diff --git a/Doc/c-api/intro.rst b/Doc/c-api/intro.rst
index 74681d2..4942b1a 100644
--- a/Doc/c-api/intro.rst
+++ b/Doc/c-api/intro.rst
@@ -17,11 +17,11 @@
 application; this technique is generally referred to as :dfn:`embedding` Python
 in an application.
 
-Writing an extension module is a relatively well-understood process,  where a
-"cookbook" approach works well.  There are several tools  that automate the
-process to some extent.  While people have embedded  Python in other
-applications since its early existence, the process of  embedding Python is less
-straightforward than writing an extension.
+Writing an extension module is a relatively well-understood process, where a
+"cookbook" approach works well.  There are several tools that automate the
+process to some extent.  While people have embedded Python in other
+applications since its early existence, the process of embedding Python is
+less straightforward than writing an extension.
 
 Many API functions are useful independent of whether you're embedding  or
 extending Python; moreover, most applications that embed Python  will need to
@@ -30,6 +30,16 @@
 application.
 
 
+Coding standards
+================
+
+If you're writing C code for inclusion in CPython, you **must** follow the
+guidelines and standards defined in :PEP:`7`.  These guidelines apply
+regardless of the version of Python you are contributing to.  Following these
+conventions is not necessary for your own third party extension modules,
+unless you eventually expect to contribute them to Python.
+
+
 .. _api-includes:
 
 Include Files
@@ -81,6 +91,48 @@
 is no need to do anything special to use the API from C++.
 
 
+Useful macros
+=============
+
+Several useful macros are defined in the Python header files.  Many are
+defined closer to where they are useful (e.g. :c:macro:`Py_RETURN_NONE`).
+Others of a more general utility are defined here.  This is not necessarily a
+complete listing.
+
+.. c:macro:: Py_UNREACHABLE()
+
+   Use this when you have a code path that you do not expect to be reached.
+   For example, in the ``default:`` clause in a ``switch`` statement for which
+   all possible values are covered in ``case`` statements.  Use this in places
+   where you might be tempted to put an ``assert(0)`` or ``abort()`` call.
+
+.. c:macro:: Py_ABS(x)
+
+   Return the absolute value of ``x``.
+
+.. c:macro:: Py_MIN(x, y)
+
+   Return the minimum value between ``x`` and ``y``.
+
+.. c:macro:: Py_MAX(x, y)
+
+   Return the maximum value between ``x`` and ``y``.
+
+.. c:macro:: Py_STRINGIFY(x)
+
+   Convert ``x`` to a C string.  E.g. ``Py_STRINGIFY(123)`` returns
+   ``"123"``.
+
+.. c:macro:: Py_MEMBER_SIZE(type, member)
+
+   Return the size of a structure (``type``) ``member`` in bytes.
+
+.. c:macro:: Py_CHARMASK(c)
+
+   Argument must be a character or an integer in the range [-128, 127] or [0,
+   255].  This macro returns ``c`` cast to an ``unsigned char``.
+
+
 .. _api-objects:
 
 Objects, Types and Reference Counts
diff --git a/Include/pymacro.h b/Include/pymacro.h
index 69577b7..3f6ddbe 100644
--- a/Include/pymacro.h
+++ b/Include/pymacro.h
@@ -95,4 +95,6 @@
 #define Py_UNUSED(name) _unused_ ## name
 #endif
 
