How to pass default value to altair encoding

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?

Solution

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)