__getitem__ and maya.cmds weird behavior

Coding
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#1

Hi all. I’m experiencing weird behavior of Python “magic” methods and maya.cmds. Below you can see 2 versions of the code. In both versions I use string representation of the class object as first argument in cmds.connectAttr() but in second version where I have implemented __getitem__ method maya throws out # TypeError: Invalid argument 1, 'transform1.translateX'. Expected arguments of type ( , ) #
Here si the code:

import maya.cmds as mc

tr1 = mc.createNode("transform", name="transform1")
tr2 = mc.createNode("transform", name="transform2")

Version 1:

class AttrClass:
    
    def __repr__(self):
        return "transform1.translateX"
    
obj = AttrClass()

mc.connectAttr(obj, "transform2.scaleX")

Version 2:

class AttrClass:
    
    def __repr__(self):
        return "transform1.translateX"
    
    def __getitem__(self, item):
        print(item)
        
obj = AttrClass()

mc.connectAttr(obj, "transform2.scaleX")

In version 2 if you try explicitly call __repr__() it works fine :

mc.connectAttr(obj.__repr__(), "transform2.scaleX")

Another weird thing happens if I try to use pymel to connect

import pymel.core as pm
pm.connectAttr(obj, "transform2.scaleX")

This will start to print integers infinitely.

What I’m doing wrong?

#2

You are not explicitly returning anything in your __getitem__ method. All it does is print the attribute. In python when you don’t explicitly use return then it returns None.
Try directly calling your __getitem__ and assign it to a variable and you’ll see the variable contains None.

#3

The original code which I can’t post here returns self. And the result is the same : # Result: <class '__main__.AttrClass'>

#4

Is this an attempt to get at the underlying attrbute? Like turning myobject.translateX in code into the string "myobject.translateX"?

In a case like that, it’s better to derive from str so there’s no shenanigans and also so the object is immutable like a string (since Maya is using immutable strings, doing something else introduces wierd bugs).

Something like this will let you add pseudo-property access

class PropDescriptor:

    def __init__(self, name ):
        self.name = name
        
    def __get__(self, obj, objtype=None):
        return obj + "." + self.name

    
class AttrClass (str):
    translate = PropDescriptor("translate")

test = AttrClass("top")

print (test.translate)
# top.translate

print(cmds.getAttr(test.translate))
# Result: [(0.0, 1000.1, 0.0)] #

This is used vey extensively in nodule which coincidentally I just updated to Python 3 today to allo things like connect (nodule_1.translateX, nodule_2.rotateY) or my_cube.visibility = True … pymel without the pymel, as it were.

2 Likes
#5

Thanks for reply. That’s partially solves the problem. Now I need to solve multi attributes(indexed). Actually __getitem__ I was using for that purpose.
By the way nice nodule! Definitely will check it out!