Imagine the init() that you'd make if this weren't a dataclass. You'd put parameters in a certain order, and you couldn't do the thing that the error message is complaining about. Answer from Deleted User on reddit.com
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Python
docs.python.org › 3 › library › dataclasses.html
dataclasses — Data Classes
1 month ago - This module provides a decorator and functions for automatically adding generated special methods such as __init__() and __repr__() to user-defined classes. It was originally described in PEP 557. The member variables to use in these generated methods are defined using PEP 526 type annotations. For example, this code: from dataclasses import dataclass @dataclass class InventoryItem: """Class for keeping track of an item in inventory.""" name: str unit_price: float quantity_on_hand: int = 0 def total_cost(self) -> float: return self.unit_price * self.quantity_on_hand
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Reddit
reddit.com › r/learnpython › dataclasses with inheritance?
r/learnpython on Reddit: Dataclasses with Inheritance?
April 18, 2023 -

I have a class Animal and Dog which inherits Animal. Why is it that I get an error if I try to give my Dog class a breed field?

TypeError: non-default argument 'breed' follows default argument

This is my code

from dataclasses import dataclass


@dataclass
class Animal:
    species: str
    arms: int
    legs: int


@dataclass
class Dog(Animal):
    breed: str
    species: str = "Dog"
    arms: int = 0
    legs: int = 4


if __name__ == '__main__':
    jake = Dog(breed="Bulldog")
    print(jake)

I did find that if I add a breed field to Animal I wouldn't get the error.

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Medium
medium.com › @aniscampos › python-dataclass-inheritance-finally-686eaf60fbb5
Python dataclass inheritance, finally ! | by Anis Campos | Medium
October 25, 2021 - If a field is marked as keyword-only, then the only affect is that the __init__() parameter generated from a keyword-only field must be specified with a keyword when __init__() is called. There is no effect on any other aspect of dataclasses. See the parameter glossary entry for details. Also see the KW_ONLY section. New in version 3.10. If against all odds you are still not understanding where this mouthful definition can help us have a better life, let me help you see it from an other angle. So let’s start by exposing a very annoying flaw of the conceptions of python dataclasss.
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Python
peps.python.org › pep-0557
PEP 557 – Data Classes - Python Enhancement Proposals
An example: @dataclass class Base: x: Any = 15.0 y: int = 0 @dataclass class C(Base): z: int = 10 x: int = 15 · The final list of fields is, in order, x, y, z. The final type of x is int, as specified in class C. The generated __init__ method ...
Top answer
1 of 16
397

The way dataclasses combines attributes prevents you from being able to use attributes with defaults in a base class and then use attributes without a default (positional attributes) in a subclass.

That's because the attributes are combined by starting from the bottom of the MRO, and building up an ordered list of the attributes in first-seen order; overrides are kept in their original location. So Parent starts out with ['name', 'age', 'ugly'], where ugly has a default, and then Child adds ['school'] to the end of that list (with ugly already in the list). This means you end up with ['name', 'age', 'ugly', 'school'] and because school doesn't have a default, this results in an invalid argument listing for __init__.

This is documented in PEP-557 Dataclasses, under inheritance:

When the Data Class is being created by the @dataclass decorator, it looks through all of the class's base classes in reverse MRO (that is, starting at object) and, for each Data Class that it finds, adds the fields from that base class to an ordered mapping of fields. After all of the base class fields are added, it adds its own fields to the ordered mapping. All of the generated methods will use this combined, calculated ordered mapping of fields. Because the fields are in insertion order, derived classes override base classes.

and under Specification:

TypeError will be raised if a field without a default value follows a field with a default value. This is true either when this occurs in a single class, or as a result of class inheritance.

You do have a few options here to avoid this issue.

The first option is to use separate base classes to force fields with defaults into a later position in the MRO order. At all cost, avoid setting fields directly on classes that are to be used as base classes, such as Parent.

The following class hierarchy works:

# base classes with fields; fields without defaults separate from fields with.
@dataclass
class _ParentBase:
    name: str
    age: int
    
@dataclass
class _ParentDefaultsBase:
    ugly: bool = False

@dataclass
class _ChildBase(_ParentBase):
    school: str

@dataclass
class _ChildDefaultsBase(_ParentDefaultsBase):
    ugly: bool = True

# public classes, deriving from base-with, base-without field classes
# subclasses of public classes should put the public base class up front.

