1 files changed, 64 insertions, 5 deletions

diff --git a/Python/compile.c b/Python/compile.c
index 9cce8aeb4e..0336959d3d 100644
--- a/Python/compile.c
+++ b/Python/compile.c
@@ -161,6 +161,11 @@ struct compiler {
     int c_optimize;              /* optimization level */
     int c_interactive;           /* true if in interactive mode */
     int c_nestlevel;
+    int c_do_not_emit_bytecode;  /* The compiler won't emit any bytecode
+                                    if this value is different from zero.
+                                    This can be used to temporarily visit
+                                    nodes without emitting bytecode to
+                                    check only errors. */
 
     PyObject *c_const_cache;     /* Python dict holding all constants,
                                     including names tuple */
@@ -340,6 +345,7 @@ PyAST_CompileObject(mod_ty mod, PyObject *filename, PyCompilerFlags *flags,
     c.c_flags = flags;
     c.c_optimize = (optimize == -1) ? config->optimization_level : optimize;
     c.c_nestlevel = 0;
+    c.c_do_not_emit_bytecode = 0;
 
     if (!_PyAST_Optimize(mod, arena, c.c_optimize)) {
         goto finally;
@@ -1152,6 +1158,9 @@ compiler_addop(struct compiler *c, int opcode)
     struct instr *i;
     int off;
     assert(!HAS_ARG(opcode));
+    if (c->c_do_not_emit_bytecode) {
+        return 1;
+    }
     off = compiler_next_instr(c, c->u->u_curblock);
     if (off < 0)
         return 0;
@@ -1305,6 +1314,10 @@ merge_consts_recursive(struct compiler *c, PyObject *o)
 static Py_ssize_t
 compiler_add_const(struct compiler *c, PyObject *o)
 {
+    if (c->c_do_not_emit_bytecode) {
+        return 0;
+    }
+
     PyObject *key = merge_consts_recursive(c, o);
     if (key == NULL) {
         return -1;
@@ -1318,6 +1331,10 @@ compiler_add_const(struct compiler *c, PyObject *o)
 static int
 compiler_addop_load_const(struct compiler *c, PyObject *o)
 {
+    if (c->c_do_not_emit_bytecode) {
+        return 1;
+    }
+
     Py_ssize_t arg = compiler_add_const(c, o);
     if (arg < 0)
         return 0;
@@ -1328,6 +1345,10 @@ static int
 compiler_addop_o(struct compiler *c, int opcode, PyObject *dict,
                      PyObject *o)
 {
+    if (c->c_do_not_emit_bytecode) {
+        return 1;
+    }
+
     Py_ssize_t arg = compiler_add_o(c, dict, o);
     if (arg < 0)
         return 0;
@@ -1339,6 +1360,11 @@ compiler_addop_name(struct compiler *c, int opcode, PyObject *dict,
                     PyObject *o)
 {
     Py_ssize_t arg;
+
+    if (c->c_do_not_emit_bytecode) {
+        return 1;
+    }
+
     PyObject *mangled = _Py_Mangle(c->u->u_private, o);
     if (!mangled)
         return 0;
@@ -1359,6 +1385,10 @@ compiler_addop_i(struct compiler *c, int opcode, Py_ssize_t oparg)
     struct instr *i;
     int off;
 
+    if (c->c_do_not_emit_bytecode) {
+        return 1;
+    }
+
     /* oparg value is unsigned, but a signed C int is usually used to store
        it in the C code (like Python/ceval.c).
 
@@ -1385,6 +1415,10 @@ compiler_addop_j(struct compiler *c, int opcode, basicblock *b, int absolute)
     struct instr *i;
     int off;
 
+    if (c->c_do_not_emit_bytecode) {
+        return 1;
+    }
+
     assert(HAS_ARG(opcode));
     assert(b != NULL);
     off = compiler_next_instr(c, c->u->u_curblock);
@@ -1519,6 +1553,17 @@ compiler_addop_j(struct compiler *c, int opcode, basicblock *b, int absolute)
     } \
 }
 
+/* These macros allows to check only for errors and not emmit bytecode
+ * while visiting nodes.
+*/
+
+#define BEGIN_DO_NOT_EMIT_BYTECODE { \
+    c->c_do_not_emit_bytecode++;
+
+#define END_DO_NOT_EMIT_BYTECODE \
+    c->c_do_not_emit_bytecode--; \
+}
+
 /* Search if variable annotations are present statically in a block. */
 
 static int
@@ -2546,13 +2591,23 @@ compiler_if(struct compiler *c, stmt_ty s)
         return 0;
 
     constant = expr_constant(s->v.If.test);
-    /* constant = 0: "if 0" Leave the optimizations to
-     * the pephole optimizer to check for syntax errors
-     * in the block.
+    /* constant = 0: "if 0"
      * constant = 1: "if 1", "if 2", ...
      * constant = -1: rest */
-    if (constant == 1) {
+    if (constant == 0) {
+        BEGIN_DO_NOT_EMIT_BYTECODE
+        VISIT_SEQ(c, stmt, s->v.If.body);
+        END_DO_NOT_EMIT_BYTECODE
+        if (s->v.If.orelse) {
+            VISIT_SEQ(c, stmt, s->v.If.orelse);
+        }
+    } else if (constant == 1) {
         VISIT_SEQ(c, stmt, s->v.If.body);
+        if (s->v.If.orelse) {
+            BEGIN_DO_NOT_EMIT_BYTECODE
+            VISIT_SEQ(c, stmt, s->v.If.orelse);
+            END_DO_NOT_EMIT_BYTECODE
+        }
     } else {
         if (asdl_seq_LEN(s->v.If.orelse)) {
             next = compiler_new_block(c);
@@ -2662,8 +2717,12 @@ compiler_while(struct compiler *c, stmt_ty s)
     int constant = expr_constant(s->v.While.test);
 
     if (constant == 0) {
-        if (s->v.While.orelse)
+        BEGIN_DO_NOT_EMIT_BYTECODE
+        VISIT_SEQ(c, stmt, s->v.While.body);
+        END_DO_NOT_EMIT_BYTECODE
+        if (s->v.While.orelse) {
             VISIT_SEQ(c, stmt, s->v.While.orelse);
+        }
         return 1;
     }
     loop = compiler_new_block(c);