I am attempting to subclass a Python dataclass, where the subclass will have all the same fields as the parent class, except certain fields will be constrained to specific values.
That is, I am attempting to do something like this:
@dataclass
class Polygon:
perimeter: float
number_of_sides: int
class Triangle(Polygon):
perimeter: float
number_of_sides: int = 3
This works until I attempt to actually use the Triangle class without specifying number_of_sides.
This works as expected:
polygon = Polygon(perimeter=5.0, number_of_sides=6)
triangle = Triangle(perimeter=5.0, number_of_sides=3)
This does not work:
triangle_using_defaults = Triangle(perimeter=5.0)
Error message is
Traceback (most recent call last):
File "/home/dmayo/PycharmProjects/playground/interval/shape.py", line 17, in <module>
triangle_using_defaults = Triangle(perimeter=5.0)
^^^^^^^^^^^^^^^^^^^^^^^
TypeError: Polygon.__init__() missing 1 required positional argument: 'number_of_sides'
How can I achieve this?
You're missing the @dataclass decorator for Triangle class;
from dataclasses import dataclass
@dataclass
class Polygon:
perimeter: float
number_of_sides: int
@dataclass
class Triangle(Polygon):
number_of_sides: int = 3
triangle_using_defaults = Triangle(perimeter=5.0)
print(triangle_using_defaults)
This will print Triangle(perimeter=5.0, number_of_sides=3)
Note: there's no need to specify perimeter: float in the subclass, as it is already defined in its parent, unless there is a need to override the parent's definition (as we do with the default value for number_of_sides).