+#define Py_UNREACHABLE() abort()
+
 #endif /* Py_PYMACRO_H */
diff --git a/Misc/NEWS.d/next/C API/2017-09-05-17-51-12.bpo-31338.LjA43Y.rst b/Misc/NEWS.d/next/C API/2017-09-05-17-51-12.bpo-31338.LjA43Y.rst
new file mode 100644
index 0000000..01878e5
--- /dev/null
+++ b/Misc/NEWS.d/next/C API/2017-09-05-17-51-12.bpo-31338.LjA43Y.rst
@@ -0,0 +1,3 @@
+Added the ``Py_UNREACHABLE()`` macro for code paths which are never expected
+to be reached.  This and a few other useful macros are now documented in the
+C API manual.
diff --git a/Modules/_datetimemodule.c b/Modules/_datetimemodule.c
index 1b68ff3..619ac84 100644
--- a/Modules/_datetimemodule.c
+++ b/Modules/_datetimemodule.c
@@ -1453,8 +1453,7 @@
         case Py_LT: istrue = diff < 0; break;
         case Py_GT: istrue = diff > 0; break;
         default:
-            assert(! "op unknown");
-            istrue = 0; /* To shut up compiler */
+            Py_UNREACHABLE();
     }
     result = istrue ? Py_True : Py_False;
     Py_INCREF(result);
diff --git a/Modules/_pickle.c b/Modules/_pickle.c
index 3165b4e..fb69f14 100644
--- a/Modules/_pickle.c
+++ b/Modules/_pickle.c
@@ -750,8 +750,7 @@
         if (entry->me_key == NULL || entry->me_key == key)
             return entry;
     }
-    assert(0);  /* Never reached */
-    return NULL;
+    Py_UNREACHABLE();
 }
 
 /* Returns -1 on failure, 0 on success. */
diff --git a/Modules/_tracemalloc.c b/Modules/_tracemalloc.c
index feb32a0..386f2f1 100644
--- a/Modules/_tracemalloc.c
+++ b/Modules/_tracemalloc.c
@@ -726,7 +726,7 @@
 
                The GIL and the table lock ensures that only one thread is
                allocating memory. */
-            assert(0 && "should never happen");
+            Py_UNREACHABLE();
         }
         TABLES_UNLOCK();
     }
diff --git a/Modules/mathmodule.c b/Modules/mathmodule.c
index c19620d..5d9fe5a 100644
--- a/Modules/mathmodule.c
+++ b/Modules/mathmodule.c
@@ -105,8 +105,7 @@
         r = sin(pi*(y-2.0));
         break;
     default:
-        assert(0);  /* should never get here */
-        r = -1.23e200; /* silence gcc warning */
+        Py_UNREACHABLE();
     }
     return copysign(1.0, x)*r;
 }
diff --git a/Objects/abstract.c b/Objects/abstract.c
index 998bcb1..38484b7 100644
--- a/Objects/abstract.c
+++ b/Objects/abstract.c
@@ -1984,7 +1984,7 @@
                 goto Done;
 
             default:
-                assert(!"unknown operation");
+                Py_UNREACHABLE();
             }
         }
 
diff --git a/Objects/bytesobject.c b/Objects/bytesobject.c
index d91cb7d..6a4eb67 100644
--- a/Objects/bytesobject.c
+++ b/Objects/bytesobject.c
@@ -868,7 +868,7 @@
                     switch(c)
                     {
                         default:
-                            assert(0 && "'type' not in [diuoxX]");
+                            Py_UNREACHABLE();
                         case 'd':
                         case 'i':
                         case 'u':
diff --git a/Objects/dictobject.c b/Objects/dictobject.c
index 81c7f7f..6ba2cc9 100644
--- a/Objects/dictobject.c
+++ b/Objects/dictobject.c
@@ -643,8 +643,7 @@
         perturb >>= PERTURB_SHIFT;
         i = mask & (i*5 + perturb + 1);
     }
-    assert(0);          /* NOT REACHED */
-    return DKIX_ERROR;
+    Py_UNREACHABLE();
 }
 
 /*
@@ -723,8 +722,7 @@
         perturb >>= PERTURB_SHIFT;
         i = (i*5 + perturb + 1) & mask;
     }
-    assert(0);          /* NOT REACHED */
-    return 0;
+    Py_UNREACHABLE();
 }
 
 /* Specialized version for string-only keys */
@@ -766,9 +764,7 @@
         perturb >>= PERTURB_SHIFT;
         i = mask & (i*5 + perturb + 1);
     }
-
-    assert(0);          /* NOT REACHED */
-    return 0;
+    Py_UNREACHABLE();
 }
 
 /* Faster version of lookdict_unicode when it is known that no <dummy> keys
@@ -810,8 +806,7 @@
         perturb >>= PERTURB_SHIFT;
         i = mask & (i*5 + perturb + 1);
     }
-    assert(0);          /* NOT REACHED */
-    return 0;
+    Py_UNREACHABLE();
 }
 
