PEP 7: Add C pre-processor macro style recommendations (python#3516)

peps/pep-0007.rst

@@ -135,6 +135,25 @@ Code lay-out
           return 0; /* "Forgive" adding a __dict__ only */
       }
 
+* Vertically align line continuation characters in multi-line macros.
+
+* Macros intended to be used as a statement should use the
+  ``do { ... } while (0)`` macro idiom,
+  without a final semicolon.
+  Example::
+
+      #define ADD_INT_MACRO(MOD, INT)                                   \
+          do {                                                          \
+              if (PyModule_AddIntConstant((MOD), (#INT), (INT)) < 0) {  \
+                  goto error;                                           \
+              }                                                         \
+          } while (0)
+
+      // To be used like a statement with a semicolon:
+      ADD_INT_MACRO(m, SOME_CONSTANT);
+
+* ``#undef`` file local macros after use.
+
 * Put blank lines around functions, structure definitions, and major
   sections inside functions.
 
@@ -170,6 +189,9 @@ Naming conventions
 * Macros should have a MixedCase prefix and then use upper case, for
   example: ``PyString_AS_STRING``, ``Py_PRINT_RAW``.
 
+* Macro parameters should use ``ALL_CAPS`` style,
+  so they are easily distinguishable from C variables and struct members.
+
 
 Documentation Strings
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