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Typing: ui.dictionary.__init__ uses invariant dict[str, UIElement] and rejects dict[str, ui.checkbox] #10631
Description
Describe the bug
marimo.ui.dictionary.__init__ currently types elements as:
dict[str, UIElement[Any, Any]]This causes static type checkers (Pylance/Pyright) to reject valid subclass dictionaries like dict[str, mo.ui.checkbox] because dict is invariant.
Observed runtime signature:
(self, elements: 'dict[str, UIElement[Any, Any]]', *, label: 'str' = '', on_change: 'Callable[[dict[str, object]], None] | None' = None) -> 'None'Error message
Pylance/Pyright:
Argument of type
dict[str, checkbox]cannot be assigned to parameterelementsof typedict[str, UIElement[Any, Any]]... Consider switching fromdicttoMapping.
Why this is problematic
- Runtime behavior is correct.
- Users must add casts like
cast(dict[str, Any], elements). UIElementappears internal/non-public, so precise public typing is hard to express externally.
Proposed fix
Use a covariant input type for constructor parameter, e.g.:
Mapping[str, UIElement[Any, Any]]Will you submit a PR?
- Yes
Environment
- marimo:
0.24.0 - Python: 3.14 (uv-managed)
- Type checker: Pylance/Pyright
Code to reproduce
from __future__ import annotations
from collections.abc import Iterable, Mapping
import marimo as mo
class Checkboxes(mo.ui.dictionary):
def __init__(
self,
options: Iterable[str] | Mapping[str, str],
preselected_options: Iterable[str] | None = None,
):
pre = set(preselected_options) if preselected_options else set()
elements: dict[str, mo.ui.checkbox]
if isinstance(options, Mapping):
elements = {
option: mo.ui.checkbox(label=label, value=(option in pre))
for option, label in options.items()
}
else:
elements = {
option: mo.ui.checkbox(label=option, value=(option in pre))
for option in options
}
super().__init__(elements) # type error
Activity
[-]Typing: ui.dictionary.__init__ uses invariant dict[UIElement] and rejects dict[str, ui.checkbox][/-][+]Typing: ui.dictionary.__init__ uses invariant dict[str, UIElement] and rejects dict[str, ui.checkbox][/+]Light2Dark commentedon Aug 24, 2026
We'd be happy to accept a contribution. You may also want to look at updating the function below too.
ui.batch also has the same issue, you may want to do a pass on similar elements like dictionary
roridemonslayer commentedon Aug 26, 2026
I'd like to take this on. Plan: change
dictionary.__init__'selementsparam toMapping[str, UIElement[Any, Any]], do the same forvalidate_and_clone's input parameter (keeping its return type asdictsince it builds a fresh one), and apply the same covariant-input fix toui.batch. Will open a PR with tests shortly.