 /* Version of lookdict for split tables.
@@ -856,8 +851,7 @@
         perturb >>= PERTURB_SHIFT;
         i = mask & (i*5 + perturb + 1);
     }
-    assert(0);          /* NOT REACHED */
-    return 0;
+    Py_UNREACHABLE();
 }
 
 int
@@ -3603,7 +3597,7 @@
         else if (Py_TYPE(di) == &PyDictIterValue_Type)
             element = dictiter_iternextvalue(&tmp);
         else
-            assert(0);
+            Py_UNREACHABLE();
         if (element) {
             if (PyList_Append(list, element)) {
                 Py_DECREF(element);
diff --git a/Objects/longobject.c b/Objects/longobject.c
index 4862b76..3b07585 100644
--- a/Objects/longobject.c
+++ b/Objects/longobject.c
@@ -1806,8 +1806,7 @@
         bits = 1;
         break;
     default:
-        assert(0); /* shouldn't ever get here */
-        bits = 0; /* to silence gcc warning */
+        Py_UNREACHABLE();
     }
 
     /* Compute exact length 'sz' of output string. */
@@ -2169,8 +2168,8 @@
         }
     }
     if (str[0] == '_') {
-	    /* May not start with underscores. */
-	    goto onError;
+        /* May not start with underscores. */
+        goto onError;
     }
 
     start = str;
diff --git a/Objects/stringlib/eq.h b/Objects/stringlib/eq.h
index f8fd384..ff22f91 100644
--- a/Objects/stringlib/eq.h
+++ b/Objects/stringlib/eq.h
@@ -10,8 +10,7 @@
     PyUnicodeObject *b = (PyUnicodeObject *)bb;
 
     if (PyUnicode_READY(a) == -1 || PyUnicode_READY(b) == -1) {
-        assert(0 && "unicode_eq ready fail");
-        return 0;
+        Py_UNREACHABLE();
     }
 
     if (PyUnicode_GET_LENGTH(a) != PyUnicode_GET_LENGTH(b))
diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
index c4d93fc..bb1c083 100644
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -768,7 +768,7 @@
         BLOOM_UPDATE(Py_UCS4, mask, ptr, len);
         break;
     default:
-        assert(0);
+        Py_UNREACHABLE();
     }
     return mask;
 
@@ -869,8 +869,7 @@
         else
             return ucs4lib_rfind_char((Py_UCS4 *) s, size, ch);
     default:
-        assert(0);
-        return -1;
+        Py_UNREACHABLE();
     }
 }
 
@@ -1520,8 +1519,7 @@
                     );
             }
             else {
-                assert(0);
-                return -1;
+                Py_UNREACHABLE();
             }
         }
         else {
@@ -2079,7 +2077,7 @@
 #endif
         break;
     default:
-        assert(0 && "Impossible state");
+        Py_UNREACHABLE();
     }
 
     return unicode_result(unicode);
@@ -2172,8 +2170,7 @@
     case PyUnicode_4BYTE_KIND:
         return 0x10000;
     default:
-        assert(0 && "invalid kind");
-        return MAX_UNICODE;
+        Py_UNREACHABLE();
     }
 }
 
@@ -2317,8 +2314,7 @@
     case PyUnicode_4BYTE_KIND:
         return ucs4lib_find_max_char(startptr, endptr);
     default:
-        assert(0);
-        return 0;
+        Py_UNREACHABLE();
     }
 }
 
