Core Python Programming (2nd Edition)

mon - tue
Traceback (most recent call last):
File "", line 1, in ?
TypeError: unsupported operand type(s) for -: 'Time60'
and 'Time60'
In-Place Addition
With augmented assignment (introduced back in Python 2.0), we may also wish to override the "inplace"
operators, for example, __iadd__(). This is for supporting an operation like mon += tue and having
the correct result placed in mon. The only trick with overriding an __i*__() method is that it has to return
self. Let us add the following bits of code to our example, fixing our repr() issue above as well as
supporting augmented assignment:
__repr__ = __str__
def __iadd__(self, other):
self.hr += other.hr
self.min += other.min
return self
Here is our resulting output:
>>> mon = Time60(10, 30)
>>> tue = Time60(11, 15)
>>> mon
10:30
>>> id(mon)
401872
>>> mon += tue
>>> id(mon)
401872
>>> mon
21:45
Note the use of the id() built-in function to confirm that before and after the in-place addition we are
indeed modifying the same object and not creating a new one. This is a great start at a class that has a
lot of potential. The complete class definition for Time60 is given in Example 13.3.
Example 13.3. Intermediate Customization (time60.py)