PEP-550: Mention why threading.local()-like interface was rejected (#377)

pep-0550.rst

@@ -1195,6 +1195,58 @@ Backwards Compatibility
 This proposal preserves 100% backwards compatibility.
 
 
+Rejected Ideas
+==============
+
+Replication of threading.local() interface
+------------------------------------------
+
+Choosing the ``threading.local()``-like interface for context
+variables was considered and rejected for the following reasons:
+
+* A survery of the standard library and Django has shown that the
+  vast majority of ``threading.local()`` uses involve a single
+  attribute, which indicates that the namespace approach is not
+  as helpful in the field.
+
+* Using ``__getattr__()`` instead of ``.get()`` for value lookup
+  does not provide any way to specify the depth of the lookup
+  (i.e. search only the top logical context).
+
+* Single-value ``ContextVar`` is easier to reason about in terms
+  of visibility.  Suppose ``new_context_var()`` returns a namespace,
+  and the consider the following::
+
+        ns = new_context_var('ns')
+
+        def gen():
+            ns.a = 2
+            yield
+            assert ns.b == 'bar' # ??
+
+        def main():
+            ns.a = 1
+            ns.b = 'foo'
+            g = gen()
+            next(g)
+            # should not see the ns.a modification in gen()
+            assert ns.a == 1
+            # but should gen() see the ns.b modification made here?
+            ns.b = 'bar'
+            yield
+
+  The above example demonstrates that reasoning about the visibility
+  of different attributes of the same context var is not trivial.
+
+* Single-value ``ContextVar`` allows straightforward implementation
+  of the lookup cache;
+
+* Single-value ``ContextVar`` interface allows the C-API to be
+  simple and essentially the same as the Python API.
+
+See also the mailing list discussion: [26]_, [27]_.
+
+
 Appendix: HAMT Performance Analysis
 ===================================
 
@@ -1340,6 +1392,9 @@ References
 
 .. [25] https://mail.python.org/pipermail/python-ideas/2017-August/046786.html
 
+.. [26] https://mail.python.org/pipermail/python-ideas/2017-August/046888.html
+
+.. [27] https://mail.python.org/pipermail/python-ideas/2017-August/046889.html
 
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