learn-python

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"""Multiple Inheritance
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@see: https://docs.python.org/3/tutorial/classes.html#multiple-inheritance
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Some classes may derive from multiple classes. This means that the derived class would have
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its attributes, along with the attributes of all the classes that it was derived from.
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"""
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def test_multiple_inheritance():
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    """Multiple Inheritance"""
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    # pylint: disable=too-few-public-methods
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    class Clock:
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        """Clock class"""
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        time = '11:23 PM'
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        def get_time(self):
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            """Get current time
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            Method is hardcoded just for multiple inheritance illustration.
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            """
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            return self.time
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    # pylint: disable=too-few-public-methods
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    class Calendar:
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        """Calendar class"""
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        date = '12/08/2018'
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        def get_date(self):
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            """Get current date
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            Method is hardcoded just for multiple inheritance illustration.
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            """
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            return self.date
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    # Python supports a form of multiple inheritance as well. A class definition with multiple
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    # base classes looks like this.
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    class CalendarClock(Clock, Calendar):
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        """Class that uses multiple inheritance.
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        For most purposes, in the simplest cases, you can think of the search for attributes
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        inherited from a parent class as depth-first, left-to-right, not searching twice in the same
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        class where there is an overlap in the hierarchy. Thus, if an attribute is not found in
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        CalendarClock, it is searched for in Clock, then (recursively) in the base classes of
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        Clock, and if it was not found there, it was searched for in Calendar, and so on.
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        In fact, it is slightly more complex than that; the method resolution order changes
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        dynamically to support cooperative calls to super(). This approach is known in some other
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        multiple-inheritance languages as call-next-method and is more powerful than the super call
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        found in single-inheritance languages.
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        Dynamic ordering is necessary because all cases of multiple inheritance exhibit one or more
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        diamond relationships (where at least one of the parent classes can be accessed through
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        multiple paths from the bottommost class). For example, all classes inherit from object,
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        so any case of multiple inheritance provides more than one path to reach object. To keep
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        the base classes from being accessed more than once, the dynamic algorithm linearizes the
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        search order in a way that preserves the left-to-right ordering specified in each class,
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        that calls each parent only once, and that is monotonic (meaning that a class can be
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        subclassed without affecting the precedence order of its parents).
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        """
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    calendar_clock = CalendarClock()
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    assert calendar_clock.get_date() == '12/08/2018'
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    assert calendar_clock.get_time() == '11:23 PM'
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