BathAfrica: Add rewritten practical lesson with steps, code and expected results

2026-10-29T21:01:01Z

Add rewritten practical lesson with steps, code and expected results

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__NOTOC__
{{OpenSite scope}}
{{OpenSite lesson
|id=opensite.help.native-property-formulas
|summary=Make one property depend on another with OpenSite’s restricted formula language.
|prerequisites=Embedded Python with the development host for the rectangular-section fixture. No Python-expression consent is needed for a native formula.
|steps==== Create the fixture ===
Start with a width of 250 mm and height of 500 mm.

=== Store a formula instead of a literal ===
Set height to <code>object(id).width * 2</code> with <code>language="formula"</code>. The object ID must be the actual ID returned by this document.

=== Change the source value ===
Set width to 300 mm. The native dependency system recomputes height as 600 mm.

=== Inspect the stored source ===
Read <code>doc.expression(id, "height")</code>. A label is a display name; use the document’s stable references rather than inventing an ID.

|result=Height changes from 0.50 m to 0.60 m after editing width. The formula source remains stored on height.
|limitations=The formula language is not arbitrary Python. A dimension mismatch or dependency cycle is rejected. Writing a literal to an expression-bound property replaces that expression.
}}

== Complete example ==
{{Code|lang=python|code=<nowiki>import opensite

created = opensite.execute("document.create_section", {
"label": "FormulaBeam",
"width": opensite.QuantityArgument(opensite.Q("250 mm"), "mm"),
"height": opensite.QuantityArgument(opensite.Q("500 mm"), "mm"),
})
doc, object_id = created["document"], created["object"]
opensite.execute("document.set_expression", {
"document": doc, "object": object_id, "property": "height",
"expression": f"object({object_id}).width * 2", "language": "formula",
})
beam = doc.get_object(object_id)
assert abs(beam.height.si_value - 0.50) < 1e-12
beam.set_property("width", opensite.Q("300 mm"))
assert abs(beam.height.si_value - 0.60) < 1e-12
print("Formula:", doc.expression(object_id, "height"))
print("Height:", beam.height.value_in("mm"), "mm")</nowiki>}}

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OpenSite/Guides/Native property formulas - Revision history

BathAfrica: Add rewritten practical lesson with steps, code and expected results