Python: Can I use a class attribute as a default value for an instance method?

Question

I am writing a new class in Python (2.5). My immediate goal is to:

GOAL: Use the value of a class (static) attribute as the default value for an instance attribute

I can add logic to the __init__ method to do this, and it works fine.

In an effort to tidy up the body of the __init__ method, however, I tried to set a default value for one of its arguments, instead of deriving the default inside the method, itself. This construct raises a NameError exception, though, and I don't understand why.

Consider the two occurrences of the variable MyClass.__DefaultName in following (admittedly nonsensical) example:

class MyClass():

    __DefaultName = 'DefaultName'


    def __init__(
            SELF
            ,name = MyClass.__DefaultName # <-- FAILS
    ):

            if name: SELF.__Name = name
            else: SELF.__Name = MyClass.__DefaultName # <-- SUCCEEDS

As the annotations suggest, I can reference MyClass.__DefaultName within the body of the __init__ method, but I get a NameError exception when I try to use that same reference as an argument default.

I've tried myriad variations on the syntax, and have read copious doc/PEPs in an effort to learn why the interpreter handles these two references differently. I'm sure I've encountered the answer somewhere along the way, but I just haven't recognized it yet.

I would greatly appreciate it if someone could help me understand why the example above produces a NameError exception - and, ideally, which syntax (if any) would avoid the exception. Thank you, in advance.

(P.S. I have an actual business case for wanting to do this. I can provide details; I just didn't want to obscure the original post with extraneous information.)

Answer 1

That's because, according to the documentation:

Default parameter values are evaluated when the function definition is executed. This means that the expression is evaluated once, when the function is defined

When __init__() is defined, the definition of MyClass is incomplete, as it's still being parsed, so you can't refer to __DefaultName yet. To work around that, you can pass a special unique value to __init__(), such as None:

def __init__(self, name=None):
    if name is None:
        name = MyClass.__DefaultName
    .
    .
    .