com_print_stmt(): Guido rightly points out that the stream expression

in extended prints should only be evaluated once.  This patch plays
stack games (documented!) to fix this.

diff --git a/Python/compile.c b/Python/compile.c
index b7ce7702ff879ac15345b60d197ebe9d18451939..e09204e46ab23c01a7af4d1a8f400551e8fae57f 100644 (file)
--- a/Python/compile.c
+++ b/Python/compile.c
@@ -2055,6 +2055,9 @@ com_print_stmt(struct compiling *c, node *n)
        /* are we using the extended print form? */
        if (NCH(n) >= 2 && TYPE(CHILD(n, 1)) == RIGHTSHIFT) {
                stream = CHILD(n, 2);
+               com_node(c, stream);
+               /* stack: [...] => [... stream] */
+               com_push(c, 1);
                if (NCH(n) > 3 && TYPE(CHILD(n, 3)) == COMMA)
                        i = 4;
                else
@@ -2062,24 +2065,38 @@ com_print_stmt(struct compiling *c, node *n)
        }
        for (; i < NCH(n); i += 2) {
                if (stream != NULL) {
-                       /* stack: [...] => [... obj stream] */
+                       com_addbyte(c, DUP_TOP);
+                       /* stack: [stream] => [stream stream] */
+                       com_push(c, 1);
                        com_node(c, CHILD(n, i));
-                       com_node(c, stream);
+                       /* stack: [stream stream] => [stream stream obj] */
+                       com_addbyte(c, ROT_TWO);
+                       /* stack: [stream stream obj] => [stream obj stream] */
                        com_addbyte(c, PRINT_ITEM_TO);
+                       /* stack: [stream obj stream] => [stream] */
                        com_pop(c, 2);
                }
                else {
-                       /* stack: [...] => [... obj] */
                        com_node(c, CHILD(n, i));
+                       /* stack: [...] => [... obj] */
                        com_addbyte(c, PRINT_ITEM);
                        com_pop(c, 1);
                }
        }
        /* XXX Alternatively, LOAD_CONST '\n' and then PRINT_ITEM */
-       if (TYPE(CHILD(n, NCH(n)-1)) != COMMA) {
+       if (TYPE(CHILD(n, NCH(n)-1)) == COMMA) {
+               if (stream != NULL) {
+                       /* must pop the extra stream object off the stack */
+                       com_addbyte(c, POP_TOP);
+                       /* stack: [... stream] => [...] */
+                       com_pop(c, 1);
+               }
+       }
+       else {
                if (stream != NULL) {
-                       com_node(c, stream);
+                       /* this consumes the last stream object on stack */
                        com_addbyte(c, PRINT_NEWLINE_TO);
+                       /* stack: [... stream] => [...] */
                        com_pop(c, 1);
                }
                else