Basic Terminology

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.