@@ -4068,7 +4064,7 @@
                     *w = *four_bytes;
 
                 if (w > wchar_end) {
-                    assert(0 && "Miscalculated string end");
+                    Py_UNREACHABLE();
                 }
             }
             *w = 0;
@@ -4120,7 +4116,7 @@
 #endif
             }
             else {
-                assert(0 && "This should never happen.");
+                Py_UNREACHABLE();
             }
         }
     }
@@ -5134,7 +5130,7 @@
 #endif
         if (ch > 0xFF) {
 #if SIZEOF_WCHAR_T == 4
-            assert(0);
+            Py_UNREACHABLE();
 #else
             assert(ch > 0xFFFF && ch <= MAX_UNICODE);
             /*  compute and append the two surrogates: */
@@ -5187,7 +5183,7 @@
 
     switch (kind) {
     default:
-        assert(0);
+        Py_UNREACHABLE();
     case PyUnicode_1BYTE_KIND:
         /* the string cannot be ASCII, or PyUnicode_UTF8() would be set */
         assert(!PyUnicode_IS_ASCII(unicode));
@@ -9294,7 +9290,7 @@
             result = ucs4lib_find_slice(buf1, len1, buf2, len2, start, end);
             break;
         default:
-            assert(0); result = -2;
+            Py_UNREACHABLE();
         }
     }
     else {
@@ -9312,7 +9308,7 @@
             result = ucs4lib_rfind_slice(buf1, len1, buf2, len2, start, end);
             break;
         default:
-            assert(0); result = -2;
+            Py_UNREACHABLE();
         }
     }
 
@@ -9386,8 +9382,7 @@
             (Py_UCS4 *) thousands_sep_data, thousands_sep_len);
         break;
     default:
-        assert(0);
-        return -1;
+        Py_UNREACHABLE();
     }
     if (unicode != NULL && thousands_sep_kind != kind) {
         if (thousands_sep_kind < kind)
@@ -9465,7 +9460,7 @@
             );
         break;
     default:
-        assert(0); result = 0;
+        Py_UNREACHABLE();
     }
 
     if (kind2 != kind1)
@@ -9881,8 +9876,7 @@
         memcpy(outdata, tmp, sizeof(Py_UCS4) * newlength);
         break;
     default:
-        assert(0);
-        break;
+        Py_UNREACHABLE();
     }
   leave:
     PyMem_FREE(tmp);
@@ -10105,7 +10099,7 @@
             for (; i_ < (length); ++i_, ++to_) *to_ = (value); \
             break; \
         } \
-        default: assert(0); \
+        default: Py_UNREACHABLE(); \
         } \
     } while (0)
 
@@ -10229,8 +10223,7 @@
             PyUnicode_GET_LENGTH(string), keepends);
         break;
     default:
-        assert(0);
-        list = 0;
+        Py_UNREACHABLE();
     }
     return list;
 }
@@ -10275,8 +10268,7 @@
                 PyUnicode_GET_LENGTH(self), maxcount
                 );
         default:
-            assert(0);
-            return NULL;
+            Py_UNREACHABLE();
         }
 
     if (PyUnicode_READY(substring) == -1)
@@ -10367,8 +10359,7 @@
                 PyUnicode_GET_LENGTH(self), maxcount
                 );
         default:
-            assert(0);
-            return NULL;
+            Py_UNREACHABLE();
         }
 
     if (PyUnicode_READY(substring) == -1)
@@ -10434,8 +10425,7 @@
     case PyUnicode_4BYTE_KIND:
         return ucs4lib_find(buf1, len1, buf2, len2, offset);
     }
-    assert(0);
-    return -1;
+    Py_UNREACHABLE();
 }
 
 static Py_ssize_t
@@ -10453,8 +10443,7 @@
     case PyUnicode_4BYTE_KIND:
         return ucs4lib_count(sbuf, slen, buf1, len1, maxcount);
     }
-    assert(0);
-    return 0;
+    Py_UNREACHABLE();
 }
 
 static void
@@ -10909,7 +10898,7 @@
             COMPARE(Py_UCS1, Py_UCS4);
             break;
         default:
-            assert(0);
+            Py_UNREACHABLE();
         }
         break;
     }
@@ -10928,7 +10917,7 @@
             COMPARE(Py_UCS2, Py_UCS4);
             break;
         default:
-            assert(0);
+            Py_UNREACHABLE();
         }
         break;
     }
@@ -10956,12 +10945,12 @@
             break;
         }
         default:
-            assert(0);
+            Py_UNREACHABLE();
         }
         break;
     }
     default:
-        assert(0);
+        Py_UNREACHABLE();
     }
 
