OpenSite/Guides/Use the expression editor
Choose a native formula or a Python expression deliberately.
- Stable identifier
opensite.help.use-the-expression-editor
Prerequisites
A practice expression-capable field and a saved checkpoint. Python mode requires the matching Python-enabled host.
Steps
Open the expression affordance
Select the quantity/expression-capable field and use its ... expression button. Read the evaluated value beside the field; the dialog edits the source, not only its formatted display.
Choose the language
Use Formula for the restricted unit-aware expression language. Use Python only for trusted code requiring its Python context. These are different languages: native if(cond, a, b) is not Python syntax.
Preview deliberately
Check destination units and dimensions. Native formula preview can validate the proposed calculation. Python preview is explicit; ordinary field refresh does not silently execute Python. An error is a reason to correct the expression, not ignore it.
Apply and verify
Accept a valid expression and compare its result with an independently known value. The OpenSite/Guides/Native property formulas and OpenSite/Guides/Python property expressions lessons provide checkable examples.
Enable Python only for a trusted document
Loading retains Python source without automatically running it. Use Edit → Enable Python Expressions and then Edit → Recalculate Python Expressions when appropriate. A busy interpreter, exception, cancellation, incompatible result or stale revision can reject evaluation.
Remove an expression intentionally
Use the dialog’s removal/literal workflow when you mean to replace the relationship. Cancelling or accepting an unchanged literal should not be used to erase a stored expression. Recheck the property afterwards.
Expected result
You can tell which language is stored and whether its evaluated value was accepted.
Limitations
Python expressions are trusted code, not a security sandbox. Tables/cell references are not available in this baseline.
