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17.7: Operator overloading

  • Page ID
    42449
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    By defining other special methods, you can specify the behavior of operators on programmer-defined types. For example, if you define a method named __add__ for the Time class, you can use the + operator on Time objects.

    Here is what the definition might look like:

    # inside class Time:
    
        def __add__(self, other):
            seconds = self.time_to_int() + other.time_to_int()
            return int_to_time(seconds)
    

    And here is how you could use it:

    >>> start = Time(9, 45)
    >>> duration = Time(1, 35)
    >>> print(start + duration)
    11:20:00
    

    When you apply the + operator to Time objects, Python invokes __add__. When you print the result, Python invokes __str__. So there is a lot happening behind the scenes!

    Changing the behavior of an operator so that it works with programmer-defined types is called operator overloading. For every operator in Python there is a corresponding special method, like __add__. For more details, see http://docs.python.org/3/reference/datamodel.html#specialnames.

    As an exercise, write an add method for the Point class.


    17.7: Operator overloading is shared under a CC BY-NC 3.0 license and was authored, remixed, and/or curated by Allen B. Downey.

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