[Tutor] unhashable objects (unrelated to previous topic)

[Jeff Shannon]

On Sun, 27 Mar 2005 18:08:41 -0500, Orri Ganel <singingxduck at gmail.com> wrote:
> Hello all,
> 
> I am attempting to implement a LinkedList in Python, and in doing so,
> have created a Node class which represents the elements of the
> LinkedList.  However, since these Nodes have custom-defined __lt__(),
> __gt__(), __le__(), __ge__(), __eq__(), and __ne__() methods, they
> are, for some reason, unhashable.  When I comment out these methods,
> Python makes no complaint. But when they are present, 'TypeError:
> unhashable instance' is raised when attempting to use a Node as a key
> in a dictionary. 

When you don't define __eq__(), Python will use an object's ID as its
hash value.  However, the standard usage of hashes implies that
objects which compare as equal should hash identically.  This means
that objects which define __eq__() and which are intended to be hashed
need to define a __hash__() method.  Be very careful if you're doing
this with mutable objects, though -- you need to either hash by object
identity or by object equality, and for mutable objects either choice
will leave you breaking an implied promise.  (Either a single object
will not hash the same at different points in its life, or objects
which are equal will not hash identically.)

Jeff Shannon