"elventear" <[EMAIL PROTECTED]> wrote in message news:[EMAIL PROTECTED] On May 12, 12:25 am, "Gabriel Genellina" <[EMAIL PROTECTED]> wrote: > En Fri, 11 May 2007 19:17:57 -0300, elventear <[EMAIL PROTECTED]> > escribió: > "The only required property is that objects which compare equal have the > same hash value; it is advised to somehow mix together (e.g., using > exclusive or) the hash values for the components of the object that also > play a part in comparison of objects. If a class does not define a > __cmp__() method it should not define a __hash__() operation either; if > it
I suspect/believe that the above should be __cmp__() or __eq__() both for uniformity with the usage below and also because objects do not need to be ordered (__cmp__) to be used as dict keys. > 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)." Thanks for the information. I have a doubt regarding the wording in the paragraph on top. Since I am defining a hash for my object, it makes sense that I should be able to define equality. But I am not sure about inequality, in my specific case. The paragraph above mentions that __cmp__ should be defined if I define a __hash__, but in the default behaviour of __cmp__ inequality is included. So what should I do? Also why is equality necessary to be defined? Do dicts only use __hash__ or do they use equality as well? =============== Dicts first compare hashes and if they are equal, then check equality. If two unequal strings have the same hash value, as is possible of course (given only 2**32 possible hashes and many more possible strings), both can still be used as different keys. Ditto for unequal numbers. Or for a number and string with equal hashes. And so on. The first quoted sentence, about mixing, is directed at minimizing such hash collisions. Terry Jan Reedy
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