Python dataclasses: What type to use if __post_init__ performs type conversion?

Question

I have a Python class, with a field which can be passed one of several sequence types. To simplify I'll stick with tuples and lists. __init__ converts the parameter to MyList.

from typing import Union
from dataclasses import dataclass, InitVar, field

class MyList(list):
    pass

@dataclass
class Struct:
    field: Union[tuple, list, MyList]

    def __post_init__(self):
        self.field = MyList(self.field)

What type should I use for the field declaration?

• If I supply a union of all possible input types, the code does not document that field is always a MyList when accessed.
• If I only supply the final MyList type, PyCharm complains when I pass Struct() a list.

I could instead use:

_field: InitVar[Union[tuple, list, MyList]] = None
field: MyList = field(init=False)

def __post_init__(self, _field):
    self.field = MyList(_field)

but this is tremendously ugly, especially when repeated across 3 fields. Additionally I have to construct a struct like Struct(_field=field) instead of Struct(field=field).

In April 2018, "tm" commented on this issue on PyCharm's announcement: https://blog.jetbrains.com/pycharm/2018/04/python-37-introducing-data-class/#comment-323957

Answer 1

You are conflating assigning a value to the attribute with the code that produces the value to assign to the attribute. I would use a separate class method to keep the two pieces of code separate.

from dataclasses import dataclass


class MyList(list):
    pass


@dataclass
class Struct:
    field: MyList

    @classmethod
    def from_iterable(cls, x):
        return cls(MyList(x))


s1 = Struct(MyList([1,2,3]))
s2 = Struct.from_iterable((4,5,6))

Now, you only pass an existing value of MyList to Struct.__init__. Tuples, lists, and whatever else MyList can accept are passed to Struct.from_iterable instead, which will take care of constructing the MyList instance to pass to Struct.