Hash collision security issue

Doc/reference/datamodel.rst

 
 .. _datamodel:
 
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 Data model
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 .. _objects:
 
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    appropriate :exc:`TypeError` when a program attempts to retrieve their hash
    value, but they will also be correctly identified as unhashable when checking
    ``isinstance(obj, collections.Hashable)`` (unlike classes which define their
    own :meth:`__hash__` to explicitly raise :exc:`TypeError`).
 
    If a class that overrides :meth:`__eq__` needs to retain the implementation
    of :meth:`__hash__` from a parent class, the interpreter must be told this
    explicitly by setting ``__hash__ = <ParentClass>.__hash__``. Otherwise the
    inheritance of :meth:`__hash__` will be blocked, just as if :attr:`__hash__`
    had been explicitly set to :const:`None`.
+
+   See also the :option:`-R` command-line option.
 
 
 .. method:: object.__bool__(self)
 
    .. index:: single: __len__() (mapping object method)
 
    Called to implement truth value testing and the built-in operation
    ``bool()``; should return ``False`` or ``True``.  When this method is not
    defined, :meth:`__len__` is called, if it is defined, and the object is
    considered true if its result is nonzero.  If a class defines neither
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 .. rubric:: Footnotes
 
 .. [#] It *is* possible in some cases to change an object's type, under certain
    controlled conditions. It generally isn't a good idea though, since it can
    lead to some very strange behaviour if it is handled incorrectly.
 
 .. [#] For operands of the same type, it is assumed that if the non-reflected method
    (such as :meth:`__add__`) fails the operation is not supported, which is why the
    reflected method is not called.