@dataclass
class Parent(_ParentDefaultsBase, _ParentBase):
    def print_name(self):
        print(self.name)

    def print_age(self):
        print(self.age)

    def print_id(self):
        print(f"The Name is {self.name} and {self.name} is {self.age} year old")

@dataclass
class Child(_ChildDefaultsBase, Parent, _ChildBase):
    pass

By pulling out fields into separate base classes with fields without defaults and fields with defaults, and a carefully selected inheritance order, you can produce an MRO that puts all fields without defaults before those with defaults. The reversed MRO (ignoring object) for Child is:

_ParentBase
_ChildBase
_ParentDefaultsBase
Parent
_ChildDefaultsBase

Note that while Parent doesn't set any new fields, it does inherit the fields from _ParentDefaultsBase and should not end up 'last' in the field listing order; the above order puts _ChildDefaultsBase last so its fields 'win'. The dataclass rules are also satisfied; the classes with fields without defaults (_ParentBase and _ChildBase) precede the classes with fields with defaults (_ParentDefaultsBase and _ChildDefaultsBase).

The result is Parent and Child classes with a sane field older, while Child is still a subclass of Parent:

>>> from inspect import signature
>>> signature(Parent)
<Signature (name: str, age: int, ugly: bool = False) -> None>
>>> signature(Child)
<Signature (name: str, age: int, school: str, ugly: bool = True) -> None>
>>> issubclass(Child, Parent)
True

and so you can create instances of both classes:

>>> jack = Parent('jack snr', 32, ugly=True)
>>> jack_son = Child('jack jnr', 12, school='havard', ugly=True)
>>> jack
Parent(name='jack snr', age=32, ugly=True)
>>> jack_son
Child(name='jack jnr', age=12, school='havard', ugly=True)

Another option is to only use fields with defaults; you can still make in an error to not supply a school value, by raising one in __post_init__:

_no_default = object()

@dataclass
class Child(Parent):
    school: str = _no_default
    ugly: bool = True

    def __post_init__(self):
        if self.school is _no_default:
            raise TypeError("__init__ missing 1 required argument: 'school'")

but this does alter the field order; school ends up after ugly:

<Signature (name: str, age: int, ugly: bool = True, school: str = <object object at 0x1101d1210>) -> None>

and a type hint checker will complain about _no_default not being a string.

You can also use the attrs project, which was the project that inspired dataclasses. It uses a different inheritance merging strategy; it pulls overridden fields in a subclass to the end of the fields list, so ['name', 'age', 'ugly'] in the Parent class becomes ['name', 'age', 'school', 'ugly'] in the Child class; by overriding the field with a default, attrs allows the override without needing to do a MRO dance.

attrs supports defining fields without type hints, but lets stick to the supported type hinting mode by setting auto_attribs=True:

import attr

@attr.s(auto_attribs=True)
class Parent:
    name: str
    age: int
    ugly: bool = False

    def print_name(self):
        print(self.name)

    def print_age(self):
        print(self.age)

    def print_id(self):
        print(f"The Name is {self.name} and {self.name} is {self.age} year old")

@attr.s(auto_attribs=True)
class Child(Parent):
    school: str
    ugly: bool = True
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191

Note that with Python 3.10, it is now possible to do it natively with dataclasses.

Dataclasses 3.10 added the kw_only attribute (similar to attrs). It allows you to specify which fields are keyword_only, thus will be set at the end of the init, not causing an inheritance problem.

Taking directly from Eric Smith's blog post on the subject:

There are two reasons people [were asking for] this feature:

  • When a dataclass has many fields, specifying them by position can become unreadable. It also requires that for backward compatibility, all new fields are added to the end of the dataclass. This isn't always desirable.
  • When a dataclass inherits from another dataclass, and the base class has fields with default values, then all of the fields in the derived class must also have defaults.

What follows is the simplest way to do it with this new argument, but there are multiple ways you can use it to use inheritance with default values in the parent class:

from dataclasses import dataclass

@dataclass(kw_only=True)
class Parent:
    name: str
    age: int
    ugly: bool = False

@dataclass(kw_only=True)
class Child(Parent):
    school: str

ch = Child(name="Kevin", age=17, school="42")
print(ch.ugly)

Take a look at the blogpost linked above for a more thorough explanation of kw_only.

