OK, I lied. On Windows, _IOLBF seems to be the same as full

gvanrossum · gvanrossum · commit b31c7dcb4330 · 1997-04-11T22:19:12.000Z
buffering, so to get the normal behavior back, I set it to
unbuffered.
diff --git a/Modules/main.c b/Modules/main.c
@@ -204,10 +204,14 @@ main(argc, argv)
 #endif
 	}
 	else if (Py_InteractiveFlag) {
-		char *ibuffer = malloc(BUFSIZ);
-		char *obuffer = malloc(BUFSIZ);
-		setvbuf(stdin,  ibuffer, _IOLBF, BUFSIZ);
-		setvbuf(stdout, obuffer, _IOLBF, BUFSIZ);
+#ifdef MS_WINDOWS
+		/* Doesn't have to have line-buffered -- use unbuffered */
+		setvbuf(stdin,  (char *)NULL, _IONBF, BUFSIZ);
+		setvbuf(stdout, (char *)NULL, _IONBF, BUFSIZ);
+#else
+		setvbuf(stdin,  (char *)NULL, _IOLBF, BUFSIZ);
+		setvbuf(stdout, (char *)NULL, _IOLBF, BUFSIZ);
+#endif
 		/* Leave stderr alone - it should be unbuffered anyway. */
   	}
 
