Updated given statement torture test to show why renaming strategies are flawed

pep-3150.txt

@@ -314,16 +314,25 @@ code at module, class and function scope::
    assert d[42] == 42 given:
        d = b
    assert "d" not in locals()
+   y = y given:
+       x = 42
+       def f(): pass
+       y = locals("x"), f.__name__
+   assert "x" not in locals()
+   assert "f" not in locals()
+   assert y == (42, "f")
 
 Most naive implementations will choke on the first complex assignment,
 while less naive but still broken implementations will fail when
-the torture test is executed at class scope.
+the torture test is executed at class scope. Renaming based strategies
+struggle to support ``locals()`` correctly and also have problems with
+class and function ``__name__`` attributes.
 
 And yes, that's a perfectly well-defined assignment statement. Insane,
 you might rightly say, but legal::
 
    >>> def f(x): return x
-   ... 
+   ...
    >>> x = 42
    >>> b = {}
    >>> a = b[f(a)] = x
@@ -349,6 +358,10 @@ two reasons mentioned in the Torture Test section above:
 * Return-based semantics struggle with complex assignment statements
   like the one in the torture test
 
+The second thought is generally some kind of hidden renaming strategy. This
+also creates problems, as Python exposes variables names via the ``locals()``
+dictionary and class and function ``__name__`` attributes.
+
 The most promising approach is one based on symtable analysis and
 copy-in-copy-out referencing semantics to move any required name
 bindings between the inner and outer scopes. The torture test above
@@ -371,6 +384,16 @@ would then translate to something like the following::
        # Nothing to copy out (not an assignment)
    _anon2()
    assert "d" not in locals()
+   def _anon3() # Nothing to copy in (no references to other variables)
+       x = 42
+       def f(): pass
+       y = locals("x"), f.__name__
+       y = y    # Assuming no optimisation of special cases
+       return y # 'y' reference copied out
+   y = _anon3()
+   assert "x" not in locals()
+   assert "f" not in locals()
+   assert y == (42, "f")
 
 However, as noted in the abstract, an actual implementation of
 this idea has never been tried.
@@ -417,7 +440,7 @@ Copyright
 This document has been placed in the public domain.
 
 
-
+
 ..
    Local Variables:
    mode: indented-text