Cheers !

PS: As it is fairly new, note that your IDE might still raise a possible error, but it works at runtime

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GitHub
github.com › ericvsmith › dataclasses › issues › 94
Please add inheritance example for __init__ from ancestor classes · Issue #94 · ericvsmith/dataclasses
November 27, 2017 - In particular, show how to ensure that super().init() is called, since (if I understand correctly), it should be called from post_init to ensure that it happens.
Author   JimJJewett
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Reddit
reddit.com › r/learnpython › modifying a dataclass's __init__() method in a sub-dataclass
r/learnpython on Reddit: Modifying a dataclass's __init__() method in a sub-dataclass
July 31, 2022 -

In the Python standard library's documentation for Dataclasses, there is a section called post init processing. In that section is this bit of code:

@dataclass
class Rectangle:
    height: float
    width: float

@dataclass
class Square(Rectangle):
    side: float

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

The accompanying text says: "The __init__() method generated by dataclass() does not call base class __init__() methods. If the base class has an __init__() method that has to be called, it is common to call this method in a __post_init__() method"

That's all fine and dandy but in my experience this has not worked as I would expect. This is because attempting to instantiate Square() causes its __init__() method to ask for 3 arguments: height, width, and side. I understand this is because of how Dataclass inheritance is advertised to work. What doesn't make sense is why the above example is given as an example of post-processing when the caller of Square() would be supplying redundant values.

It seems silly to require the caller of Square() to supply the height, width, and side if they are all going to eventually be the same value.

In [2]: @dataclass
   ...: class Rectangle:
   ...:     height: float
   ...:     width: float
   ...: 
   ...: @dataclass
   ...: class Square(Rectangle):
   ...:     side: float
   ...: 
   ...:     def __post_init__(self):
   ...:         super().__init__(self.side, self.side)
   ...: 

In [3]: s = Square(10)
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Input In [3], in <cell line: 1>()
----> 1 s = Square(10)

TypeError: Square.__init__() missing 2 required positional arguments: 'width' and 'side'

The only way I have found to overcome this is to define a custom __init__() in Square, like so:

In [4]: @dataclass
   ...: class Rectangle:
   ...:     height: float
   ...:     width: float
   ...: 
   ...: @dataclass
   ...: class Square(Rectangle):
   ...:     def __init__(self, side):
   ...:         Rectangle.__init__(self, side, side)
   ...: 

In [5]: s = Square(10)

In [6]: s
Out[6]: Square(height=10, width=10)

Is there a suggested way to make this kind of method overriding work in dataclasses? It seems like in the documentation and conference presentations that __post_init__() is the preferred option, but that does not seem to work as advertised.

