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Alternative Description of Property

John Posner

Following is a revision of the official documentation for property, which can be found at http://docs.python.org/library/functions.html#property. For an introduction to Python's property feature, see ComputedAttributesUsingPropertyObjects.

property([getfn[, setfn[, delfn[, docstr]]]])

property is a built-in data type, used to implement managed (computed) attributes. You assign the property object created by the call property(optional-args) to a class attribute of a new-style class (a class that inherits from object). When the attribute is accessed through an instance of the class, it dispatches functions that implement the managed-attribute operations.

The getfn argument is a function or method that will be dispatched to get the attribute value; likewise, setfn is a function for setting the value, and delfn is a function for deleting the attribute. docstr is a string that becomes the managed attribute's doc string; if you omit this argument, getfn 's doc string, if any, is used instead.

This example creates a managed attribute named x in class Cls:

class Cls(object):
    def __init__(self):
        self._x_storage = 100

    def get_val(self):
        return self._x_storage
    def set_val(self, value):
        self._x_storage = value
    def del_attr(self):
        del self._x_storage

    x = property(get_val, set_val, del_attr, "I'm the 'x' property.")

# using the managed attribute
obj = Cls()
print obj.x + 5   # executes get_val(obj)
obj.x = 55        # executes set_val(obj, 55)

A minimal usable property has a "get the value" function only, specified as the sole argument to property(). This makes it easy to create a read-only property using the decorator @property:

class Cls(object):
    def __init__(self):
        self._x_storage = 100

    @property
    def x(self):
        return self._x_storage

    # decorator invocation above is equivalent to placing
    # the statement "x = property(x)" here

The decorator effectively converts the "get the value" method x into a read-only managed attribute with the same name.

Although you typically don't need to use them, a property object has its own attributes, fget, fset, and fdel, which provide direct access to the functions that it dispatches. You can use these attributes only if you access the property object through the class itself (not through an instance):

>>> Cls().x
100
>>> Cls.x
<property object at 0x00E905A0>
>>> Cls.x.fget
<function get_val at 0x00E6F7F0>

These attributes are read-only, but a property object also has getter, setter, and deleter methods, designed to be used as decorators, that populate or modify its fget, fset, and fdel attributes. For example, an existing property object's setter method accepts a single function/method argument and returns a new property object that is a copy of the existing one, but with its fset attribute set to the specified function/method. Thus, class Cls in the example above could be reimplemented like this::

class Cls(object):
    def __init__(self):
        self._x = 100

    x = property()

    @x.getter
    def x(self):
        """I'm the 'x' property."""
        return self._x

    @x.setter
    def x(self, value):
        self._x = value

    @x.deleter
    def x(self):
        del self._x

Using the same name x for all the methods causes this name to be repeatedly rebound, to a series of property objects created by the decorators. The process starts with an "empty" property, and gradually builds up to a property that is "full" of dispatch functions.

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