Inherit class Decimal, and add another input argument in the constructor

Question

A minimal example to reproduce my problem:

from decimal import Decimal

class MyDecimal(Decimal):
    def __init__(self, value, dummy):
        super().__init__(value)
        print(dummy)

x = MyDecimal(5, 'test')

Throws:

TypeError: optional argument must be a context

A similar issue is described in this question, but the answer suggests that this is a bug which was fixed on Python 3.3, and I'm using Python 3.9.

Any idea what I'm doing wrong here, or how else I can inherit class Decimal while using a different constructor prototype in my class (i.e., additional input arguments before and after the input argument passed to the base class upon construction)?

Decimals are immutable, I'd guess this check is being done in `__new__` not `__init__` — jonrsharpe, Aug 31 '22 at 08:17

score 0 · Accepted Answer · answered Aug 31 '22 at 08:16

0

decimal.Decimal uses __new__ instead of __init__, because Decimal instances are immutable, and initializing in __init__ would be a mutative operation. You need to implement __new__ yourself:

import decimal

class MyDecimal(decimal.Decimal):
    def __new__(cls, value, dummy):
        print(dummy)
        return super().__new__(cls, value)

answered Aug 31 '22 at 08:16

user2357112

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1. Thank you very much for your answer!. 2. How exactly are they immutable when I can do something like `x += 5` (or am I not interpreting 'immutable' correctly)? 3. Do I need to implement `__new__` in addition to `__init__` or instead of `__init__`? – Aug 31 '22 at 08:35
@bbbbbbbbb: `x += 5` will create a new Decimal. It will not mutate the original. As for `__init__`, you do not need to implement it. Just `__new__`. Implementing both in the same class is usually a mistake, and will lead to weird initialization order issues in subclasses. – user2357112 Aug 31 '22 at 08:37
OK, something fundamental is missing in your answer, probably due to the very simple MVP that I posted. I don't really have a 'dummy' input argument that I print. I have an input argument which I store into `self`. How exactly can I achieve that when there IS no `self`? – Aug 31 '22 at 08:39
@bbbbbbbbb: What do you mean by "store into `self`"? If you mean assigning it to an attribute, then just assign it to an attribute of the new object - the one created by `super().__new__`. That'll work fine. The parts of your object structure that are inherited from `decimal.Decimal` are immutable, but attributes you add in the subclass can be assigned normally. – user2357112 Aug 31 '22 at 08:42
I mean, instead of `print(dummy)`, I need to do `self.dummy = dummy`. – Aug 31 '22 at 08:44
Then just set a `dummy` attribute on the new object. `retval = super().__new__(cls, value); retval.dummy = dummy; return retval`. – user2357112 Aug 31 '22 at 08:45

Inherit class Decimal, and add another input argument in the constructor

1 Answers1