Scripted objects with attachment

From FreeCAD Documentation


The purpose of this page is to show a minimal example of Part EditAttachment feature using Scripted objects in Python.

See full and complete minimal example below.

The following GIF demonstrates attaching our custom box to a cylinder, and automatically updating it's position when the cylinder's position changes.

NOTE: The box is our custom scripted object, and the cylinder is a regular FreeCAD object generated from the Part workbench.

Making Scripted Objects Attachable

Add Attach Extension

First, we need to add the Part::AttachExtensionPython extension to our Part::FeaturePython oject in the constructor, or __init__ method, of our custom scripted object.

class Box():
    """Custom Scripted Box Object"""

    def __init__(self, obj):
        self.Type = 'Box'

        obj.Proxy = self
        ... custom properties

        # Needed to make this object "attachable"

Without adding this code, we'll see the following warning dialog when attaching our custom scripted object to another object.

Update Position Based on Attached Object

Then, in the `execute` method of our custom scripted object, we need to call the `positionBySupport` on our `Part::FeaturePython` object.

class Box:

    def execute(self, obj):
        # Assign a Shape to obj
        obj.Shape = Part.makeBox(...)

Without calling positionBySupport, our custom scripted object won't update it's position when the position of the attached-to object changes.

Full and Complete Minimal Example

import FreeCAD as App
import Part

class Box():
    Simple Custom Box Object
    See Also:

    def __init__(self, obj):
        - obj: an existing document object or an object created with FreeCAD.Document.addObject('Part::FeaturePython', '{name}').

        self.Type = 'Box'

        obj.Proxy = self
        obj.addProperty('App::PropertyLength', 'Length',
                        'Dimensions', 'Box length').Length = 10.0
        obj.addProperty('App::PropertyLength', 'Width',
                        'Dimensions', 'Box width').Width = 10.0
        obj.addProperty('App::PropertyLength', 'Height',
                        'Dimensions', 'Box height').Height = 10.0

        # Needed to make this object "attachable",
        # or able to attach parameterically to other objects

    def execute(self, obj):
        Called on document recompute
        # Needed to update position when attached-to object changes position.
        # Reposition object based on Support, MapMode and MapPathParameter properties.
        # Returns True if attachment calculation was successful, False if object is not attached and Placement wasn't updated,

        obj.Shape = Part.makeBox(obj.Length, obj.Width, obj.Height)

def create_box(obj_name, document):
    Create a Box.
    obj = document.addObject('Part::FeaturePython', obj_name)
    obj.ViewObject.Proxy = 0  # Mandatory unless ViewProvider is coded
    return obj

document = App.ActiveDocument
if document is None:
    document = App.newDocument('Part Attachment Example')

box = create_box('CustomBox', document)


Tested With the Following FreeCAD Version

Tested with the following FreeCAD version information:

OS: Ubuntu 18.04.3 LTS
Word size of OS: 64-bit
Word size of FreeCAD: 64-bit
Version: 0.18.16146 (Git) AppImage
Build type: Release
Branch: (HEAD detached at 0.18.4)
Hash: 980bf9060e28555fecd9e3462f68ca74007b70f8
Python version: 3.6.7
Qt version: 5.6.2
Coin version: 4.0.0a
OCC version: 7.3.0
Locale: English/UnitedStates (en_US)

Note: For FreeCAD 0.19 this tutorial needs a minor update: