declaring a default value in a sub class in python

Question

Is it possible to declare a default value for an attribute in a subclass in python?

Eg something like this:

@dataclass_json
@dataclass
class BaseClass:
    b: str
    a: str


@dataclass_json
@dataclass
class SubClass(BaseClass):
    c: str
    d: str
    a: str = "my value"

I tried this and I'm getting the following error:

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

I also tried putting a before c and d in the subclass without success (same error).

Is there a way to achieve this?

It's telling you to put a before c and d. What's confusing you about the error message? — Mad Physicist
– Mad Physicist, Commented Jul 7, 2021 at 5:11
Could you please add a plain python tag as the python-3.x tag guidance suggests? — Mad Physicist
– Mad Physicist, Commented Jul 7, 2021 at 6:01
Does this answer your question? Class inheritance in Python 3.7 dataclasses — Mad Physicist
– Mad Physicist, Commented Jul 7, 2021 at 6:06

Mad Physicist · Accepted Answer · 2021-07-07 05:41:17Z

3

Your derived class will have the following order of fields in __init__:

def __init__(b, a, c, d):

This is because a appears in the base class, so the base constructor is

def __init__(b, a):

To give a a default value, can give c and d default values (e.g. None):

@dataclass_json
@dataclass
class SubClass(BaseClass):
    c: str = None
    d: str = None
    a: str = "my value"

answered Jul 7, 2021 at 5:41

Mad Physicist

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4 Comments

Tom Over a year ago

question is a duplicate of this stackoverflow.com/questions/51575931/…

Mad Physicist Over a year ago

Good find. You should close it as such then

M.Winkens Over a year ago

Type of c, d should be Optional[str] then

Mad Physicist Over a year ago

@M.Winkens. Yes

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