Beginning Python - From Novice to Professional

CHAPTER 9 ■ MAGIC METHODS, PROPERTIES, AND ITERATORS 189
class Rectangle:
def __init__(self):
self.width = 0
self.height = 0
def __setattr__(self, name, value):
if name == 'size':
self.width, self.height = value
else:
self.__dict__[name] = value
def __getattr__(self, name):
if name == 'size':
return self.width, self.height
else:
raise AttributeError
As you can see, this version of the class needs to take care of additional administrative
details. When considering this code example, it’s important to note the following:
• The __setattr__ method is called even if the attribute in question is not size. Therefore,
the method must take both cases into consideration: If the attribute is size, the same
operation is performed as before; otherwise, the magic attribute __dict__ is used. It
contains a dictionary with all the instances attributes. It is used instead of ordinary attribute
assignment to avoid having __setattr__ called again (which would cause the program
to loop endlessly).
• The __getattr__ method is called only if a normal attribute is not found, which means
that if the given name is not size, the attribute does not exist, and the method raises an
AttributeError. This is important if you want the class to work correctly with built-in
functions such as hasattr and getattr. If the name is size, the expression found in the
previous implementation is used.
ANOTHER TRAP
Just as there is an “endless loop” trap associated with __setattr__, there is a trap associated with
__getattribute__ as well. Because it intercepts all attribute accesses (in new-style classes), it will intercept
accesses to __dict__ as well! The only safe way to access attributes on self inside __getattribute__
is to use the __getattribute__ method of the superclass (using super).
Iterators
In this section, I cover only one magic method, __iter__, which is the basis of the iterator
protocol.