o_18ufhmfmq19t513t3lgmn5l1qa8a.pdf

188 CHAPTER 9 ■ MAGIC METHODS, PROPERTIES, AND ITERATORS
__metaclass__ = type
class MyClass:
@staticmethod
def smeth():
print 'This is a static method'
@classmethod
def cmeth(cls):
print 'This is a class method of', cls
Once you’ve defined these methods, they can be used like this (that is, without instantiating
the class):
>>> MyClass.smeth()
This is a static method
>>> MyClass.cmeth()
This is a class method of
Static methods and class methods haven’t historically been important in Python, mainly
because you could always use functions or bound methods instead, in some way, but also because
the support hasn’t really been there in earlier versions. So even though you may not see them
used much in current code, they do have their uses (such as factory functions, if you’ve heard
of those), and you may well think of some new ones.
__getattr__, __setattr__, and Friends
It is possible to implement properties with old-style classes, too, but you have to use magic
methods rather than the property function. The following four methods provide all the functionality
you need (in old-style classes, you only use the last three):
__getattribute__(self, name): Automatically called when the attribute name is accessed.
(Works correctly on new-style classes only.)
__getattr__(self, name): Automatically called when the attribute name is accessed and
the object has no such attribute.
__setattr__(self, name, value): Automatically called when an attempt is made to bind
the attribute name to value.
__delattr__(self, name): Automatically called when an attempt is made to delete the
attribute name.
Although a bit trickier (and less efficient) to use than property, these magic methods are
quite powerful because you can write code in one of these methods that deals with several
properties. (If you have a choice, though, stick with property.)
Here is the Rectangle example again, this time with magic methods: