defer assignment after all rhs evaluation is complete

Chetan Khanna · Chetan Khanna · commit 8d12e3daa5c5 · 2026-04-24T01:47:33.000+05:30
diff --git a/mypyc/irbuild/builder.py b/mypyc/irbuild/builder.py
@@ -932,7 +932,8 @@ def process_iterator_tuple_assignment(
         split_idx = target.star_idx if target.star_idx is not None else len(target.items)
 
         # Read values before the first starred value
-        values = []
+        left_values = []
+        right_values = []
         for litem in target.items[:split_idx]:
             ritem = self.call_c(next_op, [iterator], line)
             error_block, ok_block = BasicBlock(), BasicBlock()
@@ -948,11 +949,7 @@ def process_iterator_tuple_assignment(
 
             self.activate_block(ok_block)
 
-            values.append(ritem)
-
-        # Assign read values to target lvalues
-        for litem, ritem in zip(target.items[:split_idx], values):
-            self.assign(litem, ritem, line)
+            left_values.append(ritem)
 
         # Read the starred value and all values after it
         if target.star_idx is not None:
@@ -975,13 +972,15 @@ def process_iterator_tuple_assignment(
 
             self.activate_block(ok_block)
 
-            values = []
             for litem in reversed(post_star_vals):
                 ritem = self.primitive_op(list_pop_last, [iter_list], line)
-                values.append(ritem)
+                right_values.append(ritem)
 
-            # Assign the read values to target lvalues
-            for litem, ritem in zip(reversed(post_star_vals), values):
+            # Assign read values to target lvalues
+            for litem, ritem in zip(target.items[:split_idx], left_values):
+                self.assign(litem, ritem, line)
+
+            for litem, ritem in zip(reversed(post_star_vals), right_values):
                 self.assign(litem, ritem, line)
 
             # Assign the starred value
@@ -1003,6 +1002,10 @@ def process_iterator_tuple_assignment(
             self.add(Unreachable())
 
             self.activate_block(ok_block)
+            
+            # Assign read values to target lvalues
+            for litem, ritem in zip(target.items[:split_idx], left_values):
+                self.assign(litem, ritem, line)
 
     def push_loop_stack(self, continue_block: BasicBlock, break_block: BasicBlock) -> None:
         self.nonlocal_control.append(
diff --git a/mypyc/test-data/irbuild-statements.test b/mypyc/test-data/irbuild-statements.test
@@ -445,8 +445,9 @@ def from_any(a):
     r2 :: bool
     r3 :: object
     r4 :: bool
-    x, y, r5 :: object
+    r5 :: object
     r6 :: bool
+    x, y :: object
 L0:
     r0 = PyObject_GetIter(a)
     r1 = PyIter_Next(r0)
@@ -461,14 +462,14 @@ L3:
     r4 = raise ValueError('not enough values to unpack')
     unreachable
 L4:
-    x = r1
-    y = r3
     r5 = PyIter_Next(r0)
     if is_error(r5) goto L6 else goto L5
 L5:
     r6 = raise ValueError('too many values to unpack')
     unreachable
 L6:
+    x = r1
+    y = r3
     return 1
 
 [case testMultiAssignmentCoercions]
@@ -506,9 +507,10 @@ def from_any(a):
     r2 :: bool
     r3 :: object
     r4 :: bool
-    r5, x :: int
-    y, r6 :: object
-    r7 :: bool
+    r5 :: object
+    r6 :: bool
+    r7, x :: int
+    y :: object
 L0:
     r0 = PyObject_GetIter(a)
     r1 = PyIter_Next(r0)
@@ -523,15 +525,15 @@ L3:
     r4 = raise ValueError('not enough values to unpack')
     unreachable
 L4:
-    r5 = unbox(int, r1)
-    x = r5
-    y = r3
-    r6 = PyIter_Next(r0)
-    if is_error(r6) goto L6 else goto L5
+    r5 = PyIter_Next(r0)
+    if is_error(r5) goto L6 else goto L5
 L5:
-    r7 = raise ValueError('too many values to unpack')
+    r6 = raise ValueError('too many values to unpack')
     unreachable
 L6:
+    r7 = unbox(int, r1)
+    x = r7
+    y = r3
     return 1
 
