private API to get and set the frame evaluation function: add
+:c:func:`_PyInterpreterState_GetEvalFrameFunc` and
+:c:func:`_PyInterpreterState_SetEvalFrameFunc` C functions.
+The :c:type:`_PyFrameEvalFunction` function type now takes a *tstate*
+parameter.
diff --git a/Misc/NEWS.d/next/C API/2020-03-12-00-27-26.bpo-39884.CGOJBO.rst b/Misc/NEWS.d/next/C API/2020-03-12-00-27-26.bpo-39884.CGOJBO.rst
new file mode 100644
index 00000000000000..c65dfdc21244a9
--- /dev/null
+++ b/Misc/NEWS.d/next/C API/2020-03-12-00-27-26.bpo-39884.CGOJBO.rst	
@@ -0,0 +1,2 @@
+:c:func:`PyDescr_NewMethod` and :c:func:`PyCFunction_NewEx` now include the
+method name in the SystemError "bad call flags" error message to ease debug.
diff --git a/Misc/NEWS.d/next/Library/2020-03-12-21-59-47.bpo-39936.Ca9IKe.rst b/Misc/NEWS.d/next/Library/2020-03-12-21-59-47.bpo-39936.Ca9IKe.rst
new file mode 100644
index 00000000000000..8d3b6fd974b795
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2020-03-12-21-59-47.bpo-39936.Ca9IKe.rst
@@ -0,0 +1,5 @@
+AIX: Fix _aix_support module when the subprocess is not available, when
+building Python from scratch. It now uses new private _bootsubprocess
+module, rather than having two implementations depending if subprocess is
+available or not. So _aix_support.aix_platform() result is now the same if
+subprocess is available or not.
diff --git a/Modules/_collectionsmodule.c b/Modules/_collectionsmodule.c
index d0a381deabf5d8..fd0e4edcddfdbe 100644
--- a/Modules/_collectionsmodule.c
+++ b/Modules/_collectionsmodule.c
@@ -2133,12 +2133,8 @@ defdict_repr(defdictobject *dd)
 static PyObject*
 defdict_or(PyObject* left, PyObject* right)
 {
-    int left_is_self = PyObject_IsInstance(left, (PyObject*)&defdict_type);
-    if (left_is_self < 0) {
-        return NULL;
-    }
     PyObject *self, *other;
-    if (left_is_self) {
+    if (PyObject_TypeCheck(left, &defdict_type)) {
         self = left;
         other = right;
     }
diff --git a/Modules/unicodedata.c b/Modules/unicodedata.c
index d06ba6c6ff3509..1a9e1c0b667d39 100644
--- a/Modules/unicodedata.c
+++ b/Modules/unicodedata.c
@@ -623,7 +623,7 @@ nfd_nfkd(PyObject *self, PyObject *input, int k)
 }
 
 static int
-find_nfc_index(PyObject *self, struct reindex* nfc, Py_UCS4 code)
+find_nfc_index(const struct reindex* nfc, Py_UCS4 code)
 {
     unsigned int index;
     for (index = 0; nfc[index].start; index++) {
@@ -709,7 +709,7 @@ nfc_nfkc(PyObject *self, PyObject *input, int k)
       }
 
       /* code is still input[i] here */
-      f = find_nfc_index(self, nfc_first, code);
+      f = find_nfc_index(nfc_first, code);
       if (f == -1) {
           output[o++] = code;
           i++;
@@ -732,7 +732,7 @@ nfc_nfkc(PyObject *self, PyObject *input, int k)
                   continue;
               }
           }
-          l = find_nfc_index(self, nfc_last, code1);
+          l = find_nfc_index(nfc_last, code1);
           /* i1 cannot be combined with i. If i1
              is a starter, we don't need to look further.
              Otherwise, record the combining class. */
@@ -757,7 +757,7 @@ nfc_nfkc(PyObject *self, PyObject *input, int k)
           assert(cskipped < 20);
           skipped[cskipped++] = i1;
           i1++;
-          f = find_nfc_index(self, nfc_first, output[o]);
+          f = find_nfc_index(nfc_first, output[o]);
           if (f == -1)
               break;
       }
diff --git a/Objects/descrobject.c b/Objects/descrobject.c
index 4ebbb74151a232..b448ec642683c2 100644
--- a/Objects/descrobject.c
+++ b/Objects/descrobject.c
@@ -888,7 +888,8 @@ PyDescr_NewMethod(PyTypeObject *type, PyMethodDef *method)
             vectorcall = method_vectorcall_O;
             break;
         default:
-            PyErr_SetString(PyExc_SystemError, "bad call flags");
+            PyErr_Format(PyExc_SystemError,
+                         "%s() method: bad call flags", method->ml_name);
             return NULL;
     }
 
diff --git a/Objects/methodobject.c b/Objects/methodobject.c
index 0d4570534b1ae5..16abded3854664 100644
--- a/Objects/methodobject.c
+++ b/Objects/methodobject.c
@@ -56,7 +56,8 @@ PyCFunction_NewEx(PyMethodDef *ml, PyObject *self, PyObject *module)
             vectorcall = cfunction_vectorcall_O;
             break;
         default:
-            PyErr_SetString(PyExc_SystemError, "bad call flags");
+            PyErr_Format(PyExc_SystemError,
+                         "%s() method: bad call flags", ml->ml_name);
             return NULL;
     }
 
diff --git a/Python/ceval.c b/Python/ceval.c
index 380212a71aaf75..ccd1c06a39cfd9 100644
--- a/Python/ceval.c
+++ b/Python/ceval.c
@@ -725,9 +725,7 @@ PyEval_EvalCode(PyObject *co, PyObject *globals, PyObject *locals)
 PyObject *
 PyEval_EvalFrame(PyFrameObject *f)
 {
-    /* This is for backward compatibility with extension modules that
-       used this API; core interpreter code should call
-       PyEval_EvalFrameEx() */
+    /* Function kept for backward compatibility */
     PyThreadState *tstate = _PyThreadState_GET();
     return _PyEval_EvalFrame(tstate, f, 0);
 }
@@ -740,8 +738,10 @@ PyEval_EvalFrameEx(PyFrameObject *f, int throwflag)
 }
 
