There are several limitations when referencing items in FreeCAD via Python script. This includes needing to reference Part items such as edges and faces by name, whilst the names are liable (and likely) to change each time the model is modified.

There are also a number of exposed native methods in the Python scripting layer which expect (rarely documented) integer based constants. In this post I collate what I’ve learnt about these constants for future reference.

Body Origin Axes and Planes

The Origin of a Part Design workbench Body called myBody can be acquired via:

origin = App.getDocument('Unnamed').getObject('myBody').Origin

The axes and planes for this Origin are available as OriginFeatures which can be identified as follows:

for feat in b.Origin.OriginFeatures:
	feat.Name

which outputs:

'X_Axis'
'Y_Axis'
'Z_Axis'
'XY_Plane'
'XZ_Plane'
'YZ_Plane'

Hence to get a reference to one of these you can iterate through OriginFeatures matching against the desired feature Name.

However if you want to live dangerously, you can rely on indexing into the array of OriginFeatures with these values:

ORIGIN_X_AXIS_INDEX = 0
ORIGIN_Y_AXIS_INDEX = 1
ORIGIN_Z_AXIS_INDEX = 2
ORIGIN_XY_PLANE_INDEX = 3
ORIGIN_XZ_PLANE_INDEX = 4
ORIGIN_YZ_PLANE_INDEX = 5

The risk here is that the native code layer OriginFeatures array order may change between FreeCAD releases.

Sketch Geometry and Constraints

Geometry Item Indices

Special indices are defined as follows:

• Horizontal axis: -1
• Vertical axis: -2
• First external geometry construction item: -3.

Hence, external geometry constructions items in a sketch are indexed from -3 descending!

Vertex Indices

The start vertex of a geometry line segment has index 1 and the end vertex has index 2.

The centre vertex of a geometry circle is index 3.

Examples

Creating an DistanceX constraint between a line segment start and the origin point:

SKETCH_GEOMETRY_VERTEX_START_INDEX = 1
SKETCH_GEOMETRY_ITEM_HORIZONTAL_AXIS_INDEX = -1

my_constraint = Sketcher.Constraint("DistanceX",
                                    segment_index, SKETCH_GEOMETRY_VERTEX_START_INDEX,
                                    SKETCH_GEOMETRY_ITEM_HORIZONTAL_AXIS_INDEX, SKETCH_GEOMETRY_VERTEX_START_INDEX, 
                                    distance)

Creating a PointOnObject constraint between a line segment start and an external item used as a geometry construction item:

SKETCH_GEOMETRY_VERTEX_START_INDEX = 1
SKETCH_GEOMETRY_FIRST_CONSTRUCTION_INDEX = -3

my_sketch.addExternal(my_datum_plane.Label, '')
my_constraint = Sketcher.Constraint("PointOnObject", 
                                    segment_index, SKETCH_GEOMETRY_VERTEX_START_INDEX, 
                                    SKETCH_GEOMETRY_FIRST_CONSTRUCTION_INDEX)

Creating a DistanceY constraint between the centre of a circle and the origin point:

SKETCH_GEOMETRY_VERTEX_START_INDEX = 1
SKETCH_GEOMETRY_VERTEX_CENTRE_INDEX = 3
SKETCH_GEOMETRY_ITEM_HORIZONTAL_AXIS_INDEX = -1

my_constraint = Sketcher.Constraint("DistanceY", 
                                    segment_index, SKETCH_GEOMETRY_VERTEX_CENTRE_INDEX, 
                                    SKETCH_GEOMETRY_ITEM_HORIZONTAL_AXIS_INDEX, SKETCH_GEOMETRY_VERTEX_START_INDEX, 
                                    distance)