On Mar 3, 2008, at 11:23 AM, Michael Bayer wrote:
On Mar 3, 2008, at 11:56 AM, Patrick Hartling wrote:hash(Works()) hash(Works2())# Raises TypeError with Python 2.6 because Fails is deemed unhashable.hash(Fails()) Does anyone know of a workaround for this issue? So far, sqlalchemy.schema.PrimaryKeyConstraint and sqlalchemy.schema.Column are the two classes that have tripped me up. I have added __hash__() methods to both, but I cannot vouch for the correctness of the implementations.hmm, subtle difference between the 2.5 docs: If a class does not define a __cmp__() method it should not define a __hash__() operation either; if it defines __cmp__() or __eq__() but not __hash__(), its instances will not be usable as dictionary keys. If a class defines mutable objects and implements a __cmp__() or __eq__() method, it should not implement __hash__(), since the dictionary implementation requires that a key's hash value is immutable (if the object's hash value changes, it will be in the wrong hash bucket). and the 2.6 docs: If a class does not define a __cmp__() or __eq__() method it should not define a __hash__() operation either; if it defines __cmp__() or __eq__() but not __hash__(), its instances will not be usable as dictionary keys. If a class defines mutable objects and implements a __cmp__() or __eq__() method, it should not implement __hash__(), since the dictionary implementation requires that a key’s hash value is immutable (if the object’s hash value changes, it will be in the wrong hash bucket). both claim that if we define __eq__() but not __hash__(), it wont be useable as a dictionary key. but in the case of 2.5 this seems to be incorrect, or at least not enforced. The only difference here is that 2.6 says " if we dont define __cmp__() or __eq__(), then we shouldn't define __hash__() either" whereas 2.5 only mentions __cmp__() in that regard.
Subtle indeed.
We define __eq__() all over the place so that would be a lot of __hash__() methods to add, all of which return id(self). I wonder if we shouldn't just make a util.Mixin called "Hashable" so that we can centralize the idea.
That sounds like a reasonable approach. I have not looked much at the SQLAlchemy internals before today, but if I follow your idea correctly, I could apply that solution wherever I run into problems during testing.
-Patrick -- Patrick L. Hartling Senior Software Engineer, Priority 5 http://www.priority5.com/
PGP.sig
Description: This is a digitally signed message part
