Just for clarification on a few of these areas, I'd like to break this
down a bit, and who knows, this great info on the backend might be
useful to someone, so here goes:
On 3/22/2014 4:05 AM, Marcus Ottosson wrote:
I think the Python API 2.0 is already in effect (since Maya 2013
<http://docs.autodesk.com/MAYAUL/2013/ENU/Maya-API-Documentation/index.html?url=files/GUID-AC9B3C66-90FE-493F-A7B2-44ABBA77CED2.htm,topicNumber=d30e29422>
or so) but only applies to the API. maya.cmds on the other hand is a
wrapper around MEL independent of the underlying API and so wasn't
affected by that change, PyMEL is then a another wrapper on-top of
that wrapper, but I think PyMEL uses some of the Python API for some
of its more esoteric features; possibly to distinguish between nodes,
I'm not sure.
As I already mentioned in a previous response to this email, Maya's
Python API 2.0 is available (and has been since 2013), but is
incomplete. I'm not deep enough into programming the Maya API to know
exactly how much is not done yet, but at the very least it is missing
Iterators, which takes away the majority of the useful mesh
information/editing functions. You can still use the 1.0 API for those
within the same script, but you can't mix the two--you can't retrieve an
object with the 2.0 API and then pass it to a 1.0 function for
processing (or vice versa).
You have correctly stated that maya.cmds is a wrapper around the MEL
functionality, and is again incomplete (although much less so). You
have to go pretty deep to find the areas where maya.cmds just won't
work, and it is mostly in the UI and callback functionality, along with
some of the animation commands. For these, you must wrap a MEL command
to be called. Also, maya.cmds, like MEL, is completely string based, so
you have to do a lot of work keeping track of any changes to names.
This is where PyMEL comes in.
PyMEL is a wrapper around the overall Python functionality--not just
maya.cmds. Specifically, it uses the API to reach in and give us direct
access to the node-level information and functions, and keeps direct
references to objects instead of names. It's really nice functionality
for keeping your code simple (I find that coding in PyMel produces nice
clean code that tends to take up 1/2 to 2/3 of the lines of code to
achieve the same tasks versus maya.cmds and the API. The problem with
PyMel is twofold--first, it's slower (it's another layer of wrappers,
after all), and two, it causes compatibility issues. I've run into
situations where a bug in PyMEL effectively broke my maya installations,
and anything which even used it would drop Maya into an infinite
plugin-loading loop. Because of this, I try to stay away from PyMel
unless I know it's going to be fully available and tested for installs
I'm working on.
If you truly needed a sustainable, persistent "absolute path" to the
nodes you create, you could do what I suspect Maya does internally;
which is to assign a unique identifier (UUID, GUID or similar) to an
attribute within your node. You could then use this attribute to
distinguish nodes from each other, even though they may have the same
name.
# Psuedo-code, bear with me.
# Step 1. Create node with unique identifier
group = cmds.createNode('transform') # Will produce a unique name
cmds.addAttr(group, 'uid', 'my_unique_identifier', dt='string')
cmds.rename(group, 'myCommonName') # Absolute path is lost into
oblivion..
# Step 2. Find node by unique identifier
exact_result = None
multiple_results = cmds.ls <http://cmds.ls>('myCommonName', long=True)
# May return multiple nodes
for result in multiple_results:
uid = cmds.getAttr(result + ".uid")
if uid == my_special_id:
exact_result = result
This is actually a really useful pattern to setup, and I might implement
it in future projects. However, Maya does give us functionality that
would allow us to keep direct references to objects using the
graph--Message Nodes. This also avoids the need for loops to search for
a given GUID. What it does add is the need to maintain a node to keep
these references on, which is incidentally the exact use case this
question belonged to for me :). I'm actually creating a "Meta" node
that attaches itself to my relevant structures in the scene, and it
always has the same name--which led to challenges when I was creating a
new node. So, that's where the question came from :). I think Justin's
solution is actually the one I want to work with on this, just because I
much prefer to deal with the actual object instead of trying to keep
track of the name. As I said, I really like PyMel's way of handling
things, I've just run into enough issues that I don't want to deal with
PyMel directly :).
With that said, this is a really good explanation of why you should
create a node and rename it rather than specifying the name directly.
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