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Scripted objects with attachment

Introduction

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

{.python .numberLines} class Box(): """Custom Scripted Box Object"""

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

    obj.Proxy = self
    
    ... custom properties

    # Needed to make this object "attachable"
    obj.addExtension('Part::AttachExtensionPython', obj)

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 \

{.python .numberLines} class Box:...

def execute(self, obj):
    obj.positionBySupport()
    
    # 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

{.python .numberLines} import FreeCAD as App import Part

class Box(): """ Simple Custom Box Object See Also: https://wiki.freecadweb.org/FeaturePython_Objects """

def __init__(self, obj):
    """
    Constructor
    Arguments
    
    - 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
    obj.addExtension('Part::AttachExtensionPython', obj)

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.positionBySupport()

    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) Box(obj) 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) document.recompute()

References

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:

documentation index > [Developer Documentation](Category_Developer Documentation.md) > [Python Code](Category_Python Code.md) > Scripted objects with attachment