Descriptors and pythonic Maya properties

Tue 11 March 2014

I’m still working on the followup to Rescuing Maya GUI From Itself, but while I was at it this StackOverflow question made me realize that the same trick works for pyMel-style property access to things like position or rotation. If you’re a member of the anti-pyMel brigade you might find this a useful trick for things like pCube1.translation = (0,10,0). Personally I use pyMel most of the time, but this is a good supplement or alternative for haterz or for special circumstance where pymel is too heavy.

The goal is to be able to write something like

from xform import Xform  
example = Xform('pCube1')  
print example.translation  
# [0,0,0]  
example.rotation = (0,40, 0)

The process is about as simple as it can get thanks to the magic of descriptors. This example spotlights one advantage of descriptors over getter/setter property functions: by inheriting the two classes (BBoxProperty and WorldXformProperty) I can get 4 distinct behaviors (world and local, read-write and read-only) with very little code and no if-checks.

'''  
xform.py

Exposes the xform class: a simple way to set maya position, rotation and similar properties with point notation.

(c) 2014 Steve Theodore.  Distributed under the MIT License (http://opensource.org/licenses/MIT)  
TLDR: Use, change and share, please retain this copyright notice.  
'''

import maya.cmds as cmds

class XformProperty( object ):  
    CMD = cmds.xform  
    '''  
    Descriptor that allows for get-set access of transform properties  
    '''  
    def __init__( self, flag ):  
        self.Flag = flag  
        self._q_args = {'q':True, flag:True}  
        self._e_args = {flag: 0}


    def __get__( self, obj, objtype ):  
        return self.CMD( obj, **self._q_args )

    def __set__( self, obj, value ):  
        self._e_args[self.Flag] = value  
        self.CMD( obj, **self._e_args )


class WorldXformProperty( XformProperty ):  
    '''  
    Get-set property in world space  
    '''  
    def __init__( self, flag ):  
        self.Flag = flag  
        self._q_args = {'q':True, flag:True, 'ws':True}  
        self._e_args = {flag: 0, 'ws':True}

class BBoxProperty ( XformProperty ):  
    '''  
    Read only property for bounding boxes  
    '''  
    def __set__( self, obj, value ):  
        raise RuntimeError ( "bounding box is a read-only property!" )

class WorldBBoxProperty ( WorldXformProperty, BBoxProperty ):  
    '''  
    Read only property for bounding boxes  
    '''  
    pass


class Xform( object ):  
    '''  
    Thin wrapper providing point-notation access to transform attributes

       example = Xform('pCube1')  
       # |pCube1  
       example.translation   
       # [0,0,0]  
       example.translation = [0,10,0]

    For most purposes Xforms are just Maya unicode object names.  Note this does  
    NOT track name changes automatically. You can, however, use 'rename':  
       example = Xform('pCube1')  
       example.rename('fred')  
       print example.Object  
       # |fred

    '''

    def __init__( self, obj ):  
        self.Object = cmds.ls( obj, l=True )[0]

    def __repr__( self ):  
        return unicode( self.Object )  # so that the command will work on the string name of the object

    # property descriptors  These are descriptors so they live at the class level,  
    # not inside __init__!

    translation = XformProperty( 'translation' )  
    rotation = XformProperty( 'rotation' )  
    scale = XformProperty( 'scale' )  
    pivots = XformProperty( 'pivots' )

    world_translation = WorldXformProperty( 'translation' )  
    world_rotation = WorldXformProperty( 'rotation' )  
    world_pivots = WorldXformProperty( 'pivots' )  
    # maya does not allow 'world scale' - it's dependent on the parent scale

    # always local  
    scaleTranslation = XformProperty( 'scaleTranslation' )  
    rotateTranslation = XformProperty( 'rotateTranslation' )

    boundingBox = BBoxProperty( 'boundingBox' )  
    world_boundingBox = WorldBBoxProperty( 'boundingBox' )


    def rename( self, new_name ):  
        self.Object = cmds.ls( cmds.rename( self.Object, new_name ), l=True )[0]

    @classmethod  
    def ls( cls, *args, **kwargs ):  
        '''  
        Returns a list of Xforms, using the same arguments and flags as the default ls command  
        '''  
        try:  
            nodes = cmds.ls( *cmds.ls( *args, **kwargs ), type='transform' )  
            return map ( Xform, nodes )  
        except:  
            return []

You may note the absence of a __metaclass__. In this case, with only a single class, a meta would be an unnecessary complication. Meanwhile the code for MayaGUI itself is up on GitHub. Comments and/or contributions welcome!

tags: maya, python, gui, techart