calling class/static method from class variable in python

Question

I\'m trying to make a ImageLoader class handle the loading and processing of image resources like this:

class ImageLoader:
    TileTable = __loadTileTable(\'         


        

Answer 1

Is there a design reason you're using a static method? If so, because you're not overloading the class initialization, you'll need to declare the variable after the method definition.

But, if you do this, you'lll get a new error:

NameError: name 'arg1' is not defined

The reason for this is because you're executing the method within the class before the class is even instantiated, therefore you never have a chance to pass the arguments to the method.

So, the proper way to do this is to overload the __init__() method so that assignment to TileTable only happens when the class is constructed:

class ImageLoader(object):
    def __init__(self, arg1, arg2):
        self.TileTable = self._loadTileTable(arg1, arg2)

    @staticmethod
    def _loadTileTable(arg1, arg2):
        print arg1, arg2

This gives you the ability to call ImageLoader._loadTileTable() without having an instance, but then it also allows you to create the TileTable instance variable upon creating an instance.

Using a Class method
In response to my comment about the possible need for a classmethod, here is an example that covers this:

class ImageLoader:
    @classmethod
    def _loadTileTable(cls, arg1, arg2):
        return arg1, arg2

# We're creating the class variable outside of the class definition. If you're doing 
# this in a module, no one will ever notice.
ImageLoader.TileTable = ImageLoader._loadTileTable('foo', 'bar')

There might be a better way to do this, I don't know. But I do think that this covers what you are looking for:

>>> i = ImageLoader()
>>> i
<__main__.ImageLoader instance at 0x100488f80>
>>> ImageLoader.TileTable
('foo', 'bar')
>>> i.TileTable
('foo', 'bar')

There you have an instance i that has access to the class variable, but ImageLoader.TileTable is still available from the class object without the need for an instance.