On 3/3/2010 6:56 PM, John Posner wrote:
... I was thinking
today about "doing a Bruno", and producing similar pieces on:
* properties created with the @property decorator
* the descriptor protocol
I'll try to produce something over the next couple of days.
Starting to think about a writeup on Python properties, I've discovered
that the official Glossary [1] lacks an entry for "property" -- it's
missing in both Py2 and Py3!
Here's a somewhat long-winded definition -- comments, please:
---------------------------------------------
An attribute, *a*, of an object, *obj*, is said to be implemented as a
property if the standard ways of accessing the attribute:
* evaluation: print obj.a
* assignment: obj.a = 42
* deletion: del obj.a
... cause methods of a user-defined *property object* to be invoked. The
attribute is created as a class attribute, not an instance attribute.
Example:
class Widget:
# create "color" as class attribute, not within __init__()
color = <<property-object>>
def __init__(self, ...):
# do not define "self.color" instance attribute
The property object can be created with the built-in function
property(), which in some cases can be coded as a decorator: @property.
The property object can also be an instance of a class that implements
the descriptor protocol.
---------------------------------------------
Tx,
John
[1] http://docs.python.org/glossary.html
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