Hashability of classes, old and new

[=?iso-8859-1?Q?Fran=E7ois?= Pinard]

Hi, people. I observe that there is a difference in hashability between
old-style and new-style instances, when they are comparable. I wonder
what I meaning I should make out of this. Maybe the simplest is to first
show an example:

---------------------------------------------------------------------->
Python 2.3.3 (#1, Jan 24 2004, 09:01:30)
[GCC 3.3 20030226 (prerelease) (SuSE Linux)] on linux2
Type "help", "copyright", "credits" or "license" for more information..... __metaclass__ = type
.... def __cmp__(a, b):
.... print 'C', a, b
.... return 0
.... .... import types
.... __metaclass__ = types.ClassType
.... def __cmp__(a, b):
.... print 'D', a, b
.... return 0
.... Traceback (most recent call last):
File "<stdin>", line 1, in ?
TypeError: unhashable instance
----------------------------------------------------------------------<

As you see, `C' is new-style, `D' is old-style. (I could have written
declarations more simply, if __metaclass__ was not preset to type within
my `.pythonrc' -- in any case, the above writing is very explicit.

Is there a change in semantics in this area? For new-style, I have:

---------------------------------------------------------------------->

c1 = C()
c2 = C()
z = {}
z[c1] = 101
z[c2] = 102
c1 == c2

z[c1] 101
z[c2]

102
----------------------------------------------------------------------<

So, despite c1 equals c2, they create two different entries in a
dictionary. I thought that this is such an inconsistency that was the
reason behind requiring an explicit `__hash__' for old-style classes.
So, it seems I was not thinking right, there is something which escapes
my understanding. Would some kind soul help mine?

 

[Josiah Carlson]

So, despite c1 equals c2, they create two different entries in a

dictionary. I thought that this is such an inconsistency that was the
reason behind requiring an explicit `__hash__' for old-style classes.
So, it seems I was not thinking right, there is something which escapes
my understanding. Would some kind soul help mine?

When two objects have different hashes, they are not required to be
compared when inserted into a dictionary. They may be compared, if two
hashes drop the objects into the same bin.

- Josiah