 [case testStarUnpack]
@@ -550,28 +552,27 @@ def f(a):
     r5 :: bool
     r6 :: object
     r7 :: bool
-    r8 :: str
-    r9 :: int32
-    r10 :: bit
-    r11 :: str
-    r12 :: int32
-    r13 :: bit
-    r14 :: list
-    r15 :: ptr
-    r16 :: native_int
-    r17 :: short_int
-    r18 :: bit
-    r19 :: bool
-    r20, r21 :: object
-    r22 :: str
-    r23 :: int32
-    r24 :: bit
-    r25 :: str
-    r26 :: int32
-    r27 :: bit
-    r28 :: str
-    r29 :: int32
-    r30 :: bit
+    r8 :: list
+    r9 :: native_int
+    r10 :: short_int
+    r11 :: bit
+    r12 :: bool
+    r13, r14 :: object
+    r15 :: str
+    r16 :: i32
+    r17 :: bit
+    r18 :: str
+    r19 :: i32
+    r20 :: bit
+    r21 :: str
+    r22 :: i32
+    r23 :: bit
+    r24 :: str
+    r25 :: i32
+    r26 :: bit
+    r27 :: str
+    r28 :: i32
+    r29 :: bit
 L0:
     r0 = __main__.globals :: static
     r1 = 'it'
@@ -589,34 +590,32 @@ L3:
     r7 = raise ValueError('not enough values to unpack')
     unreachable
 L4:
-    r8 = 'x'
-    r9 = PyObject_SetAttr(a, r8, r4)
-    r10 = r9 >= 0 :: signed
-    r11 = 'y'
-    r12 = PyObject_SetAttr(a, r11, r6)
-    r13 = r12 >= 0 :: signed
-    r14 = PySequence_List(r3)
-    r15 = get_element_ptr r14 ob_size :: PyVarObject
-    r16 = load_mem r15 :: native_int*
-    keep_alive r14
-    r17 = r16 << 1
-    r18 = 4 <= r17 :: signed
-    if r18 goto L6 else goto L5 :: bool
+    r8 = PySequence_List(r3)
+    r9 = var_object_size r8
+    r10 = r9 << 1
+    r11 = int_le 4, r10
+    if r11 goto L6 else goto L5 :: bool
 L5:
-    r19 = raise ValueError('not enough values to unpack')
+    r12 = raise ValueError('not enough values to unpack')
     unreachable
 L6:
-    r20 = CPyList_PopLast(r14)
-    r21 = CPyList_PopLast(r14)
-    r22 = 'w'
-    r23 = PyObject_SetAttr(a, r22, r20)
-    r24 = r23 >= 0 :: signed
-    r25 = 'u'
-    r26 = PyObject_SetAttr(a, r25, r21)
-    r27 = r26 >= 0 :: signed
-    r28 = 'z'
-    r29 = PyObject_SetAttr(a, r28, r14)
-    r30 = r29 >= 0 :: signed
+    r13 = CPyList_PopLast(r8)
+    r14 = CPyList_PopLast(r8)
+    r15 = 'x'
+    r16 = PyObject_SetAttr(a, r15, r4)
+    r17 = r16 >= 0 :: signed
+    r18 = 'y'
+    r19 = PyObject_SetAttr(a, r18, r6)
+    r20 = r19 >= 0 :: signed
+    r21 = 'w'
+    r22 = PyObject_SetAttr(a, r21, r13)
+    r23 = r22 >= 0 :: signed
+    r24 = 'u'
+    r25 = PyObject_SetAttr(a, r24, r14)
+    r26 = r25 >= 0 :: signed
+    r27 = 'z'
+    r28 = PyObject_SetAttr(a, r27, r8)
+    r29 = r28 >= 0 :: signed
     return 1
 
 [case testMultiAssignmentNested]
