bpo-45168: change dis output to omit missing values rather than replacing them by their index (GH-28313)

iritkatriel · web-flow · commit c99fc4e53a60 · 2021-09-14T10:09:05.000+01:00
diff --git a/Doc/library/dis.rst b/Doc/library/dis.rst
@@ -293,12 +293,13 @@ details of bytecode instructions as :class:`Instruction` instances:
 
    .. data:: argval
 
-      resolved arg value (if known), otherwise same as arg
+      resolved arg value (if any), otherwise ``None``
 
 
    .. data:: argrepr
 
-      human readable description of operation argument
+      human readable description of operation argument (if any),
+      otherwise an empty string.
 
 
    .. data:: offset
diff --git a/Lib/dis.py b/Lib/dis.py
@@ -125,6 +125,13 @@ def pretty_flags(flags):
         names.append(hex(flags))
     return ", ".join(names)
 
+class _Unknown:
+    def __repr__(self):
+        return "<unknown>"
+
+# Sentinel to represent values that cannot be calculated
+UNKNOWN = _Unknown()
+
 def _get_code_object(x):
     """Helper to handle methods, compiled or raw code objects, and strings."""
     # Extract functions from methods.
@@ -315,28 +322,28 @@ def _get_const_info(const_index, const_list):
 
        Returns the dereferenced constant and its repr if the constant
        list is defined.
-       Otherwise returns the constant index and its repr().
+       Otherwise returns the sentinel value dis.UNKNOWN for the value
+       and an empty string for its repr.
     """
-    argval = const_index
     if const_list is not None:
         argval = const_list[const_index]
-    return argval, repr(argval)
+        return argval, repr(argval)
+    else:
+        return UNKNOWN, ''
 
 def _get_name_info(name_index, get_name, **extrainfo):
     """Helper to get optional details about named references
 
        Returns the dereferenced name as both value and repr if the name
        list is defined.
-       Otherwise returns the name index and its repr().
+       Otherwise returns the sentinel value dis.UNKNOWN for the value
+       and an empty string for its repr.
     """
-    argval = name_index
     if get_name is not None:
         argval = get_name(name_index, **extrainfo)
-        argrepr = argval
+        return argval, argval
     else:
-        argrepr = repr(argval)
-    return argval, argrepr
-
+        return UNKNOWN, ''
 
 def parse_varint(iterator):
     b = next(iterator)
diff --git a/Lib/test/test_dis.py b/Lib/test/test_dis.py
@@ -48,12 +48,12 @@ def cm(cls, x):
 """ % (_C.__init__.__code__.co_firstlineno + 1,)
 
 dis_c_instance_method_bytes = """\
-          0 LOAD_FAST                1 (1)
-          2 LOAD_CONST               1 (1)
+          0 LOAD_FAST                1
+          2 LOAD_CONST               1
           4 COMPARE_OP               2 (==)
-          6 LOAD_FAST                0 (0)
-          8 STORE_ATTR               0 (0)
-         10 LOAD_CONST               0 (0)
+          6 LOAD_FAST                0
+          8 STORE_ATTR               0
+         10 LOAD_CONST               0
          12 RETURN_VALUE
 """
 
@@ -105,11 +105,11 @@ def _f(a):
 
 
 dis_f_co_code = """\
-          0 LOAD_GLOBAL              0 (0)
-          2 LOAD_FAST                0 (0)
+          0 LOAD_GLOBAL              0
+          2 LOAD_FAST                0
           4 CALL_FUNCTION            1
           6 POP_TOP
-          8 LOAD_CONST               1 (1)
+          8 LOAD_CONST               1
          10 RETURN_VALUE
 """
 
diff --git a/Misc/NEWS.d/next/Library/2021-09-13-14-28-49.bpo-45168.Z1mfW4.rst b/Misc/NEWS.d/next/Library/2021-09-13-14-28-49.bpo-45168.Z1mfW4.rst
@@ -0,0 +1 @@
+Change :func:`dis.dis` output to omit op arg values that cannot be resolved due to ``co_consts``, ``co_names`` etc not being provided. Previously the oparg itself was repeated in the value field, which is not useful and can be confusing.

Original file line number	Diff line number	Diff line change
`@@ -0,0 +1 @@`
	`1`	+Change :func:`dis.dis` output to omit op arg values that cannot be resolved due to ``co_consts``, ``co_names`` etc not being provided. Previously the oparg itself was repeated in the value field, which is not useful and can be confusing.