On Sun, May 13, 2012 at 12:11 PM, Bob Grommes <bob.grom...@gmail.com> wrote: > Noob alert: writing my first Python class library. > > I have a straightforward class called Utility that lives in Utility.py. > > I'm trying to get a handle on best practices for fleshing out a library. As > such, I've done the following for starters: > > def __str__(self): > return str(type(self)) > > # def __eq__(self,other): > # return hash(self) == hash(other) > > The commented-out method is what I'm questioning. As-is, I can do the > following from my test harness: > > u = Utility() > print(str(u)) > print(hash(u)) > u2 = Utility() > print(hash(u2)) > print(hash(u) == hash(u2)) > > However if I uncomment the above _eq_() implementation, I get the following > output: > > <class 'Utility.Utility'> > Traceback (most recent call last): > File "/Users/bob/PycharmProjects/BGC/Tests.py", line 7, in <module> > print(hash(u)) > TypeError: unhashable type: 'Utility' > > Process finished with exit code 1 > > Obviously there is some sort of default implementation of __hash__() at work > and my implementation of _eq_() has somehow broken it. Can anyone explain > what's going on?
In Python, the default implementations of __hash__ and __eq__ are set to return the id of the object. Thus, an object by default compares equal only to itself, and it hashes the same everytime. In Python3, if you override __eq__, the default __hash__ is removed, however it can also be overridden to provide better hashing support. In Python2, the default removal of __hash__ did not exist, which could lead to stubble bugs where a class would override __eq__ by leave __hash__ as the default implementation. Generally, the default __eq__ and __hash__ functions provide the correct results, and are nice and convenient to have. From there, the case where __eq__ is overridden is the next most common, and if it is overridden, the default __hash__ is almost never correct, and thus the object should either not be hashable (the default in Python3) or should also be overriden to produce the correct results. The rule is that, if two objects return different results from __hash__, they should never compare equal. The opposite rule also holds true: if two objects compare equal, they should return the same value from __hash__. See http://docs.python.org/reference/datamodel.html#object.__hash__ and http://docs.python.org/reference/datamodel.html#object.__lt__ for more information. -- http://mail.python.org/mailman/listinfo/python-list
