Wire two objects together with an expression

Add an expression that reads one object’s length from another, recompute, then change the input and watch the dependent value follow.

Generated page. tools/userdocs.py writes it from tools/workflows/02_parametric_recompute.py. Edit the workflow, not this page; python3 tools/userdocs.py --check fails when the two disagree.

Requires: the FreeCAD facade; no workbench module and no GPU.

1. Two objects, one dependency

Expressions are evaluated on recompute, not on assignment. The document records the dependency edge, so it can order the objects without the script doing so.

doc = FreeCAD.newDocument("Parametric")

source = doc.addObject("App::FeaturePython", "Source")
source.addProperty("App::PropertyLength", "Length", "Base", "Input length")
source.Length = "10 mm"

derived = doc.addObject("App::FeaturePython", "Derived")
derived.addProperty("App::PropertyLength", "Result", "Base", "Computed")
derived.setExpression("Result", "Source.Length * 2")

doc.recompute()

2. Change the input

Assigning to the source does not touch Derived; the next recompute does, because the dependency is stored in the document.

source.Length = "15 mm"
doc.recompute()

3. Read the expression back

The expression is part of the document, not of the Python object.

expr = derived.ExpressionEngine

ExpressionEngine

[
  [
    "Result",
    "Source.Length * 2"
  ]
]

4. Close the document

FreeCAD.closeDocument('Parametric')