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

2026-10-29T21:01:03Z

Add rewritten practical lesson with steps, code and expected results

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__NOTOC__
{{OpenSite scope}}
{{OpenSite lesson
|id=opensite.help.python-property-expressions
|summary=Store and explicitly recalculate a Python expression on a document property.
|prerequisites=A GUI+Python build or the embedded headless interpreter. The rectangular-section fixture uses the development host. Only evaluate expression sources you trust.
|steps==== Distinguish the two languages ===
Native formulas and Python expressions are separate. Set <code>language="python"</code> for this lesson. See [[OpenSite/Concepts/Expressions]].

=== Store the expression ===
The lesson computes a projected length: width × cos(30°). Python uses <code>math.radians</code> to convert degrees before evaluating cosine.

=== Enable consent ===
Use {{MenuCommand|Edit → Enable Python Expressions}} in the desktop, or <code>document.set_python_expressions_enabled</code> in the script. Consent does not itself run the stored expression.

=== Recalculate explicitly ===
Use {{MenuCommand|Edit → Recalculate Python Expressions}} or <code>recalculate_python_expressions(doc)</code>. The operation shares the console/editor execution lane.

=== Review stale and failed results ===
After changing an input, recalculate again. A retained number marked pending, stale or invalid is not a current result. Opening a file does not silently execute its Python source.

|result=For a width of 2 m, the checked result is approximately 1.7320508076 m. The assertion uses a 1e-9 m tolerance.
|limitations=Python expressions are trusted code, not a sandbox. A busy or paused execution lane blocks another evaluation. Stop is cooperative and may wait for native code to return.
}}

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

created = opensite.execute("document.create_section", {
"label": "ProjectionBeam",
"width": opensite.QuantityArgument(opensite.Q("2 m"), "m"),
"height": opensite.QuantityArgument(opensite.Q("1 m"), "m"),
})
doc, object_id = created["document"], created["object"]
opensite.execute("document.set_expression", {
"document": doc, "object": object_id, "property": "height",
"expression": 'document.get_object("ProjectionBeam").width * math.cos(math.radians(30))',
"language": "python",
})
# Consent and evaluation are separate operations.
opensite.execute("document.set_python_expressions_enabled", {
"document": doc, "enabled": True,
})
opensite.recalculate_python_expressions(doc)
height = doc.property(object_id, "height").si_value
assert abs(height - 2.0 * math.cos(math.radians(30))) < 1e-9
print("Projected value:", height, "m")</nowiki>}}

<div class="os-page-nav">{{Docnav|OpenSite/Guides/Native property formulas|OpenSite/Guides/Dimensionless material expressions|OpenSite/Guides|All guides}}</div>
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OpenSite/Guides/Python property expressions - Revision history

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