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In-Place Addition

To also implement += in-place augmented addition, code either an __iadd__ or an __add__. The latter is used if the former is absent. In fact, the prior section’s Commuter class supports += already for this reason, but __iadd__ allows for more efficient in-place changes:

>>> class Number:
...     def __init__(self, val):
...         self.val = val
...     def __iadd__(self, other):             # __iadd__ explicit: x += y
...         self.val += other                  # Usually returns self
...         return self
...
>>> x = Number(5)
>>> x += 1
>>> x += 1
>>> x.val
7
>>> class Number:
...     def __init__(self, val):
...         self.val = val
...     def __add__(self, other):              # __add__ fallback: x = (x + y)
...         return Number(self.val + other)    # Propagates class type
...
>>> x = Number(5)
>>> x += 1
>>> x += 1
>>> x.val
7

Every binary operator has similar right-side and in-place overloading methods that work the same (e.g., __mul__, __rmul__, and __imul__). Right-side methods are an advanced topic and tend to be fairly rarely used in practice; you only code them when you need operators to be commutative, and then only if you need to support such operators at all. For instance, a Vector class may use these tools, but an Employee or Button class probably would not.