I would like to vary the shape
argument based on conditions, and in some cases would like it to remain as the default value. The following, however:
import altair as A
shape = A.Undefined
ch = A.Chart({}).encode(shape=shape)
raises TypeError: argument of type 'UndefinedType' is not iterable
. Is this a bug, or is there an appropriate 'null' value I can pass?
Altair's API does not currently support any sentinel to represent an empty encoding. If this is important to your use-case, you could open a feature request at https://github.com/altair-viz/altair/issues
The best workaround would be to do something like this:
import altair as alt
shape = None
kwds = {} if shape is None else {'shape': shape}
ch = alt.Chart().encode(**kwds)
If you are looking for defaults for tooltip and size, I used the example below.
import altair as alt
from vega_datasets import data
source = data.iris()
if size is None:
size = {}
if tooltip is None:
tooltip = []
alt.Chart(source).mark_circle().encode(
alt.X('sepalLength', scale=alt.Scale(zero=False)),
alt.Y('sepalWidth', scale=alt.Scale(zero=False, padding=1)),
color='species',
size=size,
tooltip=tooltip
)
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