“The Cleveland Tourism Board gave me 14 million dollars about 8 months ago to make a promotional video to bring people to Cleveland. As usual, I waited till the last minute and I ended up having to shoot and edit it in about an hour yesterday afternoon.” — bishopvids

Cleveland Code Co-op meeting on Sunday

The fifth meeting of the Cleveland Code Co-op will commence this Sunday, February 22, from 13:00 till 19:00. We’re expecting more participants than usual, and likely projects so far include:

• redit, a text editor in Ruby
• 80sheep, an ADC (peer-to-peer) client in Python
• a Python tutorial for beginners
• your wildest software fantasies

This month’s meeting will take place in the EECS student lounge at Case Western Reserve University, which is located in the Glennan Building. Directions are located on the wiki. You can also join us on IRC in #C3 on irc.freenode.net.

Food and drinks will be provided! Hope to see you there.

Python instance descriptors: when class descriptors aren’t dynamic enough

Python descriptors are great for customizing access to attributes on a class or instance. They are a big win for tasks like mapping Python objects to data from non-Python sources (such as SQL), since mapped attributes will need to be encoded/decoded and connected to other attributes in some way.

Below is a very simple descriptor; as you can see, accessing it from both the class and the instance invoke the descriptor protocol:

class Test(object):
    pass

class Descriptor(object):
    def __get__(self, instance, owner):
        return "Hello, world."

>>> Test.x = Descriptor()
>>> Test.x
'Hello, world.'
>>> test = Test()
>>> test.x
'Hello, world.'

However, in order to add descriptors to an object, they must be added to the object’s class. Descriptors added to an instance do not invoke the descriptor protocol:

>>> test.y = Descriptor()
>>> test.y
<__main__.Descriptor object at 0x16fe810>

This means that creating an instance with dynamic (determined at runtime) descriptors requires either the heavy-handed approach of generating a class just for that object (since adding descriptors to its class will add them to all other instances of the class), or the ad-hoc approach of redefining getattr/setattr behavior (essentially re-implementing your own descriptor protocol).

It turns out the latter approach is not as messy as it first sounds. Below is a class that enables “instance descriptors”:

class InstanceDescriptorMixin(object):
    def __getattribute__(self, name):
        value = object.__getattribute__(self, name)
        if hasattr(value, '__get__'):
            value = value.__get__(self, self.__class__)
        return value

    def __setattr__(self, name, value):
        try:
            obj = object.__getattribute__(self, name)
        except AttributeError:
            pass
        else:
            if hasattr(obj, '__set__'):
                return obj.__set__(self, value)
        return object.__setattr__(self, name, value)

class Test(InstanceDescriptorMixin):
    pass

>>> test = Test()
>>> test.z = Descriptor()
>>> test.z
'Hello, world.'

The purpose of syntax highlighting is to turn your code into a map, not The Jimi Hendrix Experience.

geopy sprint at December C³ meeting

Today (Sunday) the third meeting of the Cleveland Code Co-op will be held from 1pm to 7pm at Gypsy Beans & Bakery. We’ll be focusing on geopy again, continuing our sprint goals from last time. All are welcome to attend. You can join us via IRC in #c3 on irc.freenode.net.