pep-567: Add two more subsections to Rejected Ideas (#550)

pep-0567.rst

@@ -667,6 +667,104 @@ Given the time frame of the Python 3.7 release schedule it was decided
 to defer this proposal to Python 3.8.
 
 
+Make Context a MutableMapping
+-----------------------------
+
+Making the ``Context`` class implement the ``abc.MutableMapping``
+interface would mean that it is possible to set and unset variables
+using ``Context[var] = value`` and ``del Context[var]`` operations.
+
+This proposal was deferred to Python 3.8+ because of the following:
+
+1. If in Python 3.8 it is decided that generators should support
+   context variables (see :pep:`550` and :pep:`568`), then ``Context``
+   would be transformed into a chain-map of context variables mappings
+   (as every generator would have its own mapping).  That would make
+   mutation operations like ``Context.__delitem__`` confusing, as
+   they would operate only on the topmost mapping of the chain.
+
+2. Having a single way of mutating the context
+   (``ContextVar.set()`` and ``ContextVar.reset()`` methods) makes
+   the API more straightforward.
+
+   For example, it would be non-obvious why the below code fragment
+   does not work as expected::
+
+     var = ContextVar('var')
+
+     ctx = copy_context()
+     ctx[var] = 'value'
+     print(ctx[var])  # Prints 'value'
+
+     print(var.get())  # Raises a LookupError
+
+   While the following code would work::
+
+     ctx = copy_context()
+
+     def func():
+         ctx[var] = 'value'
+
+         # Contrary to the previous example, this would work
+         # because 'func()' is running within 'ctx'.
+         print(ctx[var])
+         print(var.get())
+
+     ctx.run(func)
+
+
+Have initial values for ContextVars
+-----------------------------------
+
+Nathaniel Smith proposed to have a required ``initial_value``
+keyword-only argument for the ``ContextVar`` constructor.
+
+The main argument against this proposal is that for some types
+there is simply no sensible "initial value" except ``None``.
+E.g. consider a web framework that stores the current HTTP
+request object in a context variable.  With the current semantics
+it is possible to create a context variable without a default value::
+
+    # Framework:
+    current_request: ContextVar[Request] = \
+        ContextVar('current_request')
+
+
+    # Later, while handling an HTTP request:
+    request: Request = current_request.get()
+
+    # Work with the 'request' object:
+    return request.method
+
+Note that in the above example there is no need to check if
+``request`` is ``None``.  It is simply expected that the framework
+always sets the ``current_request`` variable, or it is a bug (in
+which case ``current_request.get()`` would raise a ``LookupError``).
+
+If, however, we had a required initial value, we would have
+to guard against ``None`` values explicitly::
+
+    # Framework:
+    current_request: ContextVar[Optional[Request]] = \
+        ContextVar('current_request', initial_value=None)
+
+
+    # Later, while handling an HTTP request:
+    request: Optional[Request] = current_request.get()
+
+    # Check if the current request object was set:
+    if request is None:
+        raise RuntimeError
+
+    # Work with the 'request' object:
+    return request.method
+
+Moreover, we can loosely compare context variables to regular
+Python variables and to ``threading.local()`` objects.  Both
+of them raise errors on failed lookups (``NameError`` and
+``AttributeError`` respectively).
+
+
 Backwards Compatibility
 =======================