Question

I have the following code

class Transcription(object):
    WORD = 0
    PHONE = 1
    STATE = 2

    def __init__(self):
        self.transcriptions = []

    def align_transcription(self,model,target=Transcription.PHONE):
        pass

The important part here is that I would like to have a class member as default value for a variable. This however gives the following error:

NameError: name 'Transcription' is not defined

Why is this not possible and what is the right (pythonic) way to do something like this.

Answer 1

You can’t access it because Transcription isn’t defined at the time that the def statement is running.

 def align_transcription(self,model,target=PHONE):
        pass

will do the trick. The PHONE name is available in the namespace which will become the Transcription class after the class statement finishes executing.

The way this works is that class is a statement that actually gets run. When Python encounters a class statement, it enters a new local scope and executes everything that’s indented under the class statement as if it were a function. It then passes the resulting namespace, the name of the class and a tuple of the baseclasses to the metaclass which is type by default. The metaclass returns an instance of itself which is the actual class. None of this has occurred when the def statement executes.

The statement

class Foo(object):
    a = 1
    def foo(self):
        print self.a

creates a namespace ns = {'a': 1, 'foo': foo} and then executes

Foo = type('Foo', (object,), ns)

This is equivalent to

def foo(self):
    print self.a

Foo = type('Foo', (object,), {'a': 1, 'foo': foo})

You can clearly see that Foo is not defined at the time that foo is being defined so Foo.a makes no sense.