How to indicate to mypy an object has certain attributes?

0
votes

I am using some classes that derived from a parent class (Widget); among the children, some have certain attributes (posx and posy) but some don't.

import enum
from dataclasses import dataclass
from typing import List


class Color(enum.IntEnum):
    GLOWING_IN_THE_DARK = enum.auto()
    BROWN_WITH_RAINBOW_DOTS = enum.auto()


@dataclass
class Widget:
    """Generic class for widget"""


@dataclass
class Rectangle(Widget):
    """A Color Rectangle"""

    posx: int
    posy: int
    width: int = 500
    height: int = 200
    color: Color = Color.BROWN_WITH_RAINBOW_DOTS


@dataclass
class Group(Widget):
    children: List[Widget]


@dataclass
class Button(Widget):
    """A clickable button"""

    posx: int
    posy: int
    width: int = 200
    height: int = 100
    label: str = "some label"

Even after doing some filtering with only widgets with these attributes, mypy is not able to recognize that they should have.

Is there a way to indicate to mypy that we have an object with a given attribute?

For example, the following function and call:

def some_function_that_does_something(widgets: List[Widget]):
    """A useful docstring that says what the function does"""
    widgets_with_pos = [w for w in widgets if hasattr(w, "posx") and hasattr(w, "posy")]

    if not widgets_with_pos:
        raise AttributeError(f"No widget with position found among list {widgets}")

    first_widget = widgets_with_pos[0]
    pos_x = first_widget.posx
    pos_y = first_widget.posy
    print(f"Widget {first_widget} with position: {(pos_x, pos_y)}")


some_widgets = [Group([Rectangle(0, 0)]), Button(10, 10, label="A button")]
some_function_that_does_something(some_widgets)

would return a result as expected: Widget Button(posx=10, posy=10, width=200, height=100, label='A button') with position: (10, 10)

But mypy would complain:

__check_pos_and_mypy.py:53: error: "Widget" has no attribute "posx"
        pos_x = first_widget.posx
                ^
__check_pos_and_mypy.py:54: error: "Widget" has no attribute "posy"
        pos_y = first_widget.posy
                ^
Found 2 errors in 1 file (checked 1 source file)

How to do?

Maybe, one way could be to change the design of the classes:

  • Subclass of Widget with the position (e.g. WidgetWithPos)
  • Rectangle and Button would derive from this class
  • We indicate in our function: widget_with_pos: List[WidgetWithPos] = ...

... however, I cannot change the original design of the classes and mypy might still complain with something like:

List comprehension has incompatible type List[Widget]; expected List[WidgetWithPos]

Of course, we could put a bunch of # type:ignore but that will clutter the code and I am sure there is a smarter way ;)

Thanks!

pythonclassmypy