Small edits for PEP 560 (#459)

pep-0560.rst

@@ -25,19 +25,19 @@ generic types.
 Rationale
 =========
 
-The restriction to not modify the core CPython interpreter lead to some
+The restriction to not modify the core CPython interpreter led to some
 design decisions that became questionable when the ``typing`` module started
-to be widely used. There are three main points of concerns:
+to be widely used. There are three main points of concern:
 performance of the ``typing`` module, metaclass conflicts, and the large
 number of hacks currently used in ``typing``.
 
 
-Performance:
-------------
+Performance
+-----------
 
 The ``typing`` module is one of the heaviest and slowest modules in
 the standard library even with all the optimizations made. Mainly this is
-because subscripted generic types (see PEP 484 for definition of terms
+because of subscripted generic types (see PEP 484 for definition of terms
 used in this PEP) are class objects (see also [1]_). The three main ways how
 the performance can be improved with the help of the proposed special methods:
 
@@ -52,8 +52,8 @@ the performance can be improved with the help of the proposed special methods:
   (this is minor however).
 
 
-Metaclass conflicts:
---------------------
+Metaclass conflicts
+-------------------
 
 All generic types are instances of ``GenericMeta``, so if a user uses
 a custom metaclass, then it is hard to make a corresponding class generic.
@@ -70,17 +70,17 @@ but this is not always practical or even possible. With the help of the
 proposed special attributes the ``GenericMeta`` metaclass will not be needed.
 
 
-Hacks and bugs that will be removed by this proposal:
------------------------------------------------------
+Hacks and bugs that will be removed by this proposal
+----------------------------------------------------
 
-- ``_generic_new`` hack that exists since ``__init__`` is not called on
+- ``_generic_new`` hack that exists because ``__init__`` is not called on
   instances with a type differing form the type whose ``__new__`` was called,
   ``C[int]().__class__ is C``.
 
 - ``_next_in_mro`` speed hack will be not necessary since subscription will
   not create new classes.
 
-- Ugly ``sys._getframe`` hack, this one is particularly nasty, since it looks
+- Ugly ``sys._getframe`` hack. This one is particularly nasty since it looks
   like we can't remove it without changes outside ``typing``.
 
 - Currently generics do dangerous things with private ABC caches
@@ -89,14 +89,14 @@ Hacks and bugs that will be removed by this proposal:
   re-implement ``ABCMeta`` in C.
 
 - Problems with sharing attributes between subscripted generics,
-  see [3]_. Current solution already uses ``__getattr__`` and ``__setattr__``,
+  see [3]_. The current solution already uses ``__getattr__`` and ``__setattr__``,
   but it is still incomplete, and solving this without the current proposal
   will be hard and will need ``__getattribute__``.
 
-- ``_no_slots_copy`` hack, where we clean-up the class dictionary on every
+- ``_no_slots_copy`` hack, where we clean up the class dictionary on every
   subscription thus allowing generics with ``__slots__``.
 
-- General complexity of the ``typing`` module, the new proposal will not
+- General complexity of the ``typing`` module. The new proposal will not
   only allow to remove the above mentioned hacks/bugs, but also simplify
   the implementation, so that it will be easier to maintain.
 
@@ -104,9 +104,12 @@ Hacks and bugs that will be removed by this proposal:
 Specification
 =============
 
+``__class_getitem__``
+---------------------
+
 The idea of ``__class_getitem__`` is simple: it is an exact analog of
 ``__getitem__`` with an exception that it is called on a class that
-defines it, not on its instances, this allows us to avoid
+defines it, not on its instances. This allows us to avoid
 ``GenericMeta.__getitem__`` for things like ``Iterable[int]``.
 The ``__class_getitem__`` is automatically a class method and
 does not require ``@classmethod`` decorator (similar to
@@ -131,8 +134,11 @@ For example::
 Note that this method is used as a fallback, so if a metaclass defines
 ``__getitem__``, then that will have the priority.
 
+``__subclass_base__``
+---------------------
+
 If an object that is not a class object appears in the bases of a class
-definition, the ``__subclass_base__`` is searched on it. If found,
+definition, then ``__subclass_base__`` is searched on it. If found,
 it is called with the original tuple of bases as an argument. If the result
 of the call is not ``None``, then it is substituted instead of this object.
 Otherwise (if the result is ``None``), the base is just removed. This is
@@ -144,20 +150,22 @@ done by the metaclass).
 NOTE: These two method names are reserved for exclusive use by
 the ``typing`` module and the generic types machinery, and any other use is
 strongly discouraged. The reference implementation (with tests) can be found
-in [4]_, the proposal was originally posted and discussed on
+in [4]_, and the proposal was originally posted and discussed on
 the ``typing`` tracker, see [5]_.
 
 
-Backwards compatibility and impact on users who don't use ``typing``:
-=====================================================================
+Backwards compatibility and impact on users who don't use ``typing``
+====================================================================
 
 This proposal may break code that currently uses the names
-``__class_getitem__`` and ``__subclass_base__``.
+``__class_getitem__`` and ``__subclass_base__``.  (But the language
+reference explicitly reserves *all* undocumented dunder names, and
+allows "breakage without warning"; see [6]_.)
 
 This proposal will support almost complete backwards compatibility with
 the current public generic types API; moreover the ``typing`` module is still
 provisional. The only two exceptions are that currently
-``issubclass(List[int], List)`` returns True, with this proposal it will raise
+``issubclass(List[int], List)`` returns True, while with this proposal it will raise
 ``TypeError``. Also ``issubclass(collections.abc.Iterable, typing.Iterable)``
 will return ``False``, which is probably desirable, since currently we have
 a (virtual) inheritance cycle between these two classes.
@@ -172,7 +180,7 @@ References
 .. [1] Discussion following Mark Shannon's presentation at Language Summit
    (https://github.com/python/typing/issues/432)
 
-.. [2] Pull Request to implement shared generic ABC caches
+.. [2] Pull Request to implement shared generic ABC caches (merged)
    (https://github.com/python/typing/pull/383)
 
 .. [3] An old bug with setting/accessing attributes on generic types
@@ -184,6 +192,8 @@ References
 .. [5] Original proposal
    (https://github.com/python/typing/issues/468)
 
+.. [6] Reserved classes of identifiers
+   (https://docs.python.org/3/reference/lexical_analysis.html#reserved-classes-of-identifiers)
 
 Copyright
 =========