Python dataclass with inheritance: init() missing 1 required positional argument

Question

Trying my luck with inheritance with data classes (Python 3.9.13).

Given the following:

from dataclasses import dataclass
from datetime import datetime


@dataclass()
class BookMetadata():
    '''Parent class.'''
    
    isbn: str 
    title: str 
    author: str
    publisher: str
    date_published: int
    

    def __post_init__(self):
        '''Change attributes after assignment.'''
            
        # Change date from UNIX to YYYY-MM-DD HH:MM:SS
        self.date_published = datetime.fromtimestamp(int(str(self.date_published))).strftime('%Y-%m-%d %H:%M:%S')
      

@dataclass()
class RetailPrice(BookMetadata):
    '''Child class.'''
    
    def __init__(self, 
                 isbn, title, author, publisher, date_published,
                 price_usd, price_aud, price_eur, price_gbp) -> None:
        
        BookMetadata.__init__(isbn, title, author, publisher, date_published)
        
        self.price_usd: float = price_usd
        self.price_aud: float = price_aud
        self.price_eur: float = price_eur
        self.price_gbp: float = price_gbp


    def __post_init__(self):
        self.price_usd = str(self.price_usd)

and the values assigned as such:

book1 = RetailPrice(isbn='1234-5678-9000', 
                    title='My book', 
                    author='Name Surname', 
                    publisher='My publisher',
                    date_published=1670536799, 
                    price_usd=17.99, 
                    price_aud=23.99, 
                    price_eur=15.99, 
                    price_gbp=16.99)

I get a TypeError:: TypeError: __init__() missing 1 required positional argument: 'date_published', but this was provided in the assignment.

Is this due to the fact that the parent class has no __init__?

PS: this is my attempt at reproducing line 21 in the image below, having to work with data classes instead of regular classes:

Jay · Accepted Answer · 2022-12-12 14:39:23Z

1

Since you are using a subclass of BookMetadata you can just initialise the superclass from child class using super() -


@dataclass()
class RetailPrice(BookMetadata):
    '''Child class.'''
    
    def __init__(self, 
                 isbn, title, author, publisher, date_published,
                 price_usd, price_aud, price_eur, price_gbp) -> None:
        super().__init__(isbn, title, author, publisher, date_published)
        self.price_usd: float = price_usd
        self.price_aud: float = price_aud
        self.price_eur: float = price_eur
        self.price_gbp: float = price_gbp

The reason for the error in your original code is because of the missing self argument to BookMetadata.__init__(), since you don't have an instance of BookMetadata class, so one argument was missing(self + 5 additional arguments, so it was giving error on the last missing argument date_published)

edited Dec 12, 2022 at 14:39

answered Dec 12, 2022 at 14:33

Jay

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8 Comments

FaCoffee Over a year ago

Uhm, adding a __post_init__ method to the child class with exactly the same syntax as the parent class returns an AttributeError, despite that attribute being properly referenced in the code and assigned at runtime. I added def __post_init__(self): self.price_usd = str(self.price_usd), but at runtime I get AttributeError: 'RetailPrice' object has no attribute 'price_usd'

Jay Over a year ago

Can you share the corresponding code? Maybe, you are trying to access some non-existing attribute? You should be able to access the super class's attributes as self.isbn directly for example.

FaCoffee Over a year ago

Yep, look at the edited code. I think the attribute is existing, but can there be confusion between the __post_init__ of the parent class and that of the child class?

Jay Over a year ago

Moving the super().__init__(isbn, title, author, publisher, date_published) part to below the dataclass arguments fixes the issue.

Jay Over a year ago

ahh ok, you would need to update the __repr__() method of the class, the parent class being a dataclass would have the function which would only print its own fields, while child one would print its own ones. So you might have to redefine it on your own to include all fields.

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