Here’s a rare Maya post in 2019! I hate Maya, but sometimes it’s hard to avoid.

One of the most irritating things about writing scripts for Maya is that the Python commands are totally un-Pythonic… meaning, there’s no sense of “objects” that have intrinsic properties and functions (or in Python terms, “attributes” and “methods”). You could have a group node in your scene selected and get a reference to it in Python, but your reference is just a string, so you can’t do things like access the group node’s transform matrix as a property or function, and worst of all, if the name of the object changes, your reference is no longer valid! That is, if you say myGroup = cmds.ls(sl=True)[0], and at any point the name of that selected group changes, you’re SOL. This sucks, especially in situations where you’re trying to rename a list of a bunch of objects, ESPECIALLY when they’re all parented to each other, and EVEN MORESO when there’s objects with duplicate names in that stack. It makes my skin crawl.

PyMEL was developed largely to address this sort of thing, but it has a pretty serious startup time when you first initialize it, and it’s slow as hell to instantiate objects. If you’re iterating through a thousand objects and generating PyNodes for each one, you’re in for a long ride. Granted, it comes with all kinds of other neat toys, but if all you want is a consistent handle for objects regardless of name or hierarchy changes, it’s just way too much overhead.

Now, you might be saying to yourself, “what about UUIDs”? Yes, those are pretty magical, until you have two identical file references in the scene, which happens to be the case all the damned time. Those UUIDs stay the same between references, so now you’re stuck trying to identify objects by UUID and namespace.

Here’s a proposed lightweight solution: use OpenMaya to create a neat little custom Python object that stores a reference to the Maya node, regardless of the node’s name. Just get the object by name once, and as long as you keep that object in memory, it doesn’t matter what the object name changes to; you’ll always have a pointer to it.

Try this out:

import maya.OpenMaya as om

class MayaNode():
    def __init__(self, node_name):
        self._mobject = om.MObject()
        self._mdagpath = om.MDagPath()
        self._node = om.MFnDependencyNode()
        selection = om.MSelectionList()
        try:
            selection.add(node_name)
            selection.getDependNode(0, self._mobject)
            self._node = om.MFnDependencyNode(self._mobject)
            selection.getDagPath(0, self._mdagpath, om.MObject())
        except:
            pass

    def name(self, long=False):
        if self._mdagpath.isValid():
            if long:
                return self._mdagpath.fullPathName()
            return self._mdagpath.partialPathName()
        else:
            if not self._mobject.isNull():
                return self._node.name()
            return None

There’s some weird stuff in here to take in, since OpenMaya is a C++ API wrapper and follows those conventions. First, the __init__ constructor for the object. We create default “empty” containers for an MObject, an MDagPath, an MFnDependencyNode, and an MSelectionList. The MObject is a sort of generic catch-all container for Maya API objects. A lot of functions will expect to write to an MObject. In C++, unlike Python, much of the time you’ll provide arguments to the function as variables to be overwritten; these are commonly marked with a “&” in the documentation. Functions like MSelectionList::getDependNode() have these “out” arguments, and expect you to provide an object of the correct type (in this case, an MObject) as an argument to be written to. The MDagPath object is a container that describes the DAG (Directed Acyclic Graph) path of an object in Maya; that is, the path in the Outliner, like |group1|polySphere1. We’ll use this to get the full name of DAG objects later on. Next is the MFnDependencyNode object. Any object in Maya that doesn’t show up in the Outliner is often referred to as a “Dependency Graph” or “DG” node… things like shaders, textures, etc. Since we want this MayaNode class to apply to any kind of node in the scene, we’re creating containers for both types just in case. Finally, the MSelectionList object, which is used to hold a list of objects… we’re just creating this so that we can use the handy getDependNode() and getDagPath() functions from that class.

Okay, deep breaths. Now that we’ve created these objects, we can actually try to turn them into something. First, we tell our MSelectionList object to add the node name that we want… this is the only time we’ll ever need to refer to this object by it’s “MEL-like” name. Once the list is populated with this one object, we can use its getDependNode() function to take the 0th index of our selection list (since there’s only ever one object in there) and return its MObject representation, binding it to self._mobject. This MObject isn’t specific enough, though, so we want to cast it as an MFnDependencyNode, which lets us use all the tasty functions associated with that class. We can do this by just using the constructor for MFnDependencyNode, passing that MObject handle as the argument, and we bind the result to self._node. Finally, we’re going to try to get an MDagPath instance, just in case this node is a DAG node, via the getDagPath() function. This is another of those weird functions that writes to an argument, so we pass it our MDagPath instance, self._mdagpath. The third argument there is just an empty placeholder; we don’t need it.

So now we have an MObject object, which can give us a convenient “pointer” to just about any object in Maya; a MFnDependencyNode object, which can get us some basic information about just about any node; and an MDagPath object, which we can use to get the DAG path of our node, just in case it’s a DAG node. Almost there!

Next is the name() function. We want this to return the name of the node, and optionally, the “long” name of the node (if it’s actually a DAG object, since only DAG objects have these long hierarchical names). We can tell if the MDagPath object we created actually is pointing to a real DAG node by using the MDagPath::isValid() function on the instance we stored as a class member earlier (self._mdagpath). If it is in fact a DAG object, we either return the full path or the short path, depending on the value of the long argument to the function. If it’s not, we do a last check to make sure that the object actually exists (by checking our MObject handle using the isValid() function), and then return the name of the MFnDependencyNode we stored for this object.

Now as long as this MayaObject instance we’ve created is resident in memory, it doesn’t matter what we name this object, or what we parent it to, or if there’s another object elsewhere with the same name… this MayaObject will always point to that object and return its current name anytime you call the name() function. This is incredibly useful!

That’s a long explanation for what should be a very simple little node, but there’s OpenMaya for you. Still, for those of you new to the API, little tools like this can go a long way with removing some of the restrictions and annoyances imposed by MEL (and the Python cmds module, since it’s just a wrapper around MEL). Take a look at the API docs for some of these object types… there’s a lot of other little conveniences you could build into this class if you’re so inclined.

Categories: MayaPython

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