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15.7: Debugging

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  • When you start working with objects, you are likely to encounter some new exceptions. If you try to access an attribute that doesn’t exist, you get an AttributeError:

    >>> p = Point()
    >>> p.x = 3
    >>> p.y = 4
    >>> p.z
    AttributeError: Point instance has no attribute 'z'

    If you are not sure what type an object is, you can ask:

    >>> type(p)
    <class '__main__.Point'>

    You can also use isinstance to check whether an object is an instance of a class:

    >>> isinstance(p, Point)

    If you are not sure whether an object has a particular attribute, you can use the built-in function hasattr:

    >>> hasattr(p, 'x')
    >>> hasattr(p, 'z')

    The first argument can be any object; the second argument is a string that contains the name of the attribute.

    You can also use a try statement to see if the object has the attributes you need:

        x = p.x
    except AttributeError:
        x = 0

    This approach can make it easier to write functions that work with different types; more on that topic is coming up in Section 17.9.

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