Using instance attrs in init signature in Python

Question

Is there any possibility to use internal instance attributes in the signature of a class in Python3? Or if not, what would be the pythonic way to achieve the same effect. What I am thinking of is a signature like this:

  1 class C:
  2     def __init__(self, a=self._b):
  3         self._b = 1
  4         self.a = a

so that I can have a default for a based on some internal attribute _b.

Can _b be a (logically constant) class attribute? Otherwise, there's no good answer (just use a sentinel value like None as the default). — ShadowRanger
– ShadowRanger, Commented Oct 9, 2022 at 15:25

Samwise · Accepted Answer · 2022-10-09 15:30:34Z

1

Specify the default as a class variable, and use that as the default in both places:

class C:
    default_a = 1

    def __init__(self, a=default_a):
        self._b = self.default_a
        self.a = a

edited Oct 9, 2022 at 15:30

answered Oct 9, 2022 at 15:27

Samwise

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2 Comments

Samwise Over a year ago

ugh, you're right, OP's code confused me as to the actual intent. Rewrote with a class variable.

jsbueno Over a year ago

either way, default_a will be faulty in this way: if it is changed in the class after it is created, the default value in __init__ will remain the previous one, and not be affected by the change. The correct thing to do is use a sentinel value (such as None) as the default argument, and assign the class attr to the instance attr based on whether another value was passed or not.

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