About the use of **args

Mark McEahern marklists at mceahern.com
Wed Dec 10 13:37:28 CET 2003


On Wed, 2003-12-10 at 03:24, Graham Ashton wrote:
> [snip]

> I've not run that so it may have syntax errors. It should illustrate the
> principle though; you can use attributes directly until you want to take
> actions when you set them. Then you can make the attribute "private" and
> replace it with a property that access the real attribute, and does
> whatever else you want to do when it's accessed.

A couple of points on Graham's example:

1.  You have to subclass from object in order to use property.

2.  When Graham nominalizes private, he means that you can still access
the attribute variable directly--and that's by design.

See example below.

Cheers,

// m

#!/usr/bin/env python

class Shoe:

    def __init__(self):
        self._size = None
    def getSize(self):
        print "In getSize..."
        return self._size
    def setSize(self, size):
        print "In setSize..."
        self._size = size
    size = property(getSize, setSize)

class Shoe2(object):

    def __init__(self):
        self._size = None
    def getSize(self):
        print "In getSize..."
        return self._size
    def setSize(self, size):
        print "In setSize..."
        self._size = size
    size = property(getSize, setSize)

s = Shoe()
# Since we haven't subclassed from object, the property descriptor
# doesn't work; notice that setSize() isn't getting called...
s.size = 1
print s.size
# We can still access the "private" member variable.  In Python,
# private is merely a convention.
print s._size

# Now that we've subclassed from object, our get/set methods are
# called.
s2 = Shoe2()
s2.size = 1
print s2.size
# And we can still access the "private" member variable.
print s2._size








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