property

property packages up three functions that get, set, or delete an attribute, and a docstring.

property(fget=None, fset=None, fdel=None, doc=None)

property() is a succinct way of building a data descriptor that triggers function calls upon access to an attribute

The property() builtin helps whenever a user interface has granted attribute access and then subsequent changes require the intervention of a method.

For example, if you want to define a size attribute that's computed, but also settable, you could write:

class C(object):
def get_size (self):
result = ... computation ...
return result
def set_size (self, size):
... compute something based on the size
and set internal state appropriately ...

# Define a property.  The 'delete this attribute'
# method is defined as None, so the attribute
# can't be deleted.
size = property(get_size, set_size,
           None,
           "Storage size of this instance")

how property() is implemented in terms of the descriptor protocol, here is a pure Python equivalent:

class Property(object):
  "Emulate PyProperty_Type() in Objects/descrobject.c"

  def __init__(self, fget=None, fset=None, fdel=None, doc=None):
      self.fget = fget
      self.fset = fset
      self.fdel = fdel
      self.__doc__ = doc

  def __get__(self, obj, objtype=None):
      if obj is None:
          return self       
      if self.fget is None:
          raise AttributeError, "unreadable attribute"
      return self.fget(obj)

  def __set__(self, obj, value):
      if self.fset is None:
          raise AttributeError, "can't set attribute"
      self.fset(obj, value)

  def __delete__(self, obj):
      if self.fdel is None:
          raise AttributeError, "can't delete attribute"
      self.fdel(obj)