Core Python Programming (2nd Edition)

Addition
We define the addition operator functionality as adding the numbers together and concatenating the
strings; the tricky part is that the strings must be concatenated in the correct order. For example, let
NumStr1 = [n1 :: s1] and NumStr2 = [n2 :: s2]. Then NumStr1 + NumStr2 is performed as [n1 + n2 ::
s1 + s2] where + represents addition for numbers and concatenation for strings.
Multiplication
Similarly, we define the multiplication operator functionality as multiplying the numbers together and
repeating or concatenating the strings, i.e., NumStr1 * NumStr2 = [n1 * n :: s1 * n].
False Value
This entity has a false value when the number has a numeric value of zero and the string is empty, i.e.,
when NumStr = [0 :: ''].
Comparisons
Comparing a pair of NumStr objects, i.e., [n1 :: s1] vs. [n2 :: s2], we find nine different combinations
(i.e., n1 > n2 and s1 < s2, n1 == n2 and s1 > s2, etc.). We use the normal numeric and lexicographic
compares for numbers and strings, respectively, i.e., the ordinary comparison of cmp(obj1, obj2) will
return an integer less than zero if obj1 < obj2, greater than zero if obj1 > obj2, or equal to zero if the
objects have the same value.
The solution for our class is to add both of these values and return the result. The interesting thing is
that cmp() does not always return -1, 0, or 1 for us. It is, as described above, an integer less than, equal
to, or greater than zero.
In order to correctly compare our objects, we need __cmp__() to return a value of 1 if (n1 > n2) and (s1
> s2), -1 if (n1 < n2) and (s1 < s2), and 0 if both sets of numbers and strings are the same, or if the
comparisons offset each other, i.e., (n1 < n2) and (s1 > s2), or vice versa.
Example 13.6. Multi-Type Class Customization (numstr.py)
1 #!/usr/bin/env python
2
3 class NumStr(object):
4
5 def __init__(self, num=0, string=''):
6 self.__num = num
7 self.__string = string
8
9 def __str__(self): # define for str()
10 return '[%d :: %r]' % \
11 self.__num, self.__string)
12 __repr__ = __str__
13
14 def __add__(self, other): # define for s+o
15 if isinstance(other, NumStr):
16 return self.__class__(self.__num + \