control initialize order when Python dataclass inheriting a class

Question

What I kown
The Python dataclass allows inheritance, either with dataclass or class.
In best practice (and also in other languages), when we do inheritance, the initialization should be called first. In Python it is:

def __init__(self):
    super().__init__()
    ...

What I’m doing
Since the dataclass was introduced in Python 3.7, I am considering replace all of my classes with the dataclass.
With dataclass, one of its benefits is to generate __init__ for you. This is not good when the dataclass needs to inherit a base class — for example:

class Base:
    def __init__(self):
        self.a = 1

@dataclass
class Child(Base):
    a:int
    def __post_init__(self):
        super().__init__()

My problem
The problem is we have to put super initialization call inside __post_init__ which in fact is called after dataclass’s init.
The downside is that we lose the convention contract and the initialization disorder leads to that we can not override attributes of super classes.

It can be solved by concept of __pre_init__. I’ve read the document and does not see anything to do with that concept there. Am I missing something?

Asked By: WeiChing 林煒清

||

Source

Answer 1

how about:

from dataclasses import dataclass


class Base:
    def __init__(self, a=1):
        self.a = a


@dataclass
class Child(Base):

    def __post_init__(self):
        super().__init__()


ch = Child()

Answered By: naivepredictor

Answer 2

Actually there is one method which is called before __init__: it is __new__. So you can do such a trick: call Base.__init__ in Child.__new__. I can’t say is it a good solution, but if you’re interested, here is a working example:

class Base:
    def __init__(self, a=1):
        self.a = a


@dataclass
class Child(Base):
    a: int

    def __new__(cls, *args, **kwargs):
        obj = object.__new__(cls)
        Base.__init__(obj, *args, **kwargs)
        return obj


c = Child(a=3)
print(c.a)  # 3, not 1, because Child.__init__ overrides a

Answered By: sanyassh

Answer 3

In best practice […], when we do inheritance, the initialization should be called first.

This is a reasonable best practice to follow, but in the particular case of dataclasses, it doesn’t make any sense.

There are two reasons for calling a parent’s constructor, 1) to instantiate arguments that are to be handled by the parent’s constructor, and 2) to run any logic in the parent constructor that needs to happen before instantiation.

Dataclasses already handles the first one for us:

 @dataclass
class A:
    var_1: str

@dataclass
class B(A):
    var_2: str

print(B(var_1='a', var_2='b'))  # prints: B(var_1='a', var_2='b')
# 'var_a' got handled without us needing to do anything

And the second one does not apply to dataclasses. Other classes might do all kinds of strange things in their constructor, but dataclasses do exactly one thing: They assign the input arguments to their attributes. If they need to do anything else (that can’t by handled by a __post_init__), you might be writing a class that shouldn’t be a dataclass.

Answered By: Arne

Answer 4

Using dataclass inherit from dataclass:

from dataclasses import dataclass

@dataclass
class Base:
    a: int = 1
    def __post_init__(self):
        self.b = self.a * 2

@dataclass
class Child(Base):
    def __post_init__(self):
        # super().__init__()  # this cause RecursionError
        super().__post_init__()  # without this, self.b is doesn't exist
        self.c = self.b * 5

ch = Child(a=3)
print(ch.a, ch.b, ch.c)  # Output: 3 6 30

Disclaimer: I’m still learning dataclass and can’t find in the docs itself if this is recommended or not.

Answered By: Muhammad Yasirroni

control initialize order when Python dataclass inheriting a class

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