CS105: Introduction to Python

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)

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

>>> isinstance(p, Point)
True

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

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

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:

try:
    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.