bpo-47233: Fix show_caches in dis module

Author: 15r10nk

Lib/dis.py

@@ -413,6 +413,9 @@ def _get_instructions_bytes(code, varname_from_oparg=None,
             labels.add(target)
     starts_line = None
     for offset, op, arg in _unpack_opargs(code):
+        # get the next position before we skip a CACHE instruction
+        # to associate the correct position information. 
+        positions = Positions(*next(co_positions, ()))
         if not show_caches and op == CACHE:
             continue
         if linestarts is not None:
@@ -422,7 +425,6 @@ def _get_instructions_bytes(code, varname_from_oparg=None,
         is_jump_target = offset in labels
         argval = None
         argrepr = ''
-        positions = Positions(*next(co_positions, ()))
         if arg is not None:
             #  Set argval to the dereferenced value of the argument when
             #  available, and argrepr to the string representation of argval.

Lib/test/test_dis.py

@@ -1618,5 +1618,23 @@ def get_disassembly(self, tb):
         return output.getvalue()
 
 
+class TestShowCaches(unittest.TestCase):
+    def test_show_caches(self):
+        def example_code():
+            # this code is never executed
+            a.b.c(1 * x)
+            if x:
+                c = 5
+
+        def get_instructions(show_caches):
+            return [
+                instr
+                for instr in dis.Bytecode(example_code, show_caches=show_caches)
+                if instr.opname != "CACHE"
+            ]
+
+        self.assertEqual(get_instructions(True), get_instructions(False))
+
+
 if __name__ == "__main__":
     unittest.main()

Misc/NEWS.d/next/Library/2022-04-07-23-31-50.bpo-47233.JMopUw.rst

@@ -0,0 +1,2 @@
+Fix an issue where the default argument ``show_caches=False`` in :mod:`dis`
+utilities could result in incorrect position information.