Support dates in widgets such as the min/max values in sliders

[joelostblom]

What is your suggestion?

I was playing around with the much improved date support from #3653 and noticed that the min/max parameters in sliders don't accept date values:

import altair as alt
import pandas as pd
import numpy as np
from datetime import date

datelist = pd.date_range(end='2024-10-08', periods=100).to_pydatetime()

rand = np.random.RandomState(42)

df = pd.DataFrame({
    'xval': datelist,
    'yval': rand.randn(100).cumsum(),
})

slider = alt.binding_range(name='cutoff:', min=min(datelist), max=max(datelist))
selector = alt.selection_point(
    name="SelectorName",
    fields=['cutoff'],
    bind=slider,
    value=[{"cutoff": date(2020, 5, 5)}]
)

alt.Chart(df).mark_point().encode(
    x='xval',
    y='yval',
    opacity=alt.condition(
        'toDate(datum.xval) < SelectorName.cutoff[0]',
        alt.value(1), alt.value(0)
    )
).add_params(
    selector
)

SchemaValidationError: Multiple errors were found.

Error 1: '{'year': 2024, 'month': 10, 'date': 8}' is an invalid value for `max`. Valid values are of type 'number'.

Error 2: '{'year': 2024, 'month': 7, 'date': 1}' is an invalid value for `min`. Valid values are of type 'number'.

It would be convenient if these worked with dates instead of just timstamps. If I don't include min and max at all, the value of the date displays as expected initially (although formatting is a bit verbose, maybe that should be configurable or at least limit to the same level of detail as the underlying date if possible), but disappears when the slider is moved (since the slider range is the default 1 to 100).

Have you considered any alternative solutions?

I don't think there is an alternative unless we want to convert everything to timestamps again as in this SO answer.

[dangotbanned]

@joelostblom you might be able to get this working by triggering the Expression conversion instead of SchemaBase?
I'm not 100% sure what that would look like for your example though

• feat: Support datetime.(date|datetime) in Expression(s) #3654

We might be limited here by vega-lite though, since that SchemaValidationError expects an int

[dangotbanned]

Minimal Repro

https://altair-viz.github.io/user_guide/generated/api/altair.binding_range.html#altair.binding_range

import datetime as dt
import altair as alt

# SchemaBase
alt.binding_range(min=dt.date(2000, 1, 1))
# SchemaValidationError: '{'year': 2000, 'month': 1, 'date': 1}' is an invalid value for `min`. Valid values are of type 'number'.

# Expression
alt.binding_range(min=alt.expr.parseFloat(dt.date(2000, 1, 1)))
# SchemaValidationError: 'parseFloat(datetime(2000,0,1))' is an invalid value for `min`. Valid values are of type 'number'.