     if (len1 == len2)
@@ -11285,8 +11274,7 @@
         result = ucs4lib_find(buf1, len1, buf2, len2, 0) != -1;
         break;
     default:
-        result = -1;
-        assert(0);
+        Py_UNREACHABLE();
     }
 
     if (kind2 != kind1)
@@ -11511,7 +11499,7 @@
             );
         break;
     default:
-        assert(0); iresult = 0;
+        Py_UNREACHABLE();
     }
 
     result = PyLong_FromSsize_t(iresult);
@@ -12985,8 +12973,7 @@
         out = ucs4lib_partition(str_obj, buf1, len1, sep_obj, buf2, len2);
         break;
     default:
-        assert(0);
-        out = 0;
+        Py_UNREACHABLE();
     }
 
     if (kind2 != kind1)
@@ -13043,8 +13030,7 @@
         out = ucs4lib_rpartition(str_obj, buf1, len1, sep_obj, buf2, len2);
         break;
     default:
-        assert(0);
-        out = 0;
+        Py_UNREACHABLE();
     }
 
     if (kind2 != kind1)
@@ -13627,8 +13613,7 @@
     case PyUnicode_2BYTE_KIND: maxchar = 0xffff; break;
     case PyUnicode_4BYTE_KIND: maxchar = 0x10ffff; break;
     default:
-        assert(0 && "invalid kind");
-        return -1;
+        Py_UNREACHABLE();
     }
 
     return _PyUnicodeWriter_PrepareInternal(writer, 0, maxchar);
@@ -13770,7 +13755,7 @@
         break;
     }
     default:
-        assert(0);
+        Py_UNREACHABLE();
     }
 
     writer->pos += len;
@@ -14204,7 +14189,7 @@
 
     switch (type) {
     default:
-        assert(!"'type' not in [diuoxX]");
+        Py_UNREACHABLE();
     case 'd':
     case 'i':
     case 'u':
@@ -14362,7 +14347,7 @@
         switch(type)
         {
             default:
-                assert(0 && "'type' not in [diuoxX]");
+                Py_UNREACHABLE();
             case 'd':
             case 'i':
             case 'u':
@@ -15373,8 +15358,7 @@
     for (i = 0; i < n; i++) {
         s = PyList_GET_ITEM(keys, i);
         if (PyUnicode_READY(s) == -1) {
-            assert(0 && "could not ready string");
-            fprintf(stderr, "could not ready string\n");
+            Py_UNREACHABLE();
         }
         switch (PyUnicode_CHECK_INTERNED(s)) {
         case SSTATE_NOT_INTERNED:
diff --git a/Parser/grammar1.c b/Parser/grammar1.c
index 30a6f07..9e99041 100644
--- a/Parser/grammar1.c
+++ b/Parser/grammar1.c
@@ -25,8 +25,7 @@
         if (d->d_type == type)
             return d;
     }
-    assert(0);
-    /* NOTREACHED */
+    Py_UNREACHABLE();
 #endif
 }
 
diff --git a/Python/ast.c b/Python/ast.c
index 7d30e5f..fd42c00 100644
--- a/Python/ast.c
+++ b/Python/ast.c
@@ -100,8 +100,7 @@
     case Param:
         return "Param";
     default:
-        assert(0);
-        return "(unknown)";
+        Py_UNREACHABLE();
     }
 }
 
@@ -759,8 +758,7 @@
             Py_FatalError(buf);
         }
     }
-    assert(0);
-    return 0;
+    Py_UNREACHABLE();
 }
 
 /* Transform the CST rooted at node * to the appropriate AST
diff --git a/Python/ceval.c b/Python/ceval.c
index 08533a4..5b810f2 100644
--- a/Python/ceval.c
+++ b/Python/ceval.c
@@ -274,7 +274,7 @@
         if (_Py_IsFinalizing() && !_Py_CURRENTLY_FINALIZING(tstate)) {
             drop_gil(tstate);
             PyThread_exit_thread();
-            assert(0);  /* unreachable */
+            Py_UNREACHABLE();
         }
         errno = err;
     }
@@ -3430,7 +3430,7 @@
 
