What happened?

While rewriting some alt.condition(s) to alt.when(s) in #3544 I came across some examples that work correctly - but upset a type checker.

Minimal Repro

import altair as alt
from vega_datasets import data

source = data.cars()
color = (
    alt.when(alt.datum.Weight_in_lbs >= 3500)
    .then(alt.value("hotpink"))
    .otherwise(alt.value("goldenrod"))
)

chart = (
    alt.Chart(source, width=300, height=150)
    .mark_rect()
    .encode(
        x=alt.X("Cylinders:N").axis(labelColor=color),
        y=alt.Y("Origin:N", axis=alt.Axis(tickColor=color)),
        color=color,
    )
)
chart

Solution

The issue is unrelated to #3552, and can instead reuse the solution from #3458.

Not expecting this will require any runtime changes.

However, it will very likely create a huge diff since so much of the generated code will have changes like below:

Map: TypeAlias = Mapping[str, Any]

before: dict | SchemaBase
after: Map | SchemaBase

Creating an issue for visibility and to note that it will need to be merged before #3544 to avoid needing to ignore typing for examples

What would you like to happen instead?

Not require # type: ignore comments for expected use of public API.

Which version of Altair are you using?

5.4.0, 5.4.1, 5.5.0dev