 PyObject* _Py_HOT_FUNCTION
-_PyEval_EvalFrameDefault(PyFrameObject *f, int throwflag)
+_PyEval_EvalFrameDefault(PyThreadState *tstate, PyFrameObject *f, int throwflag)
 {
+    ensure_tstate_not_null(__func__, tstate);
+
 #ifdef DXPAIRS
     int lastopcode = 0;
 #endif
@@ -756,9 +756,6 @@ _PyEval_EvalFrameDefault(PyFrameObject *f, int throwflag)
     _Py_atomic_int * const eval_breaker = &ceval->eval_breaker;
     PyCodeObject *co;
 
-    PyThreadState * const tstate = _PyRuntimeState_GetThreadState(runtime);
-    ensure_tstate_not_null(__func__, tstate);
-
     /* when tracing we set things up so that
 
            not (instr_lb <= current_bytecode_offset < instr_ub)
@@ -1181,7 +1178,7 @@ _PyEval_EvalFrameDefault(PyFrameObject *f, int throwflag)
         goto error;
 
 #ifdef Py_DEBUG
-    /* PyEval_EvalFrameEx() must not be called with an exception set,
+    /* _PyEval_EvalFrameDefault() must not be called with an exception set,
        because it can clear it (directly or indirectly) and so the
        caller loses its exception */
     assert(!_PyErr_Occurred(tstate));
@@ -3702,7 +3699,7 @@ _PyEval_EvalFrameDefault(PyFrameObject *f, int throwflag)
     f->f_executing = 0;
     tstate->frame = f->f_back;
 
-    return _Py_CheckFunctionResult(tstate, NULL, retval, "PyEval_EvalFrameEx");
+    return _Py_CheckFunctionResult(tstate, NULL, retval, __func__);
 }
 
 static void
diff --git a/Python/pylifecycle.c b/Python/pylifecycle.c
index bc32eff436e68b..e63ecf70f35c1d 100644
--- a/Python/pylifecycle.c
+++ b/Python/pylifecycle.c
@@ -65,7 +65,7 @@ extern grammar _PyParser_Grammar; /* From graminit.c */
 /* Forward declarations */
 static PyStatus add_main_module(PyInterpreterState *interp);
 static PyStatus init_import_site(void);
-static PyStatus init_set_builtins_open(PyThreadState *tstate);
+static PyStatus init_set_builtins_open(void);
 static PyStatus init_sys_streams(PyThreadState *tstate);
 static PyStatus init_signals(PyThreadState *tstate);
 static void call_py_exitfuncs(PyThreadState *tstate);
@@ -1025,7 +1025,7 @@ init_interp_main(PyThreadState *tstate)
         return status;
     }
 
-    status = init_set_builtins_open(tstate);
+    status = init_set_builtins_open();
     if (_PyStatus_EXCEPTION(status)) {
         return status;
     }
@@ -1888,7 +1888,7 @@ create_stdio(const PyConfig *config, PyObject* io,
 
 /* Set builtins.open to io.OpenWrapper */
 static PyStatus
-init_set_builtins_open(PyThreadState *tstate)
+init_set_builtins_open(void)
 {
     PyObject *iomod = NULL, *wrapper;
     PyObject *bimod = NULL;
@@ -2056,9 +2056,8 @@ _Py_FatalError_DumpTracebacks(int fd, PyInterpreterState *interp,
    Return 1 if the traceback was displayed, 0 otherwise. */
 
 static int
-_Py_FatalError_PrintExc(int fd)
+_Py_FatalError_PrintExc(PyThreadState *tstate)
 {
-    PyThreadState *tstate = _PyThreadState_GET();
     PyObject *ferr, *res;
     PyObject *exception, *v, *tb;
     int has_tb;
@@ -2220,7 +2219,7 @@ fatal_error(const char *prefix, const char *msg, int status)
     int has_tstate_and_gil = (tss_tstate != NULL && tss_tstate == tstate);
     if (has_tstate_and_gil) {
         /* If an exception is set, print the exception with its traceback */
-        if (!_Py_FatalError_PrintExc(fd)) {
+        if (!_Py_FatalError_PrintExc(tss_tstate)) {
             /* No exception is set, or an exception is set without traceback */
             _Py_FatalError_DumpTracebacks(fd, interp, tss_tstate);
         }
diff --git a/Python/pystate.c b/Python/pystate.c
index 504f5f456dd2a5..9cf6bea1a027d1 100644
--- a/Python/pystate.c
+++ b/Python/pystate.c
@@ -1722,6 +1722,20 @@ _register_builtins_for_crossinterpreter_data(struct _xidregistry *xidregistry)
 }
 
 
+_PyFrameEvalFunction
+_PyInterpreterState_GetEvalFrameFunc(PyInterpreterState *interp)
+{
+    return interp->eval_frame;
+}
+
+
+void
+_PyInterpreterState_SetEvalFrameFunc(PyInterpreterState *interp,
+                                     _PyFrameEvalFunction eval_frame)
+{
+    interp->eval_frame = eval_frame;
+}
+
 #ifdef __cplusplus
 }
 #endif