How can I set a color scheme as a theme in Altair

Question

It is easy to use a color scheme (for example set2 ) in each Altair diagram:

import pandas as pd 
import altair as alt
from sklearn.datasets import load_iris

data_iris = load_iris()
df = pd.DataFrame(data_iris["data"], columns=data_iris["feature_names"])
df["species"] = [data_iris["target_names"][i] for i in data_iris["target"]]

alt.Chart(df).mark_circle().encode(
    x=alt.X("sepal width (cm)", scale=alt.Scale(zero=False)), 
    y=alt.Y("sepal length (cm)", scale=alt.Scale(zero=False)),
    color=alt.Color("species", 
                    scale=alt.Scale(scheme="set2") ## should be in theme
                   )
)

I want to use the color theme for all diagrams, but I do not find a way to configure a color scheme as an Altair scheme.

import altair as alt

def my_theme():
    return {"config": None} ## How can we set the color scheme here?

alt.themes.register('my_theme', my_theme)
alt.themes.enable('my_theme')

By reading Documentation , I tried several, but no success.

Answer 1

You will want to configure the range parameter. See Scale Range Properties for more details.

import altair as alt
from vega_datasets import data

# define the theme by returning the dictionary of configurations
def my_theme():
    return {
        'config': {
            'view': {
                'height': 300,
                'width': 400,
            },
            'range': {
                'category': {'scheme':'set2'}
            }
        }
    }

# register the custom theme under a chosen name
alt.themes.register('my_theme', my_theme)

# enable the newly registered theme
alt.themes.enable('my_theme')

# draw the chart
cars = data.cars.url
alt.Chart(cars).mark_point().encode(
    x='Horsepower:Q',
    y='Miles_per_Gallon:Q', 
    color='Origin:N'
)