update from Atlas with major reorg

attic/interfaces/pypy_difference.rst

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+
+====================================
+Differences between PyPy and CPython
+====================================
+
+Note: this is an excerpt from the `PyPy`_ documentation. On Nov. 19, 2014 I ran this test on PyPy 2.4.0 and PyPy3 2.4.0 and the result was not as described, but was the same as with CPython: 'foo'.
+
+.. _PyPy: http://pypy.readthedocs.org/en/latest/cpython_differences.html#subclasses-of-built-in-types
+
+Subclasses of built-in types
+----------------------------
+
+Officially, CPython has no rule at all for when exactly
+overridden method of subclasses of built-in types get
+implicitly called or not.  As an approximation, these methods
+are never called by other built-in methods of the same object.
+For example, an overridden ``__getitem__()`` in a subclass of
+``dict`` will not be called by e.g. the built-in ``get()``
+method.
+
+The above is true both in CPython and in PyPy.  Differences
+can occur about whether a built-in function or method will
+call an overridden method of *another* object than ``self``.
+In PyPy, they are generally always called, whereas not in
+CPython.  For example, in PyPy, ``dict1.update(dict2)``
+considers that ``dict2`` is just a general mapping object, and
+will thus call overridden ``keys()``  and ``__getitem__()``
+methods on it.  So the following code prints ``42`` on PyPy
+but ``foo`` on CPython::
+
+    >>> class D(dict):
+    ...     def __getitem__(self, key):
+    ...         return 42
+    ...
+    >>>
+    >>> d1 = {}
+    >>> d2 = D(a='foo')
+    >>> d1.update(d2)
+    >>> print(d1['a'])
+    42