         /* This should never be reached. Every opcode should end with DISPATCH()
            or goto error. */
-        assert(0);
+        Py_UNREACHABLE();
 
 error:
 
diff --git a/Python/compile.c b/Python/compile.c
index e547c2f..df5070a 100644
--- a/Python/compile.c
+++ b/Python/compile.c
@@ -1350,8 +1350,7 @@
     case NameConstant_kind:
         return e->v.NameConstant.value;
     default:
-        assert(!is_const(e));
-        return NULL;
+        Py_UNREACHABLE();
     }
 }
 
diff --git a/Python/formatter_unicode.c b/Python/formatter_unicode.c
index 9192bfd..2df7494 100644
--- a/Python/formatter_unicode.c
+++ b/Python/formatter_unicode.c
@@ -351,8 +351,7 @@
         *n_lpadding = 0;
     else {
         /* We should never have an unspecified alignment. */
-        *n_lpadding = 0;
-        assert(0);
+        Py_UNREACHABLE();
     }
 
     *n_rpadding = *n_total - nchars - *n_lpadding;
@@ -569,9 +568,7 @@
             break;
         default:
             /* Shouldn't get here, but treat it as '>' */
-            spec->n_lpadding = n_padding;
-            assert(0);
-            break;
+            Py_UNREACHABLE();
         }
     }
 
diff --git a/Python/pyhash.c b/Python/pyhash.c
index a2ec230..8a6bd60 100644
--- a/Python/pyhash.c
+++ b/Python/pyhash.c
@@ -175,7 +175,7 @@
             case 2: hash = ((hash << 5) + hash) + *p++; /* fallthrough */
             case 1: hash = ((hash << 5) + hash) + *p++; break;
             default:
-                assert(0);
+                Py_UNREACHABLE();
         }
         hash ^= len;
         hash ^= (Py_uhash_t) _Py_HashSecret.djbx33a.suffix;
diff --git a/Python/pystrtod.c b/Python/pystrtod.c
index 64d0c52..f19d239 100644
--- a/Python/pystrtod.c
+++ b/Python/pystrtod.c
@@ -431,8 +431,8 @@
   error:
     PyMem_Free(dup);
     PyErr_Format(PyExc_ValueError,
-		 "could not convert string to %s: "
-		 "%R", what, obj);
+                 "could not convert string to %s: "
+                 "%R", what, obj);
     return NULL;
 }
 
@@ -1061,7 +1061,7 @@
                something starting with a digit, an 'I',  or 'N' */
             strncpy(p, "ERR", 3);
             /* p += 3; */
-            assert(0);
+            Py_UNREACHABLE();
         }
         goto exit;
     }
diff --git a/Python/pytime.c b/Python/pytime.c
index 7edb534..8f275d2 100644
--- a/Python/pytime.c
+++ b/Python/pytime.c
@@ -630,10 +630,7 @@
     _PyTime_t t;
     if (pygettimeofday(&t, NULL, 0) < 0) {
         /* should not happen, _PyTime_Init() checked the clock at startup */
-        assert(0);
-
-        /* use a fixed value instead of a random value from the stack */
-        t = 0;
+        Py_UNREACHABLE();
     }
     return t;
 }
@@ -663,7 +660,7 @@
             return -1;
         }
         /* Hello, time traveler! */
-        assert(0);
+        Py_UNREACHABLE();
     }
     *tp = t * MS_TO_NS;
 
@@ -771,10 +768,7 @@
     if (pymonotonic(&t, NULL, 0) < 0) {
         /* should not happen, _PyTime_Init() checked that monotonic clock at
            startup */
-        assert(0);
-
-        /* use a fixed value instead of a random value from the stack */
-        t = 0;
+        Py_UNREACHABLE();
     }
     return t;
 }
diff --git a/Python/wordcode_helpers.h b/Python/wordcode_helpers.h
index cce81c1..c8f7a0f 100644
--- a/Python/wordcode_helpers.h
+++ b/Python/wordcode_helpers.h
@@ -39,6 +39,6 @@
             *codestr++ = PACKOPARG(opcode, oparg & 0xff);
             break;
         default:
-            assert(0);
+            Py_UNREACHABLE();
     }
 }