Returning String Data From Python Back To Mel

[nextgencoleslaw]

Greetings All,

I know MEL well, lol...but Python less so...

Still,..I have what would appear to be a very simple task, but it is confounding me every which way.

In a simple prototype, here's what I've got....

1) MEL opens window, with textfield and button
2) hitting the button calls a Mel function that in turn loads a Python script (using loadPlugin)
and passes a string containing a single character
3) Won't go into detail, but Python code computes a 16 individual characters,
then concatenates them into one simple string.
4) While I can print the string, I cannot for the life of me figure out how to "return" it.

  The launch line is like:

      string $result[] = `PythonPlugInName -flag string`;

 Just like normal MEL,etc...

5) In the Python script, as I mentioned, all runs well in the computation stage.
Then, I can
print varName
and can see correct results...but if I do a
return varName
or
return self.varName
or
a ream of other permutations....I get no output back in the MEL script.

Am using Maya 2009, and Python 2.5.2

I suspect this is something stupid I'm doing,...or some weird quirk of the Python/Mel relationship,
but I would greatly appreciate any help or light you can shed on this...

Thank you very much,
NextGenColeslaw

P.S. At the website: http://mayamel.tiddlyspot.com/ there is a related topic, titled
"Casting variable data from Python to Maya". (Bink on "return" on right sidebar)
Does this article have any bearing on the problem?

[joojaa]

QUOTE
Still,..I have what would appear to be a very simple task, but it is confounding me every which way.

Thats because your making it into a command and dont understand that your now in c++ land.

Mel only has integers, strings,floats,vectors, and matrices anlists of all but matrices. So whatever you return must be of one of those types no automatic type conversion in python.

Now theres 2 different paths to interface with python.

Maya api (but in python as opposed to c++)

the mel python command

the mel python command

Ok ill show the second first because its easier to understand. I know for fact your going for the api solution because you load it as a plug. You probably should not load a plug because what you do is a command and thats just extra overhead on you. And basically because YOU DONT EXPECT to use API really

CODE
python("import maya.cmds as mc");
$return=python("mc.ls(sl=1)");

Ok thats fine and simple so you should probably implement your code as a module instead of a plugin, if your looking for the easy option. (a pyhon module is just a file named yourModuleName.py in your Python\Lib\site-packages directory).

PS: theres a catch 13, it only returns if you call one line!

Maya api

The API way has NOTHING to do with how python works. Its just how maya api works your bound to the API because thats what your using. No returning whatsoever (or really just succes, failure actually)! The conduit of communication in maya api does not work that way. The api is C++ not python you just use python wrappers around thees e things, its still rooted in how C++ works just wirtten in python. Having good knowledge in python only takes you so far here. Basically its not just knowing python anymore.

So let us make a simple exmple Taht takes as arguments a string and return another, for this you will need to make a external file lets call it simpleApiCommandExample.py.

simpleApiCommandExample.py:
CODE
import maya.OpenMaya as OpenMaya
import maya.OpenMayaMPx as OpenMayaMPx

#change kPluginCmdName
kPluginCmdName = "simpleApiCommandExample"

class scriptedCommand(OpenMayaMPx.MPxCommand):
  def __init__(self):
    OpenMayaMPx.MPxCommand.__init__(self)

  #basically all you do is re implement this function
  def doIt(self, args):
    if(args.length()>0):
      string=args.asString(0)*2
      #following lines are the return
      OpenMayaMPx.MPxCommand.clearResult()
      OpenMayaMPx.MPxCommand.setResult(string)

# Creator
def cmdCreator():
  return OpenMayaMPx.asMPxPtr( scriptedCommand() )

# Initialize the script plug-in
def initializePlugin(mobject):
  mplugin = OpenMayaMPx.MFnPlugin(mobject)
  try:
    mplugin.registerCommand( kPluginCmdName, cmdCreator )
  except:
    sys.stderr.write( "Failed to register command: %s\n" % kPluginCmdName )

# Uninitialize the script plug-in
def uninitializePlugin(mobject):
  mplugin = OpenMayaMPx.MFnPlugin(mobject)
  try:
    mplugin.deregisterCommand( kPluginCmdName )
  except:
    sys.stderr.write( "Failed to unregister command: %s\n" % kPluginCmdName )

Once you load this call:

CODE
$return=simpleApiCommandExample "test";
print ("got: '"+$return[0]+"'")

you can go all elegant and also just return one thing (if you omit the .clearResult() line) Do read MPxCommand's manual entry in the api reference.

PS: threes one thing of wisdom about api development that states if your doing a command your doing it wrong (coined by Bateman, I happen to agree) so maybe jsut maybe you want to implement it as a module its less code and hassle. Reserve api for nodes, displays and exporters.

PPS: "With great power comes great responsibility"- Stan Lee. More power does not in this case mean simpler.

[nextgencoleslaw]

Ahhhhhhh i see now

First, Thank you very much for taking the time to reply, and doubly so for taking the time
to write such a thoughtful and informative response.

I had not considered or realized that what I was doing was writing a python wrapper around
the C++ API. Once you said this, I had a "doh!" moment...lol
I've written some C++ API plugins before, but since I only do that once in a blue moon,
I didn't make the connection. Once I saw your "setResult(string)", it made perfect sense.

Thank you also for clearing up the relationship between python, MEL, and the API....
I was a little fuzzy about how python fit between the two, now I have a better understanding.

I get your point about "command" vs. plugin,..or "node", as I would say.
It probably does appear to a command at present,...but I once I get this prototype working
properly, I intend to turn it into a dynamic node, because I will need multiple instances of
it in the scene.

Again, Thank you for very much for taking the time to write such a complete, educational,
and informative reply. I am extremely grateful.

Cheers,
-NextGenColeslaw

[joojaa]

No problem i wrote the answer to myself really. I'm trying to hit some softspot in good answer so i can post a article on this. Testing them out is part of the process.