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Emilearthur
emilearthur.github.io › fastblog › python › oop › data › 2021 › 08 › 18 › DataClass-Inheritance-and-Composition.html
Dataclass - Inheritance and Composition | Emile’s Blog
August 18, 2021 - We further modify some classes implemented earlier, which make use of the roles and policies created (Multiple inheritance here) from dataclasses import dataclass from typing import Union import SalaryPolicy, CommissionPolicy, HourlyPolicy import ManagerRole, SecretaryRole, SalesRole, FactoryRole @dataclass class Employee: id: int name: str @dataclass class Manager(Employee, ManagerRole, SalaryPolicy): def __post_init__(self): SalaryPolicy.__init__(self, self.weekly_salary) super().__init__(self.id, self.name) @dataclass class Secretary(Employee, SecretaryRole, SalaryPolicy): def __post_init__
Find elsewhere
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Python Morsels
pythonmorsels.com › customizing-dataclass-initialization
Customizing dataclass initialization - Python Morsels
October 18, 2024 - Since we're working with a frozen dataclass, our class instances should be immutable, meaning we should be able to store this property as a concrete attribute on each instance of our dataclass. Let's add a __post_init__ method that does exactly that:
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GeeksforGeeks
geeksforgeeks.org › python › data-classes-in-python-set-4-inheritance
Data Classes in Python | Set 4 (Inheritance) - GeeksforGeeks
July 11, 2025 - When a DataClass inherits a normal class, the __init__() from the super-class is overridden in sub-class.
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GitHub
github.com › python › cpython › issues › 139497
Improve documentation on `dataclass` inheritance · Issue #139497 · python/cpython
October 2, 2025 - class C will inherit fields from both A and B (because it's decorated as a dataclass), but class D will only inherit fields from A. >>> C.__init__ <function __main__.C.__init__(self, bar: int, foo: str) -> None> >>> D.__init__ <function ...
Author   MarkRotchell
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Python.org
discuss.python.org › ideas
Default __post_init__ Implementation in Dataclasses - Ideas - Discussions on Python.org
July 4, 2024 - When using inheritance with ... to illustrate the issue: from dataclasses import dataclass, field @dataclass class Parent: greetP: str = field(default="Hello from Parent") # No __post_......
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Python.org
discuss.python.org › python help
Dataclasses and non-dataclasses inheritance - Python Help - Discussions on Python.org
April 19, 2025 - My codebase included (over many modules) code like: class C0: pass @dataclass class DC2: my_field: bool = True class C1(C0, DC2): # original state pass And lo it was good: print(len(C1.__dataclass_fields__)) # 1, life is good But then I tried ...
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Real Python
realpython.com › python-data-classes
Data Classes in Python (Guide) – Real Python
March 8, 2024 - A Python dataclass lets you define classes for storing data with less boilerplate. Use @dataclass to generate .__init__(), .__repr__(), and .__eq__() automatically. Dataclasses allow you to create classes quickly, but you can also add defaults, custom methods, ordering, immutability, inheritance, and even slots.
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PyPI
pypi.org › project › dataclassy
dataclassy · PyPI
If true (the default), generate an __init__ method that has as parameters all fields up its inheritance chain. These are ordered in definition order, with all fields with default values placed towards the end, following all fields without them.
      » pip install dataclassy
    
Published   Jan 14, 2023
Version   1.0.1
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Mattdood
mattdood.com › 2023 › 1 › python-dataclass-inheritance-with-default-values-is-wonky-20230124114200
python dataclass inheritance with default values is wonky | Matthew Wimberly
January 24, 2023 - from dataclasses import dataclass, field @dataclass class Example var_one: int var_two: int sum_one_two: int = field(init=False) def __post_init__(self) -> None: self.sum_one_two = self.var_one + self.var_two · Handling inheritance was an interesting issue, wherein the default values of the parent class are not considered during instantiation due to the ordering of how they're created.
Top answer
1 of 4
19

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
2 of 4
13

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.

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Mimo
mimo.org › glossary › python › data-class
Python Data Class: Syntax, Usage, and Examples
By using frozen=True, you can make ... Data class objects can be compared directly using ==: ... @dataclass class Rectangle: width: int height: int r1 = Rectangle(10, 20) r2 = Rectangle(10, 20) print(r1 == r2) # True...
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Dataquest
dataquest.io › blog › how-to-use-python-data-classes
How to Use Python Data Classes (A Beginner's Guide) – Dataquest
May 12, 2025 - Notice that we used default attributes to make the example shorter. In this case, the comparison is valid because the dataclass creates behind the scenes an __eq__ method, which performs the comparison. Without the decorator, we'd have to create this method ourselves. The same comparison would result in a different outcome if using a standard Python class, even though the classes are in fact equal to each other: class Person(): def __init__(self, name='Joe', age=30, height=1.85, email='[email protected]'): self.name = name self.age = age self.height = height self.email = email print(Person() == Person())
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Studytonight
studytonight.com › post › inheritance-in-python-dataclass
Inheritance in Python Dataclass - Studytonight
June 24, 2023 - Python_StudyTonight(name: str, type_of_website: str, no_of_characters: str, languages_covered: str) Observe that the attribute name, even though present in both the classes, is passed as one argument to the init method. Also, the order of the parameters is such because they are passed in the order of the superclass and then the subclass. from dataclasses import dataclass @dataclass class StudyTonight: name: str type_of_website: str no_of_characters: str @dataclass class Python_StudyTonight(StudyTonight): name: str languages_covered: str new_instance = Python_StudyTonight("Studytonight", "Technical", 12, "Python, C, C++") print(new_instance)