Added PEP 549, "Instance Properties".

pep-0549.rst

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+PEP: XXXX
+Title: Instance Properties
+Version: $Revision$
+Last-Modified: $Date$
+Author: larry@hastings.org (Larry Hastings)
+Status: Draft
+Type: Standards Track
+Created: 4-Sep-2017
+Python-Version: 3.7
+Post-History: 4-Sep-2017
+
+
+Abstract
+--------
+
+    Python's descriptor protocol requires that descriptors
+    be members of the *type* of an object.  This PEP proposes
+    an extension to the descriptor protocol allowing use of
+    the descriptor protocol for members of *instances.*  This
+    would permit using properties in modules.
+
+Rationale 
+---------
+
+    Python's descriptor protocol guides programmers towards
+    elegant API design.  If your class supports a data-like
+    member, and you *might* someday need to run code when
+    changing the member's value, you're encouraged to
+    simply declare it as a simple data member of the class
+    for now.  If in the future you do need to run code, you
+    can change it to a "property", and happily the API doesn't
+    change.
+
+    Unfortunately this doesn't work with modules.  Modules are
+    instances of a single generic ``module`` type, and it's not
+    feasible to modify or subclass this type to add a property
+    to one's module.  This means that programmers facing this
+    API design decision, where the data-like member is a singleton
+    stored in a module, must preemptively add ugly "getters"
+    and "setters" for the data.
+
+    Adding support for module properties in pure Python is possible
+    only by using hacks like:
+
+      * peeking in ``sys.getframe()``,
+
+      * changing the type of an object after it's created, or
+
+      * replacing the object stored in ``sys.modules``.
+
+    These techniques can be made to work, but they're fragile.
+
+    This PEP proposes a per-type opt-in extension to the descriptor
+    protocol specifically designed to enable properties in modules.
+    The mechanism is a way to honor the descriptor protocol for
+    members of *instances* of a class without the member being declared
+    as a class variable.
+
+    Although this is being proposed as a general mechanism, the author
+    currently only forsees this as being useful for module objects.
+
+Implementation
+--------------
+
+    The basic idea is simple: modify the ``tp_descr_get`` and ``tp_descr_set``
+    functions exposed by ``PyModule_Type`` to inspect the attribute interacted
+    with, and if it supports the descriptor protocol, call the relevant
+    exposed function.
+
+    Our implementation faces two challenges:
+
+    1. Since this code will be run every time an attribute is looked up on a
+       method, it needs to add very little overhead in the general case,
+       where the object stored in the attribute is not a descriptor.
+
+    2. Since functions are descriptors, we must take care to *not* honor
+       the descriptor protocol for all objects.  Otherwise, all module-level
+       functions will suddenly become bound to the module instance as if
+       they were method calls on the module object.  The module handle would
+       be passed in as a "self" argument to all module-level functions.
+
+    Both challenges can be solved with the same approach: we define a new
+    "fast subclass" flag that means "This object is a desciptor, and it
+    should be honored directly when this object is looked up as an
+    attribute of an instance".  So far the only placed where this flag
+    is set is on ``PyProperty_Type``.
+
+Prototype
+---------
+
+    A prototype of this functionality is under development
+    at GitHub [github]_.
+
+Acknowledgements
+----------------
+
+    Armin Rigo essentially proposed this mechanism when presented
+    with the idea of "module properties", and educated the author
+    both on the complexities of the problem and the proper solution.
+
+References
+----------
+
+.. [github]
+   https://github.com/larryhastings/cpython/tree/module-properties
+
+Copyright
+---------
+
+    This document has been placed in the public domain.
+
+
+
+Local Variables:
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