dataclass inheritance: Fields without default values cannot appear after fields with default values

Question

Context

I created two data classes to handle table metadata. TableMetadata apply to any kind of tables, while RestTableMetadata contains information relevant for data extracted from REST apis

@dataclass
class TableMetadata:
    """
    - entity: business entity represented by the table
    - origin: path / query / url from which data withdrawn
    - id: field to be used as ID (unique)
    - historicity: full, delta
    - upload: should the table be uploaded
    """

    entity: str
    origin: str
    view: str
    id: str = None
    historicity: str = "full"
    upload: bool = True
    columns: list = field(default_factory=list)


@dataclass
class RestTableMetadata(TableMetadata):
    """
    - method: HTTP method to be used
    - payloadpath: portion of the response payload to use to build the dataframe
    """

    method: str
    payloadpath: str = None

Problem

Because of inheritance, method (without default values) comes after columns, resulting in the following Pylance error: Fields without default values cannot appear after fields with default values

I'm looking for a way to fix it without overriding __init__ (if there is such a way). I also noticed a method called __init_subclass__ (This method is called when a class is subclassed.) that might affect how RestTableMetadata.__init__ and other subclasses is generated.

I think this answer has a lot of detail, such as use of __post_init__ and attrs library: stackoverflow.com/questions/51575931/…. EDIT: Looks like Python 3.10 offers some new options as well. Details in the linked question. — Mr. Fegur
– Mr. Fegur, Commented Jun 2, 2022 at 7:39

zar3bski · Accepted Answer · 2022-06-22 12:31:03Z

10

Here is a working solution for python > 3.10

@dataclass(kw_only=True)
class TableMetadata:
    """
    - entity: business entity represented by the table
    - origin: path / query / url from which data withdrawn
    - id: field to be used as ID (unique)
    - historicity: full, delta
    - upload: should the table be uploaded
    """

    entity: str
    origin: str
    view: str
    id: str = None
    historicity: str = "full"
    upload: bool = True
    columns: list = field(default_factory=list)


@dataclass(kw_only=True)
class RestTableMetadata(TableMetadata):
    """
    - method: HTTP method to be used
    - payloadpath: portion of the response payload to use to build the dataframe
    """

    method: str
    payloadpath: str = None

answered Jun 22, 2022 at 12:31

zar3bski

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3 Comments

FaCoffee Over a year ago

What about version 3.9?

zar3bski Over a year ago

@FaCoffee : have a look at this https://stackoverflow.com/a/53085935/7648881

tekknolagi Over a year ago

This answer worked better for me for 3.8/3.9: stackoverflow.com/a/58525728/569183

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