alt.datum and alt.value generates incorrect error messages

[joelostblom]

Originally reported in #2883 (comment). When using the datum encoding then the message is incorrect:

alt.Chart().mark_point().encode(x=alt.datum(1, wrong_name=1))

SchemaValidationError: Invalid specification

        altair.vegalite.v5.schema.channels.X, validating 'additionalProperties'

        Additional properties are not allowed ('datum', 'wrong_name' were unexpected)

'datum' should not be included above as an incorrect property to the X channel, instead wrong_name should be mentioned as an incorrect parameter to datum. The way to handle this is probably with special error checking inside the datum function (although maybe it needs to also consider which encoding channel it is used for?).

---

Also mentioned in #2568 (comment) for alt.value. This can look more confusing now that the error message gives parameter hints:

I found one spec that gives an incorrect hint:

import altair as alt
from vega_datasets import data
cars = data.cars.url

alt.Chart(cars).mark_point().encode(
    x='Acceleration:Q',
    y='Horsepower:Q',
    color=alt.value(1)  # should be eg. alt.value('red')
)

SchemaValidationError: `Color` has no parameter named 'value'

Existing parameter names are:
shorthand      bin         legend   timeUnit   
aggregate      condition   scale    title      
bandPosition   field       sort     type       

See the help for `Color` to read the full description of these parameters

It is allowed to use alt.value() in this context, but in this case it should be a string and not a number.

[joelostblom]

@binste This might be an issue to keep in mind when working on #2915 since I believe it is also affected of additionalProperties

[joelostblom]

This seems to have been fixed.partially The error message for alt.value(1) in the second example is now more helpful: SchemaValidationError: '1' is an invalid value for value. Valid values are of type 'object', 'string', or 'null'.

The error for the second example also makes sense now:

SchemaValidationError: `X` has no parameter named 'wrong_name'

Existing parameter names are:
shorthand      bin      scale   timeUnit   
aggregate      field    sort    title      
axis           impute   stack   type       
bandPosition                               

However, when specifying a property that exists, it still shows an error message which is self-contradictory, alt.Chart().mark_point().encode(x=alt.datum(1, bin=True)):

SchemaValidationError: `X` has no parameter named 'bin'

Existing parameter names are:
shorthand      bin      scale   timeUnit   
aggregate      field    sort    title      
axis           impute   stack   type       
bandPosition     

The error message is similar to what we see here #2563 (comment)