dangotbanned
commented
1 month ago Thanks for opening this @mattijn
The reason I wanted an issue was to investigate why the definitions marked as # BUG were not working.
Code block from review
```py import altair as alt from vega_datasets import data source = data.barley() base = alt.Chart(source).encode( x=alt.X("sum(yield):Q").stack("zero"), y=alt.Y("site:O").sort("-x"), text=alt.Text("sum(yield):Q", format=".0f"), ) # NOTE: Original `str` expressions # tooltip = alt.expr("luminance(scale('color', datum.sum_yield))") # color = alt.expr("luminance(scale('color', datum.sum_yield)) > 0.5 ? 'black' : 'white'") luminance = alt.expr.luminance(alt.expr.scale("color", alt.datum.sum_yield)) # BUG: These definitions alone *should* work, but don't evaluate as expressions tooltip = luminance color = alt.expr.if_(luminance > 0.5, "black", "white") # HACK: Actual additional steps required tooltip = alt.expr(repr(luminance)) color = alt.expr(repr(color)) bars = base.mark_bar(tooltip=tooltip).encode(color="sum(yield):Q") text = base.mark_text(align="right", dx=-3, color=color) bars + text ```I'm now seeing that this is the distinction between Expr (str) and ExprRef ({"expr": str}).
I think handling this implicitly could be error prone.
A simple alternative could be adding Expression.to_expr:
Diff altair/expr/core.py
```diff diff --git a/altair/expr/core.py b/altair/expr/core.py index e7872ada..095385b4 100644 --- a/altair/expr/core.py +++ b/altair/expr/core.py @@ -168,21 +168,24 @@ class OperatorMixin: class Expression(OperatorMixin, SchemaBase): """ Expression. Base object for enabling build-up of Javascript expressions using a Python syntax. Calling ``repr(obj)`` will return a Javascript representation of the object and the operations it encodes. """ _schema = {"type": "string"} def to_dict(self, *args, **kwargs): return repr(self) + def to_expr(self): + return {"expr": repr(self)} + def __setattr__(self, attr, val) -> None: # We don't need the setattr magic defined in SchemaBase return object.__setattr__(self, attr, val) # item access def __getitem__(self, val): return GetItemExpression(self, val) ```
This would then allow the following syntax:
import altair as alt
from vega_datasets import data
source = data.barley()
base = alt.Chart(source).encode(
x=alt.X("sum(yield):Q").stack("zero"),
y=alt.Y("site:O").sort("-x"),
text=alt.Text("sum(yield):Q", format=".0f"),
)
# NOTE: Original `str` expressions
# tooltip = alt.expr("luminance(scale('color', datum.sum_yield))")
# color = alt.expr("luminance(scale('color', datum.sum_yield)) > 0.5 ? 'black' : 'white'")
luminance = alt.expr.luminance(alt.expr.scale("color", alt.datum.sum_yield))
# NOTE: More ergonomic? # <-------------------------------------------------
tooltip = luminance.to_expr()
color = alt.expr.if_(luminance > 0.5, "black", "white").to_expr()
bars = base.mark_bar(tooltip=tooltip).encode(color="sum(yield):Q")
text = base.mark_text(align="right", dx=-3, color=color)
bars + textIt would only be needed as the last operation, so it makes sense to be appearing as the final method call.
We could instead use (expr|ExprRef), this was just the quick and easy solution I saw
mattijn
What is your suggestion?
As requested by @dangotbanned here: https://github.com/vega/altair/pull/3614#pullrequestreview-2333267989. https://github.com/vega/altair/pull/3614 uses an expression as a
strwith plain Vega Expression syntax. It would be nice to use the equivalent python syntax instead.Have you considered any alternative solutions?
No response