Unexpected return value from __add__

[Magnus Lycka]

The code below puzzles me. Am I just blind, or is there something
really strange here...

The code below is a small test case for the problem. Don't
expect it to make any sense... Anyway, I have an IP address
stored as a long, and presented as '192.0.1.1' etc. I have
an __add__ method where I can add an integer value to an IP
address, so that I ought to get:

>>> a = IPAddr((192,0,0,0))
>>> b = a + 257
>>> print b
192.0.1.1

What actually happens is that I get an object of the right
class back, but not with the right value. The internal long
has the value 0! But if I remove the # in __add__, I see a
printout of just the value I expected! Somehow it seems to
get lost when I return it! Some strange scope thingie???

I'm using ActivePython build 2.1.211

--

class IPAddr:
    def __init__(self,addr):
        if type(addr) == type(0L):
            self.addr = addr
        else:
            self.addr = 0L
            for digit in addr:
                self.addr = 256*self.addr+digit
    def __str__(self):
        res = []
        while self.addr:
            self.addr, digit = divmod(self.addr,256)
            res.append(str(digit))
        res.reverse()
        return ".".join(res)
    def __add__(self, number):
        newAddress = self.addr+number
        new = IPAddr(newAddress)
        #print "New IP", new, new.__class__
        return new

import unittest

class IPAddrTest(unittest.TestCase):
    def testAdd(self):
        a = IPAddr((192,0,0,0))
        b = a + 257
        # Check that a stays the same
        assert str(a) == "192.0.0.0", "Expected '192.0.0.0', \
           got '%s'" % str(a)
        # Check that we get the right type back
        assert isinstance(b,IPAddr)
        # Check that we get correct value
        assert str(b) == "192.0.1.1", "Expected '192.0.1.1', \
           got '%s' (%d)" % (str(b), b.addr)
        
unittest.main()    

-- 
Magnus Lyckå | Älvans väg 99 | magnus at thinkware.se | tel: 070-582 80 65
Thinkware AB | 907 50  UMEÅ  | www.thinkware.se    | fax: 070-612 80 65