Chart.encode() returns confusing error message when using an invalid channel and selections

[dechamps]

Consider:

selection = alt.selection_single()
(alt.Chart(data=None)
    .mark_circle()
    .encode(color=alt.value('red'), invalidChannel=None))

Predictably, this fails with:

SchemaValidationError: Invalid specification
        altair.vegalite.v4.schema.core.FacetedEncoding, validating 'additionalProperties'
        Additional properties are not allowed ('invalidChannel' was unexpected)

So far so good. But now watch what happens if x is set to a selection:

selection = alt.selection_single()
(alt.Chart(data=None)
    .mark_circle()
    .add_selection(selection)
    .encode(
        color=alt.condition(selection, alt.value('red'), alt.value('green')),
        invalidChannel=None))

This time, the error message is way more confusing and doesn't point to the actual cause of the problem:

SchemaValidationError: Invalid specification
        altair.vegalite.v4.schema.channels.ColorValue, validating 'additionalProperties'
        Additional properties are not allowed ('selection' was unexpected)

The misleading error message made me waste a lot of time chasing down dead ends simply because I mistyped tooltip as tooltips :(

[jakevdp]

Yes, this is pretty misleading. It’s difficult to specialize, because validation errors come from validating the schema of the full chart.