Brian Brian - 1 month ago 6
Python Question

Why can't class variables be used in __init__ keyword arg?

I can't find any documentation on when exactly a class can reference itself. In the following it will fail. This is because the class has been created but not initialized until after

__init__
's first line, correct?

class A(object):
class_var = 'Hi'
def __init__(self, var=A.class_var):
self.var = var


So in the use case where I want to do that is this the best solution:

class A(object):
class_var = 'Hi'
def __init__(self, var=None)
if var is None:
var = A.class_var
self.var = var


Any help or documentation appreciated!

Answer

Python scripts are interpreted as you go. So when the interpreter enters __init__() the class variable A isn't defined yet (you are inside it), same with self (that is a different parameter and only available in function body).

However anything in that class is interpreted top to bottom, so class_var is defined so you can simply use that one.

class A(object):
    class_var = 'Hi'
    def __init__(self, var=class_var):
        self.var = var

but I am not super certain that this will be stable across different interpreters...

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