TypeError: 'UndefinedType' object is not callable

Question

I'm getting a TypeError: 'UndefinedType' object is not callable when running the following Altair code.

import altair as alt
from vega_datasets import data
cars = data.cars()

alt.Chart(cars).mark_point().encode(
    x=alt.X('Horsepower').axis(tickMinStep=50),
    y=alt.Y('Miles_per_Gallon').title('Miles per Gallon'),
    color='Origin',
    shape='Origin'
)

Answer 1

Vega-Altair introduced in version 5 a method-based syntax for setting encoding channel options (see docs).

The following line from the OP specification is using a method-based syntax:

x=alt.X('Horsepower').axis(tickMinStep=50)

Where this is written as such using an argument-based syntax:

x=alt.X('Horsepower', axis=alt.Axis(tickMinStep=50))

If you are facing this error you are likely on Vega-Altair version 4.x. You have two options:

• Rewrite the method-based syntax to argument-based syntax.
• Upgrade to a recent version of Vega-Altair.

For the first, use the documentation of Vega-Altair version 4.

For the latter, currently Vega-Altair version 5 can be installed using the following command:

pip install altair>=5