python - Doubts in Hashing -

i reading post , had question regarding __cmp__().

my code

class book(object):       def __init__(self, title, year):          self.title = title         self.year = year      def __hash__(self):         # hash function          print "self = ", self         print "hash value of self = ", hash(str(self))         print "hash value of title = ", hash(self.title)         print "hash value of year = ", hash(self.year)         return 0     def __cmp__(self, other):         return self.title == other.title  books = []  books.append(book("abc", 123)) print hash(books[0])  books.append(book("def", 456)) print hash(books[1])   books.append(book("abc", 123)) print hash(books[len(books)-1])  print len(books)  print cmp(books[0],books[2]) 

output

self =  <__main__.book object @ 0x0000000001e77b00> hash value of self =  -1040857764 hash value of title =  826005955 hash value of year =  123 0 self =  <__main__.book object @ 0x0000000001e77ba8> hash value of self =  -992414627 hash value of title =  -589261154 hash value of year =  456 0 self =  <__main__.book object @ 0x0000000001e77be0> hash value of self =  1901105233 hash value of title =  -2015893559 hash value of year =  789 0 self =  <__main__.book object @ 0x0000000001e77c18> hash value of self =  -228580758 hash value of title =  826005955 hash value of year =  123 0 4 1  # how ?  

== operator on strings returns boolean value.
in code cmp() compares 2 strings (namely titles) return value of cmp() must bool.

• how come getting integer ? (i looking explanation respect comparison of objects x > y)
• even when integer obtained, how value decided ?
• how can make 1st , 3rd object equal ?

how come getting integer ?

__cmp__ returns int. __cmp__ returning bool, subclass of int, python interprets int.

even when integer obtained, how value decided ?

• -1 means first operand less second
• 0 means both operands equal
• +1 means first operand greater second

how can make 1st , 3rd object equal?

the problem code __cmp__ returns result of ==, returns bool, subclass of int, , true 1, instead of returning 0 __cmp__ returning true/1 -- wrong.

the methods should using (as __cmp__ gets removed in later pythons) __eq__, __ne__, __le__, __lt__, __ge__, , __gt__.

using __eq__:

def __eq__(self, other):     if not isinstance(other, self.__class__):         return notimplemented     return self.title == other.title 

note comparing title, , not considering year (which may fine, or may not be, depending on how using book class).

you can find out more them here.