Serialization of expr

[ChristopherDavisUCI]

This is an attempt to resolve the issues @mattijn raised in #2573. Now an expression like theta_shift+5 does not yield a Parameter but instead a new class ParameterExpression. If we have something defined like x = theta_shift+5, we can't use something like add_parameter(x), because x is not a Parameter, but if we have used add_parameter(theta_shift), then we can use x within the chart construction.

Please let me know any comments!

[mattijn]

Yes this works logically. I test a parameter expression as string and typed (through altair.expr.*):

import altair as alt
from altair.expr import PI, length, data

data_wind = [
    {'winddirection': 0, 'label': 'North'},
    {'winddirection': 90, 'label': 'East'},
    {'winddirection': 180, 'label': 'South'},
    {'winddirection': 270, 'label': 'West'},
]

source_wind = alt.DataSource(alt.InlineData(values=data_wind, name='source_wind'))

par_expr_str = alt.parameter(expr="-PI/length(data('source_wind'))")
par_expr_type = alt.parameter(expr=-PI/length(data('source_wind')))

chart = alt.Chart(source_wind).mark_arc(
    tooltip=True,
    thetaOffset=par_expr_type + par_expr_str
).encode(
    theta='winddirection:N',
    color='label:N'
).add_parameter(par_expr_str, par_expr_type)
chart.to_dict()

{'$schema': 'https://vega.github.io/schema/vega-lite/v5.2.0.json',
 'config': {'view': {'continuousHeight': 300, 'continuousWidth': 400}},
 'data': {'name': 'source_wind',
  'values': [{'label': 'North', 'winddirection': 0},
   {'label': 'East', 'winddirection': 90},
   {'label': 'South', 'winddirection': 180},
   {'label': 'West', 'winddirection': 270}]},
 'encoding': {'color': {'field': 'label', 'type': 'nominal'},
  'theta': {'field': 'winddirection', 'type': 'nominal'}},
 'mark': {'thetaOffset': {'expr': '(parameter002 + parameter001)'},
  'tooltip': True,
  'type': 'arc'},
 'params': [{'expr': "-PI/length(data('source_wind'))",
   'name': 'parameter001'},
  {'expr': "((-PI) / length(data('source_wind')))", 'name': 'parameter002'}]}

Both are added as an unique parameter within the params and are added together by parameter name within the ExprRef. Nice!

[ChristopherDavisUCI]

Thank you for looking this over @mattijn!

@jakevdp I think this portion is finished, but I'm always happy for any suggestions.

[joelostblom]

@ChristopherDavisUCI Could you remind me what was the reason for closing this PR? Was it superseded by one of the others you opened? I just want to make sure it wasn't closed accidentally.

[ChristopherDavisUCI]

Hi @joelostblom, hmm, I honestly don't remember. Is there any easy way to tell what date it was closed? It might help to know what I was working on at the time. (Was it definitely closed by me?) I checked @mattijn's example from #2591 (comment) and in my current version it is not working the way @mattijn reported above. In Mattijn's version we have {'thetaOffset': {'expr': '(parameter002 + parameter001)'} and in my current version we have {'expr': "(((-PI) / length(data('source_wind'))) + -PI/length(data('source_wind')))"}.

I will try to think some more.

[joelostblom]

It looks like you might have deleted the repository that the PR was opened from. When I hover over the repo name above that is what it tells me:

Because it was deleted that way, I don't think there is a way to know when it happened. But it does seem like the changes are still there in the files tab so maybe you can create a new branch on your current repo, copy in the changes and open a new PR?

[ChristopherDavisUCI]

Thanks @joelostblom! I think I didn't realize I was closing this PR when I deleted that repository. I will try to re-create these changes.